Progress on management of parasitic weeds - Abuelgasim Elzein and Jürgen Kroschel

INTRODUCTION

Parasitic weeds of the families Orobanchaceae (Aeginetia, Orobanche, broomrape) and Scrophulariaceae (Alectra, Striga, witchweed) are considered to be among the most serious agricultural pests of economic importance in many parts of the world. The genus Striga includes about 40 species, of which 11 species are parasites on agricultural crops. The genus Orobanche has more than 100 species but only seven are considered as economically significant (Parker and Riches, 1993; Raynal-Roques, 1996).

Geographical distribution and main host plants

Parasitic weeds have evolved specificity to crops and plants in the natural vegetation. Striga hermonthica (Del.) Benth., S. asiatica (L.) Kuntze and S. gesnerioides (L.) Vatke, in the given order, are the most economically important species in the semi-arid to sub-humid tropics. The former two species are almost entirely specific to grasses (cereals) such as sorghum (Sorghum bicolor (L.) Moench), maize (Zea mays L.), pearl millet (Pennsisetum americanum L.), rice (Oryza sativa L.), sugar cane (Saccharum officinarum L.) and others, while the third one is parasitizing dicot hosts, mainly cowpea (Vigna unguiculata (L.) Walp.), tobacco (Nicotiana tabacum L.) and sweet potato (Ipomea batatas (L.) Lam.) (Ejeta et al. 1992). Africa was described as the place of origin of the agriculturally important Striga species, particularly the Sudano-Ethiopia region, where also sorghum was postulated to be originated (Vasudeva Rao and Musselman, 1987). S. hermonthica is widespread in the semi-arid zones of northern tropical Africa and it is also found in the south-western part of the Arabian Peninsula (Mohamed et al. 2001). S. asiatica, on the other hand, has a wide distribution in the eastern to southern part of Africa, Asia, Australia and the United States (Musselman, 1987). The third species, S. gesnerioides, occurs in Africa, the Arabian Peninsula, the Indian subcontinent, and has been introduced to the United States (Musselman and Parker, 1981).

The species of the genus Alectra are found mainly in tropical Africa and subtropical southern Africa. A. sessiliflora, and A. fluminensis are also found in subtropical Asia and tropical and subtropical South America, respectively (Parker and Riches 1993). A. vogelii Benth. is the most important species parasitizing mainly grain legumes in sub-Saharan Africa, which include cowpea, bambara groundnut (Vigna subterranea (L.) Verdc.), soybean (Glycine max (L.) Merr.), mung bean (Vigna radiata (L.) Wilczek), groundnut (Arachis hypogaea L.) and common bean (Phaseolus vulgaris L.).

The Mediterranean region is considered to be one of the centres of origin of Orobanche species. The species are distributed worldwide from temperate climates to the semi-arid tropics. The distribution of Orobanche crenata Forsk. is restricted to the Mediterranean regions, the Middle East and East Africa (Ethiopia), while other species have a wider spread. Today, the species O. crenata, O. ramosa L., O. aegyptiaca Pers., O. cernua Loefl., O cumana Wallr., O. minor Sm. and O. foetida Poir. are one of the major biotic limiting factors to the production of legumes such as faba bean (Vicia faba L.), chickpea (Cicer arietinum L.), lentil (Lens culinaris Medick.), and to crops of the family Solanaceae [tomato (Lycopersicon esculentum Mill.), potato (Solanum tuberosum L.), and tobacco (Nicotiana tabacum L.)] and Asteraceae, mainly sunflower (Helianthus annuus L.) (Parker and Riches, 1993).

Life cycle

The seeds of the root-parasitic weeds vary in their ability to germinate immediately after they have reached maturity. Seeds of Striga and Orobanche are dormant and require a period of after-ripening or so called post-harvest ripening period, whereas seeds of Alectra vogelli can germinate immediately after harvest when germination requirements are met (Riches, 1989). Seed germination occurs when ripened seeds are preconditioned by exposure to warm moist conditions for several days followed by exogenous chemical signals produced by host roots and some non-hosts (germination stimulant) (Worsham, 1987). Upon germination, a germ tube, which is in close proximity to the host roots, elongates towards the root of the host, develops an organ of attachment, the haustorium, which serves as a bridge between the parasite and its host, and deprives it of water, mineral nutrients and carbohydrates, causing drought stress and wilting of the host. Stunted shoot growth, leaf chlorosis and reduced photosynthesis are also phenomena that can be observed on susceptible host plants which contribute to reduction of grain yield (Frost et al. 1997). Most of the seeds in the soil will not be reached by the stimulant, but will remain viable for up to 15 years, forming a seed reservoir for the next cropping seasons. The penetration of haustorial cells into host tissue (xylem and/or phloem system) is carried out mechanically by pressure on the host endodermal cells and by hydrolytic enzymes. Conditioning, germination, parasitic contact (attachment) and penetration are mediated by elegant systems of chemical communication between host and parasite (Maass, 1999). After several weeks of underground development the parasite emerges above the soil surface and starts to flower and produce seeds after another short period of time. Seed production is prodigious, up to 100 000 seeds or even more can be produced by a single plant and lead to a re-infestation of the field. Thus, if host plants are frequently cultivated, the seed population in the soil increases tremendously and cropping of host plants becomes more and more uneconomical (Kroschel, 2001).

Agricultural significance and yield losses

A considerable loss in growth and yield of many food and fodder crops is caused by root-parasitic flowering plants. Globally, Striga have a greater impact on human welfare than any other parasitic angiosperms because their hosts are subsistence crops in areas marginal for agriculture. In general, low soil fertility, nitrogen deficiency, well-drained soils, and water stress accentuate the severity of Striga damage to the hosts. These are typically the environmental conditions for Striga-hosts in the semi-arid to subhumid tropics. Nowadays, Striga is considered as the greatest single biotic constraint to food production in Africa, where the livelihood of 300 million people is adversely affected. In infested areas, yield losses associated with Striga damage are often significant, ranging from 40-100 percent (Bebawi and Farah, 1981; Lagoke et al. 1991; Ejeta et al. 1992). Moreover, it is predicted that grain production in Africa is potentially at even increasing risk in the future. This is because several factors that influence the occurrence and may accelerate the future spread and the infestation intensity of Striga species in agricultural cropping systems. These include the future adaptation of Striga to crops and to a wide ecological amplitude, and a drop in soil fertility in tropical soils (Kroschel, 1998). The significant yield reductions result in little or no food at all for millions of subsistence farmers and consequently aggravate hunger and poverty.

Alectra vogelii is a serious pest in cowpea production in Africa. The parasite infection did not decrease cowpea dry matter production, but it significantly altered dry matter partioning by increasing the proportion of root dry matter (Rambakudzibga et al. 2002). Crop yield losses resulting from A. vogelii infestation range from 41 percent to total crop loss of highly susceptible cultivars (Lagoke et al. 1993). The yield reduction is mediated through the delayed onset of flowering, reduced number of flowers and pods, and reduced mass of pods and grain (Mugabe, 1983).

The damage caused by the parasites Orobanche on field and vegetable crops is significant in the Near East, South and East Europe and in various republics of the former Soviet Union. It causes yield losses ranging from 5-100 percent (Linke et al. 1989). For example, in Morocco, the infestation of O. crenata in food legumes caused yield losses of 32.7 percent on an average in five provinces in the year 1994, which was equal to a production loss of 14 389 tonnes (US$8.6 million. (Geipert et al. 1996). As a result of the complete devastation caused by Orobanche in many areas, production methods had to be modified and/or cultivation of some susceptible hosts had to be abandoned.

Control methods: possibilities and constraints

Compared with non-parasitic weeds, the control of parasitic weeds has proved to be exceptionally difficult. The ability of the parasite to produce a tremendously high number of seeds, which can remain viable in the soil for more than ten years, and their intimate physiological interaction with their host plants, are the main difficulties that limit the development of successful control measures that can be accepted and used by subsistence farmers. However, several control methods have been tried for the control of parasitic weeds, including cultural and mechanical (crop rotation, trap and catch cropping, fallowing, hand-pulling, nitrogen fertilization, time and method of planting, intercropping and mixed cropping), physical (solarization), chemical (herbicides, artificial seed germination stimulants, e.g. ethylene, ethephon, strigol), use of resistant varieties, and biological. These methods of control were well reviewed by Parker and Riches (1993), and recently summarized in Kroschel (2001), and Omanya (2001). At on-farm level, the management of parasitic weeds is still unsatisfactory since - with the exception of the use of glyphosate in faba bean to control O. crenata - present control methods are not efficient enough to control already the underground development stages of the parasites. At present, the restoration of infested fields can only succeed through the improvement of existing farming systems based on a sound analysis of the parasitic weed problem and the development of a sustainable long-term integrated control programme consisting of the more applicable control approaches that are compatible with existing farming systems and with farmer preference and income (Kroschel, 1999). The success of cultural measures becomes evident only in the long run and will not improve yields in the present crop, because of the long underground developmental phase as well as the high seed production and longevity (Parker and Riches, 1993). The income of the subsistence farmers is usually too low to justify the use of highly sophisticated technical inputs such as ethylene to trigger ineffective Striga seed germination, as used in North Carolina to eradicate S. asiatica, or with soil solarization. In addition to the cost, selectivity, low persistence and availability are major constraints that limit the successful usage of herbicides. In addition, the use of synthetic germination stimulants and application of high dosage of nitrogen fertilizer (more than 80 kg N ha^-1, mainly as ammonium sulfate or urea), are not readily applicable in African farming systems (Kroschel et al. 1997). Few resistant lines for some host-parasite associations were reported (Lane et al. 1997) but resistance is often interfered by the large genetic diversity of the parasites. Recent successes have been achieved in biological control, but it has not led to practical field application owing to the difficulties associated with mass rearing, release, formulation and delivery systems. Accordingly, the principal objective of this chapter is to summarize the progress that has been achieved recently in the management of parasitic weeds. Strategies will be proposed on how to utilize this progress to formulate successful control methods that are economically accessible and acceptable to subsistence farmers

ADVANCEMENTS IN PARASITIC WEED CONTROL

Progress in cultural control: indigenous practices and possible improvement

Transplanting

In some areas of Africa and Asia, transplanting is a traditional practice in sorghum and millet cultivation, either to fill the gaps after crop emergence and thinning or to compensate for a growth period that is too short for a complete crop cycle (Rehm, 1989). However, transplanting maize has not been practiced in rainfed conditions of the tropics and subtropics, probably because of the lack of an appropriate technique and the lack of irrigation to control moisture at the time of transplanting. Transplanting a host might be an effective method in Striga management because the crop seedling would be older and more resistant to Striga attack. Cechin and Press (1993) found that fewer Striga plant attached, and less damage was inflicted on the host with the increasing age of the sorghum. In in vitro experiments, germination and underground development of Striga hermonthica was reported to be low in transplanted sorghum compared with directly-sown sorghum (Dawoud et al. 1996). The method significantly reduced the damage caused by Striga to sorghum and increased the crop yield. More recently, Oswald et al. (2001) assessed the effect of transplanting maize and sorghum on grain yield and Striga parasitism under rain-fed field condition of western Kenya. They found a significant increase in grain yield of the transplanted maize and less Striga attachment and emergence compared with the direct seeding. However, the transplanted sorghum failed to reduce either Striga emergence or to improve grain yield. Considerably, low densities of Striga were observed when maize seedlings were 17 days old at transplanting. With the increasing age of maize seedlings, a further decrease in the level of Striga infestation was recorded.

It could be concluded that transplanting of sorghum and maize seems to be a possible method that might lead to an increase in crop tolerance to the parasite infestation. The method is simple and requires low skill for its implementation in such a way that it can be done by the subsistence farmers and their families. Because of the high labour requirement, transplanting maize under rainfed conditions is probably only suitable for small areas (0.1 ha) that are highly infested with Striga. Under these conditions, crop yield can be more than doubled. An incentive to using this method by small-scale farmers would be that the main input at risk is their own labour. Maize transplanting could also be extended to large production mechanized farms, if the positive results of transplanting maize developed by Scheffer (1988) in Germany could be adapted under rainfed conditions. However, the establishment of nurseries and the timing of the transplanting operation require a certain level of farm management that could restrict the adoption of this technique.

Crop rotation with trap and catch crops

Crop rotation with trap and catch crops has long been proposed and practised as control measure for parasitic weeds. The most favourable rotation as far as Striga control is concerned is a rotation with a trap crop; a crop which stimulates Striga seed germination but which cannot be infected by the parasite. The most promising trap crops reported for Striga and Orobanche species are recently listed in Kroschel (2001). It is obvious that farmers from countries or regions with possibilities for marketing Striga trap crops like cotton and soybean have an overall advantage in adopting such farming systems (Kroschel and Sauerborn, 1996). It has been concluded that large differences between cultivars in their ‘trapping’ ability exists and that the characterization and recommendation of the best cultivars should be a routine activity for researchers focusing on Striga and Orobanche infested areas (Berner et al. 1995). It should also be borne in mind, however, that farmers prefer many traits in a particular cultivar such as high yield potential, earliness, colour, cooking time, etc., which need to be considered when making recommendations to farmers (Ransom, 1999). Catch crops are crops that are susceptible to the parasite and thus become infected. Before the parasite has the chance to flower and to set seed, the farmer should up-root the catch crop to destroy the parasite. The most promising catch crop for S. hermonthica control is the use of Sudan grass (Sorghum sudanense L.) (Last, 1961, Oswald et al. 1999). However, catch cropping has rarely been used by small farmers to control Striga because the technique is not well known or understood and holds serious disadvantages if not adapted to a specific cropping system (Oswald et al. 1999). In spite of several reports of successes, it has been shown over the years that these two methods (trap and catch crops) only have a reasonable effect in areas where the parasite infestation level of the soil is very low. Unfortunately, experiments in western Kenya have shown that even after four years of continuous cropping with cowpea or cotton, damaging levels of Striga still remain in the soil (Odhiambo and Ransom, 1996).

Intercropping

Intercropping cereals with legumes and other crops is a common practice in most areas of Africa, and has been reported as influencing Striga infestation. Recent result showed that intercropping maize with cowpea and sweet potato can significantly reduce the emergence of Striga in Kenya (Oswald et al. 2002). In Kenya, more recently, it was discovered that inhibition of Striga hermonthica, was significantly greater in maize-silverleaf [Desmodium uncinatum (Jacq.) DC.] intercrop than that observed with other legumes, e.g. sun hemp (Crotolaria spp.), soybean or cowpea (Khan et al. 2000). Consequently, the yield of maize was significantly increased by two tonnes per ha. Desmodium species are legumes that can easily be controlled by regular cutting in order to avoid or minimize the competition with the crop if any. The mechanisms by which D. uncinatum reduce Striga infestation in intercropping was found to be the allelopathic effect inhibiting the development of haustoria of Striga (Khan et al. 2001). Identification of the compounds released from D. uncinatum involved in the suppression of the parasite may give more exploitation for developing reliable intercropping strategies, as well as new approaches for molecular biology in S. hermonthica (Gressel, 2000). In conclusion, intercropping has the potential to reduce Striga infestation and reproduction.

Managed fallows as a new technique for Striga management

Managed fallows are a new concept that is being introduced into areas where the soils are severely depleted. Leguminous trees and shrubs possess many advantages for being used as candidates for managed fallow. Once established, they need very little management and can produce much needed fuel-wood and fodder with some species, while significantly enhancing the nitrogen status of the soil (Rao and Gacheru, 1998). Multipurpose trees grown on farms (agroforestry) may have the potential to increase soil fertility and/or cause suicidal germination of Striga seeds and thereby help to reduce the level of Striga infestation. A number of tree species have been identified as good candidates for managed fallows and have been shown to stimulate Striga germination in pot and root chamber studies (Oswald et al. 1996). Sesbania sesban (L.) Merr. is the most promising one, both in terms of ease of establishment and in ease of removal later from the field as well as stimulating Striga to germinate (ICRAF 1996). Further laboratory tests by (Rao and Gacheru 1998) confirmed the potential of a number of tree/shrub species in stimulating the germination of preconditioned Striga seeds. The most promising species in these tests were Senna spp., Crotalaria agatiflora Schweinf., and C. grahamiana Wight and Arn., Calliandra calothyrsus Meis, Leucaena leucocephala (Lam.) de Wit, and Desmodium distortum (Aubl.) Macbr. In field trails in Kenya, the same authors reported less Striga infestation on maize after 12-month fallows of Sesbania spp., S. didymobotrya (Fresen.), Tephrosia vogelii (Hemsley) A. Gary, C. grahamiana, and D. distortum than after natural fallows. In the field in eastern Zambia, maize following three-year Sesbania sesban, Leucaena leucocephala and Senna siamea (Lam.) H.S. Irwin and Barneby fallows did not show any infestation of S. asiatica (ICRAF, 1996).

According to Rao and Gacheru (1998), improved fallows by adopting agroforestry technology reduced Striga infestation by some or all the following mechanisms:

• the trees in the fallow system acting as false hosts and causing suicidal germination of Striga seeds;

• increased mineral nitrogen in the topsoil at the end of fallows and nitrogen mineralization in the subsequent cropping phase;

• production of Striga stimulant in the process of leaf litter decomposition;

• enhanced microbial activity following the incorporation of organic residues in the soil affecting Striga seed conditioning and seed viability; and

• improved soil fertility enabling maize plant to better compete with Striga and reduce the damaging potential of Striga. Thus, an appropriate agroforestry system can provide a significant supplementary strategy to deplete the Striga seed bank through suicidal germination, even during the fallow phase.

Progress in chemical control: accessibility and economic feasibility

Herbicides

In order to be effective and beneficial within the growing season, herbicides for the control of root parasitic weeds, which are characterized by long underground developmental stages, may be applied pre- or post-emergence of the crop but should always be pre-emergence of the parasites. During the last decades, some potential useful chemical interventions have become available for parasitic weed control (Garcia-Torres, 1998). However, lack of application technology, marginal crop selectivity, low persistence and availability are major constraints that limit the successful usage of herbicides in developing countries where the income of subsistence farmers is usually too low to afford them.

Striga: Dicamba, as a post-emergence herbicide, has been shown to control Striga when applied soon after attachment, but timing is very critical to maximize its effectiveness, both in term of Striga control and in safety of the crop (Odhiambo and Ransom, 1997). 2, 4-D is the herbicide most widely used to prevent further Striga seed production. Therefore, the herbicide needs to be sprayed several times directly on the parasites during the growing season, because Striga seedlings that are still in their subterranean stage are unaffected by it (Pare et al. 1996). Because of its low selectivity, however, 2, 4-D cannot be used in intercropping.

Commercial seed treatments with fungicides and insecticides are very commonly used in modern agriculture. However, herbicide seed treatments have not been commercially developed and could be of interest in the case of root parasitic weeds, since the parasites infections only occur in the root system of the host plants. An interesting outcome of recent efforts has been the delivery of the acetolactate synthase (ALS)-inhibiting herbicides as seed dressing on maize genotypes which possess target-site resistance. The advantage of these maize genotypes allows the use of an extensive family of effective herbicides against Striga which would otherwise kill maize. Among these herbicides, imazapyr and pyrithiobac, which were safe to ALS-resistant maize as seed drenches, primings, and coating, were found to provide excellent and effective Striga control (Kanampiu et al. 2001a). Even under heavy Striga infestation these treatments can improve maize yields by almost threefold (Abayo et al. 1998). In further laboratory investigations, almost complete destruction of viable Striga seeds in the upper 10 cm, and 80 percent supression of the seed germination at 30 cm depth was reported when imazapyr and pyrithiobac was dressed to the seed of imidazolinone resistant (IR) maize (Kanampiu et al. 2001b). Systemical translocation of both herbicides through, and their exudation from, the roots of maize was also confirmed by the destruction of attached Striga and decimating its seed bank in soil, respectively. However, herbicide movement in the soil profile may cause a detrimental effect on susceptible intercrops, since intercropping of Striga hosts is commonly practiced in Africa. However, imazapyr and pyrithiobac-sensitive cowpea and yellow gram (Vigna radiata L.) were unaffected when interplanted in the field at more than 15 cm from maize seeds coated with 0.4 mg a.i. pyrithiobac and 0.84 mg a.i. imazapyr seed^-1, respectively (21 and 45 g a.i. ha^-1), double the recommended dose. Since these legumes are typically sown equidistant between maize hills within the row (i.e. 30 cm from each hill), or between the maize rows (spaced at 75 cm), the technique is safe for the intercrops. The simplicity of the technology, its adaptability to the agronomic Striga control methods, and, more specially, its compatibility with intercropping farming systems, might contribute to a more meaningful application of an integrated control approach for Striga in the future, provided that herbicide-resistance will not be developed by new races of Striga. However, currently it is not an option for resource-poor farmers. Continuity in the exploitation of genetic engineering techniques into crops to obtain herbicide-resistant plants will widen the use of inherently safer herbicides, and provide cost-effective, broad-spectrum parasitic weed control.

Orobanche: Several attempts and intensive research were made in different countries to screen for potential herbicides against Orobanche. In general, the most limiting factor in the use of the promising herbicides is their degree of selectivity among the crops at the required rate for parasite control, and the critical time of application, especially of the foliarly applied systemic herbicides. However, glyphosate is the first promising herbicide developed for Orobanche crenata control in faba bean (Schmitt et al. 1979), and is still the most important herbicide used. Since then, a considerable number of studies have been performed in trying to clarify the selectivity and use of glyphosate in other legume and non-legume crops that are susceptible Orobanche hosts other than faba bean. Some degree of success has been achieved in some crops under specific conditions such as time and rate of application. Hence, the crops in which the use of glyphosate is well confirmed, so far, include faba bean, carrot (Daucus carota L.) and celery (Apium graveolens L.). However, the corresponding yield increase is inconsistent especially under heavy infestation. In addition to glyphosate, other herbicides, e.g. sulfonylurea, imazethapyr and imidazolinone proved to be effective in many host crops, both at the demonstration or commercial levels used in many locations in the Near East (Israel, Jacobsohn, 2002) and the Mediterranean region (Spain, Garcia-Torres et al. 1999). It has been shown that the phytotoxicity of some post-emergence herbicides on crops can be avoided or reduced by altering their application and delivery technique. For example, by chemigation chlorsulfuron and triasulfuron into the soil, O. aegyptiaca can be successfully controlled and the yield of tomato increased without damaging the crop (Kleifeld et al. 1999). In Spain, promising results were also obtained (60-80 percent O. crenata control) by soaking or coating faba bean and pea (Pisum sativum L.) seeds, or lentil seeds in low concentration of imazethapyr or imazapyr, respectively (Jurado Exposito et al. 1996, 1997; Garcia-Torres et al. 1999). The seed germination and crop growth were not affected by the phytotoxicity of both herbicides. However, this technology has not yet been developed or optimized for application at the field level.

Germination stimulants

The application of germination stimulants to induce suicidal seed germination of parasitic weeds appear attractive because of their safety, decomposition in the soil within a short period of time and their high biological activity at a very low rate. An outstanding example so far was the use of ethylene gas in North Carolina to eradicate S. asiatica, through injection of the gas below the soil surface using special sophisticated machinery. At research level, the method was also tested in Africa. However, the result achieved in Kenya was only 50-60 percent reduction of the seed bank, which was much less than that routinely achieved in the United States (Egley et al. 1990). However, in some parts of Africa. ethylene was found to be more effective than trap crops in reducing the Striga seed bank (Odhiambo and Ransom, 1997). Thus, its use in heavily infested areas would likely be highly economical provided that its distribution challenges and application technology were to be resolved, since the actual cost of the chemical is small ($12.4 per ha) (Ransom, 1999).

Nijmegen 1, recently synthesized by Zwanenburg and Wigchert (1998) and GR 24, which are structurally related to the natural stimulant Strigolactone, are the most potent active synthetic germination stimulants for many species of Striga and Orobanche at low concentration (10^-9-10^-6 mol l^-I) (Wigchert et al. 1999). The naturally-produced Strigolactone cannot be isolated for commercial application because of its extremely low concentration in sorghum (Wegmann, 2002). However, large-scale preparation of Nijmegen 1 is relatively easy and possible at low cost, which may facilitate field application (Nefkens et al. 1997; Wigchert et al. 1999). Under field conditions, application of 2.5 l m^-2 of 50 ppm solution of Nijmegen 1 depleted the soil seed bank of Orobanche by an average of 35 percent (0-30 cm depth) (Miele et al. 2001) which corresponded to maximum germination induced by it in in vitro studies. Proper formulation of the stimulant could perhaps increase efficacy and distribution in the soil. Recently, Wegmann (2002) reported very promising results when the formulated Nijmegen 1 (by the BASF AG), was applied under field conditions. In-depth knowledge of Nijmegen 1 behaviour in the soil such as its adsorption, mobility and fate and how to spray the compound with a manageable carrier volume may help in understanding its relative efficacy under contrasting soil and environmental conditions (Streibig, 2002).

Progress in breeding for resistant crop varieties

Host plant resistance would probably be the most feasible and potential method for parasitic weed control. Using biotechnological approaches (including biochemistry, tissue culture, plant genetics and breeding, and molecular biology) significant progress has been made in developing screening methodologies and new laboratory assays, leading to the identification of better sources of parasitic weed host resistance (Ejeta et al. 2000; Haussmann et al,. 2000; Omanya, 2001; Mohamed et al. 2001). Full immunity of host plants to Striga or Orobanche has not yet been found. However, several resistant crop varieties are used nowadays in various parts of Africa, Europe and Asia. As the reported resistant or tolerant cultivars are often not accepted by farmers because of their low yield, low seed and storing quality, poor adaptation to a wide range of agro-ecological zones and their sensitivity to pest and diseases, the newly developed techniques significantly contribute to overcoming these problems by permitting transfer of resistance genes into adapted cultivars with high-yielding potential. This will lead to a lower parasite infestation and to a higher crop yield.

Striga: Resistance to Striga has been documented in cowpea, upland rice and sorghum (Hess and Haussmann 1999). Over the past few years, the INTSORMIL PRF-213 project has led to identification of a number of sorghum varieties to be officially released for commercial cultivation in several countries (Ejeta, 2002). In 1999, two Striga resistant cultivars were officially released for a wide cultivation in Striga endemic regions of northwest Ethiopia. In 2002, another superior variety, called Brhan, was identified and recommended for official release in the Amhara region of Ethiopia. In Tanzania, two white-grained, early-maturing Striga resistant sorghum varieties have been approved for official release and cultivation in sorghum- growing areas where Striga has been a significant production constraint. Both lines (P9405 and P9406) which have been registered with the names KAKIKA and WAHI respectively, were developed at Purdue University, United States (Mbwaga and Riches, 2002). In highly infested fields, however, sorghum resistance level is often not sufficient to reduce yield losses, e.g. the resistance mechanism of SRN39, which is based on low stimulant production, is not effective in fields with a high Striga seed bank.

In maize, considerable progress has been achieved at the International Institute of Tropical Agriculture (IITA) and by the International Maize and Wheat Improvement Center (CIMMYT) in developing open-pollinated varieties, inbred lines and hybrids that have both reduced host plant damage symptoms (tolerance) and reduced parasite emergence under artificial infestation with S. hermonthica (Kim, 1994; Berner et al. 1995; Lane et al. 1995). Some of these varieties have also shown moderate levels of resistance to S. aspera (Willd.) Benth. and S. asiatica. Resistant varieties have been developed with adaptation to the lowland and mid-altitude ecologies, and with a range of maturity, grain colour and grain texture characteristics (for more details see Kling et al. 2000). In order to ensure that the developed varieties are adapted to farmer circumstances and satisfy end-user preferences, the best varieties are being actively extended to farmers through the efforts of the regional maize and Striga networks and several collaborative projects, including the African maize stress project, and these are now being evaluated.

In rice, Oryza glaberrima lines ‘ACC102196’, ‘Makassa’, and ‘IG 10’, as well as O. sativa lines ‘IR49255-BB-5-2’ and IR47255-BB-5-4’ showed partial resistance to S. aspera and S. hermonthica under field conditions in Cote d’Ivoire (Riches et al. 1996; Johnson et al, 2000).

In cowpea, the landrace ‘B 301’ from Botswana; ‘IT82D-849’ (improved breeding line from IITA); ‘Gorom Local’ from Burkina Faso; and ‘58-57’ from Senegal were found to be completely resistant, with some variation, to different strains of S. gesnerioides (Singh and Emechebe 1990). By crossing, a number of new varieties have been developed with combined resistance to all five strains of S. gesnerioides (Singh, 2000). Some of these lines also showed resistance to Alectra vogelii, bacterial blight, aphid, bruchid, thrips, and virus diseases with yield potential over two tonnes ha^-1.

Orobanche: Resistant/tolerant varieties to Orobanche have been developed in several crops and used for some years. The most outstanding example has been the development of sunflower varieties resistant to O. cernua/cumana. Unfortunately, this resistance has often been overcome by new, more virulent ‘races’ of the parasite in many countries across the Mediterranean region, eastern Europe and the former Soviet Union. Recently, the ‘“ of O. cumana (Rodrigue-Ojeda et al. 2001). Two cultivars of faba been (Giza 429 and Giza 674) with a good level of resistance to O. crenata have been released in middle- and upper-Egypt (Khalil et al. 1993). Furthermore, a new faba bean genotype (X-843) resistance to O. crenata, derived from Giza 402, was reported to have a good yield performance and was recommended for release in north Egypt (Saber et al. 1999). A well-adapted, high-yielding faba bean cultivar ‘Baraca’ has been developed in Spain under field conditions, with a high level of resistance to O. crenata (Cubero 1994). So far, seeds of the released and recommended faba bean cultivars are not commercialized and are thus not available for farmers. Alonso (1998) has intensively reviewed the most significant results achieved in the breeding for resistance to other Orobanche spp. However, most of the reports on other crops referred to tolerance and moderate levels of resistance, and were unable to detect a reliable high level of resistance. This has been the case for the following parasite-host relationship: O. aegyptiaca and O. ramosa (tomato), O. ramosa (hemp), O. crenata (lentils, chickpea and pea), O. aegyptiaca (cucurbits), O. aegyptiaca, O. ramosa and O. cernua (tobacco). In some cases complete resistance under field conditions of some vetch lines (473A) (Gil et al. 1987; Goldwasser et al. 1997), and tomato lines (PZU-11) (Foy et al. 1988) was confirmed. Nevertheless, under artificial inoculation with a very high dose of Orobanche, even the field resistant lines were attacked by broomrape.

More recently, Sauerborn et al. (2002) induced resistance in sunflower against O. cumana by using the synthetic chemical benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH), the active ingredient of Bion^®, under controlled growth camber conditions. However, the application of BTH failed to induce resistance in sorghum and maize against Striga (Zain, 2002).

Progress in biological control: implementation and future prospectives

Biological control of weeds with insects and microbial agents means the utilization of living organisms to manipulate (suppress, reduce, or eradicate) weed densities. Biological control, especially using insects and fungal antagonists against parasitic weeds, has gained considerable attention in recent years and appears to be promising as a viable supplement to other control methods.

Insects

Many phytophagous insects have been collected on Striga and Orobanche species but most are polyphagous and the target weed species are often not their principal host plants. Oligo- and monophagous herbivores with potential in biocontrol of Striga spp. are larvae of the butterfly Eulocastra argentisparsa Hampson (Lepidoptera: Noctuiidae) in India; Smicronyx spp. (Coleoptera: Curculionidae) in India and Africa; and the shoot miner Ophiomyia strigalis Spencer (Diptera: Agromyzidae) in East Africa (Kroschel et al. 1999; Traoré et al. 1999). The fly Phytomyza orobanchia Kalt. (Diptera: Agromyzidae) and Smicronyx cyaneus Gyll. are of great interest in biocontrol of Orobanche spp. (Klein and Kroschel, 2002). As a single method, biological control with herbivores will hardly be fully successful in tackling the parasitic weed problem. But in combination with other methods in an integrated control package, herbivores could play a role in lowering the parasitic weed population and reducing its reproductive capacity and spread. Most promising are inundative mass releases of P. orobanchia. However, also classical biological control might be an option by introducing P. orobanchia into countries where Orobanche spp. are not endemic and have been unintentionally introduced, such as Chile (Norambuena et al. 1999, 2001; Klein and Kroschel, 2002).

Microbial agents

Plant pathogens are proposed for use in a non-classical inundative approach as ‘bioherbicides’ for biological control of parasitic weeds. The protocol for their use involves: to survey the weed for pathogens; isolation; identification and classification; inoculum production; screening for efficacy (pathogenicity testing); host specificity and safety testing; inoculum mass production; preliminary field testing; formulation and delivery to target weed. Accordingly, a variety of fungal and very few bacterial agents applied to the seeds, foliage and/or soil have been explored as potential candidates for parasitic weeds of the genus Striga and Orobanche since the early 1990s (well reviewed in Kroschel and Müller-Stöver, 2003).

The promising fungal isolates currently under investigation for the biocontrol of Orobanche and Striga were found to have a different width of host range (Table 1). FOO exclusively attacks O. cumana, the very closely related O. cernua and some susceptible biotypes of O. aegyptiaca (Bedi 1994; Thomas et al. 1998). Müller-Stöver, (2001) observed a reduced germination rate of O. crenata seeds caused by FOO, although no pathogenicity was observed towards later developmental stages of the parasitic weeds. FOXY and FARTH, isolated in Israel attack O. aegyptiaca, O. cernua and O. ramosa, but are avirulent against O. cumana (Amsellem et al. 2001). Recent observations showed that Foxy 2 significantly reduced the emergence of S. hermonthica and S. asiatica, whereas disease symptoms could only be observed on S. hermonthica (Elzein and Kroschel, 2003). This advantage and the ability of the potential fungal isolates to control more than one Orobanche or Striga species provides an opportunity to control more parasites simultaneously in those regions where they are co-existing, which may encourage the regulatory authorities to accept and implement inundative biological control of parasitic weeds.

The type of formulation used for delivering a bioherbicide depends on the biology of the target weed, the type, biology and mode of action of the pathogen as well as on the available application technology. For fungal pathogens, the simplest method is the use of a spore or mycelial suspension in water, which can be used as a soil drench or a post-emergence spray application. For parasitic weeds, this formulation was used in the early stages for the evaluation of the efficacy of potential fungal antagonists either in greenhouse or field conditions. Solid or granular formulations are more suitable for pathogens that infect their target weeds at or below the soil surface (i.e. attacking weed seedlings as they emerge from the soil), a system more appropriate for root parasitic weeds and best suited for pre-emergence application. Hence, several solid substrates have often been used as carriers to deliver mycoherbicides of parasitic weeds. These include infested cereal grains or straw (barely, wheat, sorghum), soil/maize feed mixtures, a mixture of straw and maize flour, and river sand and maize meal. Although these simple granular (solid) formulations were proved to be very effective under greenhouse and field conditions in both control of the parasites and improvement of the performence of host plants, it was found that very high levels of fungal inoculum (approximately 800 kg ha^-1) were required for successful parasite control. In addition, many undesirable characteristics are encountered with the use of cereal grains and straw as substrates for delivering pathogenic fungi for parasitic weed control, especially under large-scale heavy infestation, including:

• difficulty in the preparation and application of large amounts;

• inadequate shelf-life;

• difficulty in controlling the production of the appropriate (desired) fungal propagules;

• high cost;

• not suitable for commercial use.

It is evident that research efforts to develop an appropriate mycoherbicidal formulation of the potential fungal antagonists for Orobanche and Striga control are both justified and needed for facilitating field application.

Mycoherbicides: Challenges and limitations of success

A number of biological, environmental and technological limitations, variability of field performance, cost of production and market size are the major constraints that potentially affect the economic feasibility of any given biological control product (Auld and Morin, 1995). Up to date, a large number of microbial biological control agents have been, or are being, evaluated for their pathogenicity against parasitic weeds. Despite the high number of organisms investigated, only two isolates for the control of Striga and three for Orobanche are being considered as effective and promising candidates (Table 1), but have not yet reached large-scale field application. However, it should be kept in mind that discovery of a pathogen is only one step in the long process of development of a bioherbicide. The final biocontrol product is one where all processes in production and formulation, delivery, and information on the pathogenicity, mode of action and host specificity have been evaluated. There are many inherent obstacles to overcome in order to establish biocontrol efficacy These include basic aspects of strain selection, efficient production of biomass, formulation, storage ability, and method of application. Also, a better understanding of mechanisms of action, nutrition and ecology of the biocontrol agent is needed. In addition, avoidance of unintentional adverse effects on non-target cultivated and wild plant species, related to the target weed or grown within the range of dissemination of that pathogen, so called ‘host specificity’ needs to be ascertained. Further, the potential for genetic manipulation of microbial agents to create genetically superior strains or hybrids that can perform better than the wild types needs to be considered. The progress in biological control of parasitic weeds microbially is intended to be addressed with regard to the achievements in overcoming the above mentioned obstacles.

SUCCESSFUL APPROACHES FOR OVERCOMING LIMITATIONS OF BIOCONTROL: A STEP TOWARDS PRACTICAL APPLICATION

Mass rearing and release of insects: the case of phytomyza orobanchia

Biology and natural efficacy

P. orobanchia (Diptera, Agromyzidae) has co-evolved with Orobanche species. Therefore, its distribution is related to the natural occurrence of plants of the genus Orobanche. The damage on Orobanche is caused by the larvae, which mine in shoots and seed capsules. Thereby P. orobanchia has an enormous impact on the seed production of Orobanche spp. Depending on the site, the host plant and the Orobanche species, up to 95 percent of the seed capsules can be infested by P. orobanchia. A natural reduction of the seed production between 11-79 percent is reported from several countries of the Mediterranean region, North and East Africa and the Near East (Klein and Kroschel, 2002). However, the efficacy of P. orobanchia to reduce the Orobanche population is mainly limited by cultural practices (by destroying pupae in soil through soil cultivation methods or by using insecticides) and the occurrence of natural enemies. Most important are larval and pupal parasitoids of the order Hymenoptera with, worldwide, 24 identified species belonging to seven families (Klein and Kroschel, 2002). The natural population of P. orobanchia is too low to be able to reduce sufficiently the Orobanche population to the point that no economic losses occur. The natural equilibrium between Orobanche and P. orobanchia as exists in natural vegetation is disturbed by the extensive cropping of Orobanche host plants. Therefore, an increase in the efficacy of P. orobanchia can only be achieved by augmentation of the natural population.

Efficacy of inundative

Biocontrol of Orobanche spp. by P. orobanchia is based on the prerequisite that Orobanche shoots are loaded with P. orobanchia pupae at the time of harvest of the Orobanche host plants and that no rearing methods on artificial diets have been successfully developed. This means that Orobanche shoots have to be collected and stored until the next season. By doing so, the mechanical destruction of pupae by tillage or other field operations can be avoided. Storage can preferably be done in a so-called ‘Phytomyzarium’, which offers special advantages for the collection and release of hatched adults (Klein and Kroschel, 2002; Kroschel and Klein, 2003). However, the hatching rate of adults from collected and stored pupae is only 4 percent because a high proportion of the pupae are in diapause, which can last for three years. If applicable, storage under colder conditions in the refrigerator for a certain period of time can increase the hatching rate to 10 percent.

In calculating the number of flies to be released in the field, the expected Orobanche infestation level as well as the reproductive capacity of the fly has to be taken into consideration. The efficacy of inundative releases of P. orobanchia to reduce the seed production of O. cernua or O. aegyptiaca parasiting sunflower and other crops has already been demonstrated in the 1960s and 1970s in different regions of the former Soviet Union. Releases of 500-1 000 adults per ha resulted in a reduction of up to 96 percent of the Orobanche seed production (Natalenko, 1969; Bronstejn, 1971; Kapralov, 1974). In Morocco, inundative releases significantly reduced the reproduction of O. crenata in faba bean. During the 3-year research period, only 3.7 percent to 6.2 percent of the seeds were viable after releases, in comparison to 94.9 percent (1996), 57.1 percent (1997) and 36.5 percent (1998) in the control plots without inundative releases (Kroschel and Klein, 2003).

Inundative releases require an efficient mass rearing method or in the case of P. orobanchia a sufficient provision of flies from collected Orobanche shoots. In Morocco, the number of hatched flies from stored shoots in a Phytomyzarium with a hatching rate of only 4 percent will still be sufficient even for the treatment of highly infested fields. However, more effective methods should be developed to manipulate diapause and hatching of adults, which would make the collection and storage of shoots more efficient and practical.

Taking the enormous seed bank and the longevity of Orobanche seeds into consideration, inundative releases of P. orobanchia will not be sufficient as a single control method in heavily infested fields. On the contrary, in weakly infested areas it could prevent further dissemination. However, especially for the control of parasitic weeds, a successful and sustainable weed management system must be based on combinations of different techniques. Biological control with P. orobanchia could be an important part in an integrated Orobanche management system and, therefore, especially part of Farmers’ Field Schools, e.g. the combination of hand-weeding of Orobanche shoots before seed ripening and the storage of the collected shoots to rear P. orobanchia could be very effective. Furthermore, the bioagent could limit the seed production of escaping Orobanche shoots which develop when resistant varieties are used and may prevent the development of more aggressive Orobanche races.

Formulation and delivery of mycoherbicides: the case of Fusarium isolates

Risk assessment: Pathogenicity towards non-target plants.

The acceptance and implementation of inundative biological control by regulatory authorities are based on safety issues, which include avoidance of non-target adverse effects associated with the use of biological control agents, whether the agent is indigenous or non-indigenous, naturally occurring or genetically modified. Therefore, it is very important that host-specificity testing and risk assessment methodologies should both lead to prevention of the release of any organism that is likely to have detrimental impacts on non-target plants and/or on the environment. In recent studies the effect of F. oxysporum Foxy 2 on 25 non-target plant species was investigated in the greenhouse (Elzein and Kroschel, 2003). The choice of plants was based on the relatedness of the test species to S. hermonthica and the host plant sorghum (Wapshere, 1974). Economically important cultivated crops, as well as plant species reported to be highly susceptible to F. oxysporum disease in the tropics and subtropics were also considered. None of the tested plant species showed any sign of infection when inoculated with Foxy 2. This host specificity was also confirmed by other host range studies in which the indigenous F. oxysporum isolates from Burkina Faso, Mali and Niger were found to infect only Striga spp. and none of the crops and vegetables tested (Abbasher et al. 1998; Ciotola et al. 1995). Also the Orobanche-pathogenic Fusarium strains did not attack any of various tested crop plants (Bedi, 1994; Amsellem et al. 2001; Müller-Stöver, unpublished data).

Production of phytotoxins

Many species of Fusarium produce a range of phytotoxic compounds, such as fumonisin, enniatin, moniliformin, zerealenone, trichotecene derivates, fusaric acid and others (Nelson et al. 1994). Some of these compounds have a marked toxic effect on humans, animals and plants and are probably carcinogenic (e.g. moniliformin and fumonisin B₁) (Nelson et al. 1993). Because of health concerns and potential risks associated with mycotoxin contamination, the use of bioherbicide candidates that produce carcinogenic mycotoxins cannot be recommended. For example, the use of F. nygamai, a potential candidate for the control of S. hermonthica (Abbasher and Suerborn, 1992) was rejected because of the production of fumonisin B₁ (Zonno et al. 1996). Fortunately, fusaric acid, 10-11-dehydrofusaric acid and their methyl esters, which do not present health risks, were the only metabolites produced by potential F. oxysporum isolates under current investigation for biological control of Striga and Orobanche (Savard et al. 1997; Thomas, 1998; Amalfitano et al. 2002) making these isolates very interesting candidates for the biological control of parasitic weeds.

Inoculum mass production

Once a microbial agent has shown potential for parasitic weed control in laboratory, greenhouse, and field tests, mass production of viable, infective and genetically stable propagules becomes a major concern for the development of a bioherbicide. For laboratory and greenhouse research and even small-scale field trials, production of a sufficient quantity of fungal inoculums may often be easily achieved. However, mass production methods and techniques must be developed for large-scale practical use. Some fungal biocontrol isolates perform best if their inoclum contain chlamydospores, while others cause a pathogenic effect with conidia and/or mycelia. Aimed at the development of economically feasible mass production methods and techniques, recent investigations emphasized the use of agricultural by-products as substrates, including sorghum straw, maize straw, cotton seed cake and wheat-based stillage, to mass produce F. oxysporum isolates of Striga and Orobanche. These substrates are readily available, inexpensive and do not require any further processing prior to their use. Using sorghum straw (Ciotola et al. 2000), a mixture of sorghum straw and wheat-based stillage (Müller-Stöver, 2001) and maize straw, plus wheat stillage (Elzein, 2003), abundant chlamydospores of three F. oxysporum isolates (M12-4A, FOO and Foxy 2, respectively) were produced in liquid fermentation systems. On the other hand, mass production of other propagules, e.g. conidia and mycelia and of all promising fungal antagonists of Striga or Orobanche, easily achieved using artificial culture media such as potato dextrose broth, malt extract agar or agricultural by-products (Abbasher, 1994; Bedi, 1994; Ciotola et al. 1995; Kroschel et al. 1996a; Thomas, 1998, Amsellem et al. 2001, Müller-Stöver, 2001; Elzein, 2003).

Formulation and delivery of mycoherbicides

Proper formulation and delivery systems are the key elements in performance of mycoherbicides. A number of challenges are encountered in the formulation of bioherbicides, including ease of production and application, as well as a guaranteed propagule viability and efficacy over the long term since the products need to be storable for long periods of time, often in facilities with variable environmental conditions (Lumsden et al. 1995). Therefore, a selection of the appropriate formulations that can enhance the efficacy and speed of weed control by overcoming environmental constraints, reducing the required end-use concentration of biological agents, increasing shelf-life and/or interfering with the defence mechanisms of the target weed may reduce inconsistency of field performance of many potential biological control agents (Connick et al. 1991; Boyette et al. 1996; Auld and Morin, 1995). Formulation involves the preparation of the active ingredient, i.e. propagules of the microbial agent, a carrier, and often other adjuvants to provide a viable and effective product which can be delivered to the target weed.

Several techniques have been developed to encapsulate fungal propagules in a solid matrix which can also help to buffer the organism from environmental constraints like rapid desiccation or microbial competition (Walker and Connick, 1983; Connick et al. 1991). Some efforts and progress were made to apply simple encapsulation methods suitable for pre-planting soil applications to the biocontrol agents pathogenic to parasitic weeds. Amsellem et al. (1999) successfully used the ‘Stabileze’ formulation method of Quimby et al. (1999), a formulation of absorbent starch, corn oil, sugar and silica, to formulate conidia and mycelia of FOXY and FARTH to control O. aegyptiaca grown in polyethylene bags. Another successful example of granular formulation called ‘Pesta’, containing propagules of Foxy 2 and FOO was prepared, and showed high efficacy in controlling S. hermonthica and O. cumana in the greenhouse (Kroschel et al. 2000, Müller-Stöver, 2001; Elzein, 2003) (Fig. 1). The method consists of mixing fermented fungal biomass (wet or dry) in a matrix composed of agricultural commodities (semolina, kaolin, and sucrose) using an extrusion process (for more details see Müller-Stöver 2001; Elzein 2003). With an application of 0.5 g of the formulated ‘Pesta’ granules per kg of soil, the authors achieved the same efficacy in controlling Striga and Orobanche as with the use of 10 g of inoculum propagated on wheat grains. (Table 2a, 2b, Fig. 2). Such a reduction (95 percent) in the amount of fungal inoculum as a result of adopting formulation technology could be economically very significant when large-scale application is considered. Using 0.5 g granules per kg of soil, approximately 300 kg of ‘Pesta’ granules, at a cost of approximately US$86 (ingredients only), is required for the control of 1 ha infested with O. cumana or S. hermonthica. Nevertheless, and from a practical perspective, one possibility to reduce the dose may be the proper placement of the formulated product onto the target weed, e.g. in the planting hole, in-furrow application, or coating the sorghum seeds with the antagonist before delivery. For example, if 0.5-1g ‘Pesta’ granules is applied per seed pocket, only 14-28 kg (US$4-8) granules will be required for the control of 1 ha infested with Orobanche or Striga.

Table 1: Fusarium isolates currently under investigation for the biocontrol of Striga and Orobanche spp.

Table 2a: Efficacy of ‘Pesta’ formulation containing chlamydospore-rich biomass of F. oxysporum ‘Foxy 2’ in comparison to wheat grain inoculum in controlling Striga hermonthica and improving sorghum performance

Data from Elzein, (2003).

Table 2b: Efficacy of ‘Pesta’ formulation containing chlamydospore-rich biomass of F. oxysporum f.sp. orthoceras “FOO” in comparison to wheat grain inoculum in controlling Orobanche cumana and improving sunflower performance

¹ Control (-) = host (sorghum or sunflower) plus parasite; Control (C+) = sorghum only.

² Efficacy was calculated as percent reduction of healthy emerged parasite (Striga, or Orobanche) shoots comared to the negative control (C-). Data from Müller-Stöver (2001).

Fig. 1: ‘Pesta’ granular formulation made with different propagules of Fusarium oxysporum ‘Foxy 2’: Microconidia (top left); Mycelia + Microconidia (top right); Fresh chlamydospore-rich biomass (bottom right), and dried chlamydospore-rich biomass (bottom left).

Fig. 2: Disease symptoms on S. hermonthica shoots caused by Fusarium oxysporum ‘Foxy 2’ (right), control (left)

The type of inoculum used in the formulations greatly influenced their shelf-life. Amsellem et al. (1999) found out that formulated chopped mycelia were better storable than conidia, and Elzein et al. (2000) as well as Müller-Stöver (2001) reported a longer shelf-life for formulated chlamydospore-rich biomass than microconidia of the biocontrol agents. Up to 100 percent viability of fungal propagules in ‘Pesta’ granules for at least one year was achieved by incorporating chlamydospore-rich biomass and storing it at a low storage temperature (4° C), which might be sufficient for commercialisation.

The promising levels of Orobanche and Striga control obtained with ‘Pesta’ granules containing FOO and Foxy 2 propagules justify a further development of ‘Pesta’ granules for field testing. Commercial-scale production is possible using twin-screw extrusion (Daigle et al. 1997). Wheat flour-kaolin granules Pesta’ can be modified easily to accommodate ingredients from indigenous agricultural resources of sub-Saharan countries such as other cereal grain flours which may give a more effective and economic product. The preparation of ‘Pesta’ as free-flowing granules has several advantages: it is non-toxic; relatively cost effective; can be produced on a large scale; convenient to store; simple to use; can be applied using agricultural machinery; and can be easily integrated with existing Striga and Orobanche control methods, e.g. cultural and mechanical, and the use of resistant varieties.

Another attractive approach to deliver the biocontrol agents directly into the rhizosphere of the host plants is the use of seed dressings. Seed treatment with a variety of antagonistic micro-organisms has been used successfully to control some seed- and soil-borne diseases. By coating the seeds of hosts of Striga and Orobanche with antagonistic fungi, the fungal pathogens could be introduced to the infection sites of the parasites. Poor survival of antagonists during seed treatment processing and subsequent storage as well as the inability to colonize the rhizosphere during the early seedling development has been described as the main reasons for poor performance of the antagonist when applied as a seed treatment (Berger et al. 1996). Sorghum seeds coated with air-dried F. oxysporum M12-4A inoculum completely prevented S. hermonthica emergence in a pot experiment (Ciotola et al. 2000). In recent studies, Foxy 2 survived the seed treatment processing and showed excellent viability on seeds for at least one year of storage (Elzein et al. 2003). The fungus was also able to quickly colonize the root system of sorghum, and showed promising efficacy (75-100 percent) in controlling Striga in pot and root chamber trials. Therefore, F. oxysporum isolates (M12-4A, Foxy 2) met the criteria of being a promising candidate for controlling Striga when applied as a seed treatment. However, problems may arise with incompatible fungicides or other biocontrol agents that are intended to be applied as seed dressings as well (Amsellem et al. 2001). The cost of precoating sorghum seeds sufficient for the cultivation of 1 ha is only US$21, which includes the cost of coating material as well as the industrial processing (production cost) (Elzein, 2003). Further costs are not expected since the precoated seeds are more appropriate to be sown using the existing agricultural machinery and are also compatible with adapted cultural practices. By using naturally occurring coating materials (e.g. arabic gum) in the Sahelian and sub-Saharan zones of Africa, a further reduction in costs of the seed treatment is possible. If these results were to be confirmed under field conditions, seed treatment might contribute to a more meaningful application of F. oxysporum isolates (M12-4A, Foxy 2) as an antagonist for Striga within an integrated control approach.

Microbially-derived phytotoxic metabolites

Considering the increasing awareness of herbicide resistance, novel compounds from micro-organisms may provide new chemistries for weeds that may otherwise be difficult to control, e.g. parasitic weeds. Microbially-derived compounds may be pursued either as templates for new synthetic chemical herbicides or as pathogens applied directly to the target weed (Boyetchko 1999). Toxins produced by Fusarium spp. are also phytotoxic to several plants (Duke, 1986) and therefore, their bioherbicidal effects were tested against various weeds and crops (Hoagland, 1990; Abbas and Boyette, 1992). This provided an impetus to investigate whether the potential Fusarium isolates of Striga and Orobanche are able to produce phytotoxic metabolites that have bioherbicidal effects against different developmental stages of parasitic weeds, and to evaluate whether these compounds play a significant role in the antagonistic effect of the isolates. Most of the promising Fusarium isolates of Striga and Orobanche investigated were able to produce small amounts of fusaric acid, 10-11-dehydrofusaric acid and their methyl esters. The toxic effect of these metabolites on Striga seeds was previously proved by Zonno et al. (1996). Further investigations showed that fumonisin B₁, which is produced by F. nygamai isolated from S. hermonthica, has a herbicidal effect on Striga spp., especially when applied as post-emergence. This might indicate the role of fumonisin B₁ either in the infection process and/or in the course of the disease when the fungus is applied to control Striga (Kroschel and Elzein, 2003).

Genetic engineering of biocontrol agents

All examples of microbial agents for parasitic weeds have dealt with indigenous, naturally occurring antagonists, which will continue to be a major source for screening for more efficient new isolates (strains) in the future. However, molecular approaches and genetic engineering to develop a weed biological control agent with enhanced bioactivity have been investigated (Sands et al. 2001). Transgenic biocontrol agents (Gressel, 2001; Sharon et al. 2001) or transgenic plants containing pathogen genes encoding enzymes and toxins to produce pest resistence (Harman and Donzelli, 2001), are other ways that have been used for further enhancing the virulence, survival and efficacy of weed and other biological pest control agents, which are also options that appear to be appropriate for parasitic weeds. Virulence of biocontrol agents may generally be enhanced transgenically, using exogenous genes in combination with failsafes to prevent their spread and introgression into crop pathogens. For example, the transformed pathogenic fungi F. oxysporum (FOXY) and F. arthrosporioides (FARTH) of O. aegyptiaca with two genes of the indole-3-acetamide (IAM) pathway, were more effective in suppressing the number and size of Orobanche shoots than the natural one on tomato plants (Amsellem et al. 2001; Gressel, 2001). However, the creation of genetically engineered mycoherbicides must rely on detailed knowledge of the biology of the organisms, both the antagonists and their hosts. Any immature attempt to introduce genetical changes into mycoherbicides without an extensive preliminary study may result in a lack of success or worse, in products that may cause agricultural and environmental risks.

Future prospects

Crop production systems in which parasitic weeds constitute major problems are subject to a variety of farm management practices. To date, the impact of agro-ecosystem management on the efficacy of biocontrol agents has rarely been evaluated. Many bioherbicide candidates are still in the developmental stage, including evaluation of formulations and delivery. Therefore, the next step would be to consider how to integrate these biocontrol agents in different crop management sytems.

ACHIEVEMENTS IN INTEGRATED CONTROL APPROACHES

Considering the constraints to a successful control of parasitic weeds so far, it is well recognized that no single method of control can provide an effective and economically acceptable solution. Therefore, an integrated control approach is essential, ideal and useful to small-scale farmers, in order to achieve sustainable crop production. The progress achieved in individual parasitic control measures. as has been previously summarized and discussed in this chapter, may be of significance in contributing to the success of any proposed integrated approach through the accommodation of newly adaptive and applicable components. No standard integrated control package for parasitic weeds can be put forward; therefore it needs to be adjusted to individual cropping systems, local needs and preferences. In this context, the development and use of mathematical modelling tools may be helpful in adapting and optimizing control strategies to different agro-ecosystems (Manschadi et al. 1999).

Striga

Certain “key” factors of farming systems are directly related to the occurrence and infestation intensity of Striga, including: i) length of fallow; ii) weeding practice; iii) maintenance of soil fertility with the use of crop residues and organic manure; iv) crop rotation and the proportion of cereals (hosts) in the rotation; and v) the use of, and access to, external inputs (herbicides, fertilizers) and improved seeds (Kroschel, 1999). Analysing these key factors may provide possibilities for improving the cropping system, as well as identifying the best-suited control approaches according to farmers’ specific situations. With regard to Striga, any ideal integrated control strategy should consider containment and control as well as the need to improve soil fertility in order to be successful in achieving sustainable crop production. Specific examples of how the various components currently available to farmers, which can be evaluated for inclusion in an integrated Striga control programme, are given by Ransom (1999) (Examples I and II), Ejeta (2002) (Example III) and Louise et al. (2001) (Example IV).

Example 1: For an area that is heavily infested with Striga, where soybeans have a ready market, and maize, the staple cereal can be easily purchased on the local market, an integrated approach might include the following components:

• Rotation with soybeans, using a variety that is high yielding and well adapted to the region. In order to maintain on-farm productivity and production, it is probably more important to select the most productive and/or easily marketed variety as opposed to one that stimulates the most Striga seeds but is otherwise a poor performer. With yield and adaptation being equal, however, the most effective Striga seed germination stimulator should be recommended.

• Since maize is the staple cereal, probably only half of the farm can be rotated to a crop other than maize. Within the maize crop, the farmer should be encouraged to apply as much organic fertiliser as possible (i.e. crop stover, manure, etc.) and the recommended (or the maximum amount affordable) of the inorganic fertilizers.

• The farmer can intercrop his maize crop with other species if desired, but it is not necessary to achieve maximum control.

• If the numbers emerging are not overwhelming, Striga should be hand-weeded to ensure that there are no new additions to the Striga seed bank.

• After harvest, all crop residues should be ploughed back into the soil.

Example II: For an area where there is a limited market for grain legumes, wheat or teff as non-Striga hosts can be grown. The following integrated approach might be considered: i) Wheat or teff should be grown in the most severely affected fields. Growing these non-hosts would allow for a staple cereal to be produced. ii) Every effort to improve the fertility of soil should be utilized, which should include fertilizers, manure and crop residues. iii) If sorghum or maize is required by the farmer, local varieties of sorghum which show considerable tolerance to Striga should be grown in preference to maize. iv) Hand-weeding or the use of 2, 4-D to stop reproduction of any emerged Striga should also be used, depending on the Striga pressure and availability of labour and chemicals.

Example III: In Ethiopia, an integrated Striga management package was recently begun to be implemented through funds provided by the Office of Foreign Disaster Assistance (OFDA) of the USAID. It includes seed of Striga-resistant sorghum (INTSORMIL varieties or Brhan), nitrogen fertilizer, and the use of tied ridging as a water conservation measure. In the summer of 2003, a total of one thousand Ethiopian farmers in four Striga endemic regions will participate in this management programme.

Example IV: In Cote d’Ivoire, the S. hermonthica control package including the use of Striga tolerant maize varieties (ACR 94TZL Comp 5-w) intercropped or in rotation with legumes cultivars (soybean, cowpea) was reported to be effective in reducing the parasite infestation and increasing yields of maize.

Orobanche

A detailed review by Pieterse et al. (1992) and Parker and Riches (1993) suggesting the possible combinations of relevant control methods for Orobanche in a number of susceptible individual crops, still remains very important. However, the following integrated control approach was suggested by Dhanapal et al. (2001) for O. cernua control in tobacco in India:

• Grow trap crops (sunnhemp/greengram) in the early spring and incorporate in situ 45 days after sowing.

• Transplant tobacco after 15-20 days.

• Take up general weeding within 45 days after transplanting (DAT).

• Apply glyphosate at 60 DAT at 0.5 kg a.i. ha^-1 (or less).

• Remove the remaining few broomrape spikes by hand or apply plant oils to prevent seed formation.

For O. crenata control in faba bean in Morocco, the package should include:

a) treatment with glyphosate;

b) crop rotation with non-host and avoidance of planting host crops for at least 3-4 years in the same field;

c) hand weeding of the remaining Orobanche shoots before and after crop harvest and removal of shoots from the field and burning (Kroschel, 2001).

PROGRESS IN THE IMPLEMENTATION OF CONTROL MEASURES

In the previous parts of this chapter, emphasis has been laid on various control methods. It is obvious that research has made enormous progress in widening our knowledge and on the possibilities for parasitic weed control. However, control options which can be adopted by farmers are still very limited. In this section we focus on the progress of how to make use of research findings and how to assist farmers in reducing the threat to their livelihood of parasitic weeds.

Training of researchers, extension workers and farmers

The process of generating and dissemination of agricultural innovations involves several actors, among which the most important are farmers, extension workers and researchers. Hence, training of all three parties will help to improve their capacities in their respective fields of action in parasitic weed control (Kachelriess, 2001). Researchers need information and training on scientific methods. Additionally, they should acquire and train their skills to understand farmers’ rationale so as to assist them in the development of appropriate technologies. Extension workers have the important task of disseminating innovations, and acting as a link between farmers and researchers. But, as various surveys have shown, extension workers are often barely equipped with specific indepth knowledge about the biology and control of parasitic weeds (Ghana: Sprich, 1994; Morocco: Kroschel et al. 1996b) and therefore, require targeted training. In northern Ghana, for instance, only 54.2 percent of the extension workers interviewed knew that Striga is a parasite. In Morocco, the use of glyphosate to control O. crenata in faba bean was propagated for more than 15 years but only 15 percent of the interviewed extension workers were able to give a correct description of its application technology. Therefore, training of extension staff is as an important component in facilitating effective advisory work and in assisting farmers to assess and adopt an integrated control approach. Apart from technical knowledge, extension workers may also require training on the appropriate use of extension material and on how to improve their communication skills. Farmers have a more universal knowledge about agriculture. They may require support, advice and training while developing, experimenting with, or adopting innovations in parasitic weed control (Kachelriess, 2001). In the 90s, survey results from various countries (Egypt: Müller-Stöver et al. 1999; Ghana: Sprich 1994; Kenya: Frost 1995; Madagascar: Geiger et al. 1996; Malawi: Shaxson et al. 1993; Morocco: Lutzeyer et al. 1994; Kroschel et al. 1996; Tanzania: Reichmann et al. 1995) have clearly shown that farmers’ knowledge on the biology of parasitic weeds is very poor, which is obviously the reason why farmers are often not successfully implementing available control methods. Hence, training of farmers in parasitic weed biology is needed in each extension programme so that they can understand and apply control methods correctly.

Training courses

Within the framework of the supra-regional GTZ project Ecology and Management of Parasitic Weeds, and according to the needs of all three actors in parasitic weed control, specific training curricula were developed and implemented during the period 1989 to 1999 (Kachelriess and Kroschel, 2001). Researchers and high-level extension workers were trained by University lecturers and scientists from the IARC in annual international training courses, which took place twice a week from 1994 to 1998. A total of 76 participants attended from 19 countries, mainly from Africa but also a number from Asia. The participants were trained in the biology and ecology of parasitic weeds, their control methods, economic and socio-economic aspects and limitations involved in the introduction of control methods to small-scale farmers, as well as research methodologies, participatory technology development, communication and agricultural extension. Participants at the international courses were often involved as resource persons in national training courses for extension workers. These courses were conducted in the pilot regions of the project in northern Ghana and Morocco as well as in Benin, Madagascar and Malawi with regard to Striga, and in Algeria, Egypt and Tunisia with regard to Orobanche, respectively. The training courses had the following objectives for participants (Kachelriess et al. 1999):

• Be competent in and able to explain, the biology and ecology of parasitic weeds.

• Have knowledge on different control methods and to be able to select and combine adapted control methods.

• Be aware of appropriate extension material and their use.

• Be able to stimulate a discussion with, and amongst, farmers.

• Be prepared to discuss on-farm trials with farmers and researchers in the field.

• Have the opportunity to exchange experiences with colleagues.

• Be able to train other staff as trainers.

Experiences were exchanged between the resource persons and the participants in the form of oral presentations. Participants had the opportunity to bring in their experiences in discussions. The trainers did not teach in a top-down approach but through animated reflection and discussion. Group work allowed for more intensive discussions and helped to deepen the knowledge. Practical demonstrations are very important for enhancing the comprehension of the scientific information presented. It is also valuable to include field visits to exchange ideas with farmers. Ideally, demonstration plots and on-farm experiments should also be included and discussed. Generally, the length of these training courses should not exceed three full days (Kachelriess and Kroschel, 2001).

Development and use of appropriate extension aids for parasitic weed control

Extension materials can be made available in printed form, such as technical leaflets, manuals, brochures but also through the media of role plays, picture series or mass communication, such as radio or audio visuals, video tapes and others. An overview of their comparative advantages and inconveniences is given by Albrecht et al. (1990). The potential users are extension workers. The material should be available for farmers during extension activities (e.g. pictures) but also as information material for extension workers (e.g. technical leaflets) (Kachelriess et al. 2001). Farmers’ awareness on the biology (reproduction, number and longevity of seeds, differences between host plants and trap crops, etc.) of parasitic weeds is often a bottleneck for the introduction of control methods. Furthermore, farmers lack knowledge on the appropriate use of control methods. Hence, the adaptation of control methods to individual on-farm situations is difficult and farmers are rarely in a position to take sound decisions on the control of parasitic weeds. Research reports and scientific publications are not suitable for the use of farmers. Therefore, it is essential to present this information in a way that extension workers and farmers are able to make use of it. Taking a high rate of illiteracy into consideration, the method of communicating this information should be suited to the local socio-cultural conditions (Kachelriess et al. 2001). In the extension work for parasitic weeds in northern Ghana and in Morocco valuable experiences have been carried out with the use of visual extension materials (Feil et al. 1997, Fischer, 1999, Kachelriess et al. 1996, Loudie et al. 1999, Kroschel et al. 1997). These were developed as a series of more than 40 hand-coloured pictures intended for the use of extension workers and farmers groups. The extension series is divided into three parts: (i) awareness creation; (ii) biology; and (iii) control methods. The pictures are shown to a group of farmers and the extension worker acts as a moderator and stimulates discussions within the group. Necessary information and hints for the speaker are written on the back of the pictures. After the presentation of each picture, a series of pictures is then attached accordingly to a felt board, indicating the life cycle of the parasite, for example. However, it has to be considered and noted that visual aids cannot simply be copied to be used under various farming conditions, but have to be adapted through a creative process, by participatory material development with researchers, extension workers and farmers.

CONCLUSIONS

Over the last decade and with the help of innovative technologies, basic and applied research efforts have generated a wealth of scientific knowledge for the better understanding and improvement of sustainable integrated parasitic weed management. As has been summarized and discussed, the significant progress achieved in the various individual parasitic weed control measures are highly relevant to the success of any proposed and/or applied integrated control approach through the accommodation of newly adaptive and applicable components. However, although the words ‘integrated control’ have become ‘magic words’ in parasitic weed management, no long-term studies exist in which integrated control has been tested and proved to be the key to their control in the field. Since no standard integrated control ‘package’ for parasitic weeds can be put forward, relevant control options need to be adjusted to individual cropping systems, local needs and farmers’ preferences. Despite the high number of possible control options, only a few have been adopted by farmers. In addition to other reasons, the lack of active channels (extension staff) and links between farmers and researchers for disseminating innovations and transferring available technologies are seen as the main constraints. Therefore, training of extension staff is an important component in facilitating effective advisory work and in assisting farmers to assess and adopt an integrated control approach. To achieve field applications, the key factors remain on-farm implementation and experimentation of accessible control options as well as an exchange of ideas, experiences and information between different groups of researchers and farmers. Through such a system of feedback, a continuous and rapid improvement of adaptable control strategies could be realized, and, it is hoped, will be the key to successful parasitic weed management in the future.

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