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Conclusions were presented by several speakers including the organizers of the Conference. Their main points are summarized in the following statements.
If this Conference is to be judged a success, as many questions were raised as were answered. If it accomplished its goals, new applications, techniques, and ideas for use of solar energy in agriculture will soon follow. Questions were raised during the Conference that will stimulate involvement of scientists from other disciplines, including soil physicists and chemists, and climatologists, as well as others involved in pest management. The objectives of the Conference were to stimulate new understanding and to open new horizons, not only in soil solarization but in other aspects of applied and basic agricultural sciences. We learn from each other and this is important.
Soil solarization is a developing technology; improvements are needed in its application, effectiveness, and economic feasibility in open fields and greenhouses and in preplant and postplant situations. Progress may be manifested in advancements such as improved mulching materials and techniques, adaptation of optimal cropping practices, and development of compatible integrated pest management methods. Although soil solarization was first developed in the Mediterranean countries for open field use, its use has been extended to applications in closed plastic houses which is also effective in cooler climates.
Among the topics addressed during the Conference was the specific control of various agricultural plant pathogens and pests. Following field applications of soil solarization, the most obvious result is the control or great reduction in weed populations. Winter annual species which germinate in the cool season almost uniformly have been controlled. Only certain species of summer annuals, particularly those like Portulaca and Melilotus are difficult to control by solarization. Weeds such as nutsedge and black nightshade also have been difficult to control by soil solarization. Orobanche species have shown considerable variation in control in different applications of soil solarization. These variable results may be due to lack of uniform concern for all the characteristics that are required for excellent solarization practice, such as adequate soil moisture and soil preparation, so that the plastic sheeting lies close to the soil surface. Soil solarization for the control of weeds has an advantage over herbicides in that it kills weed seeds and allows the crop plants to get off to a competitive start, thus eliminating the need for reliance on pre-and post-emergence herbicides. Future uses of soil solarization in weed control will probably emphasize the control of specific weed species as well as a general application for weed control.
In regard to nematode control, some parasitic nematodes such as Ditylencus species and PratyIencus thornei have been controlled effectively by soil solarization However, as in the control of other pathogens and pests, results may be contradictory. Such is the case for Meloidogyne species. Sometimes good results are achieved, whereas in other situations, the results are less satisfactory. Although we try to explain the reason for our failures and can understand the physical and chemical phenomena involved, the biological phenomena are more difficult since they involve a greater multiplicity of interacting factors.
During the Conference, considerable emphasis was given to the control of fungal and bacterial plant pathogens. Convincing examples were discussed involving the management of species such as Phytophthora, Pythium, Pyrenochaeta, Fusarium, Verticillium, Sclerotinia, Sclerotium and many others. Among bacterial pathogens, Agrobacterium tumefaciens is highly sensitive to soil solarization while less success has been achieved with Pseudomonas solanacearum. Most plant pathogenic organisms are limited to growth within temperature limits of about 5 to 31 C. and have a threshold killing temperature of about 37°C during extended time periods. The temperatures during the period of soil solarization effectively target these plant pathogens without greatly reducing the populations of beneficial organisms, such as mycorrhizae, in contrast to the broader killing action and harsh effects that soil steaming and soil fumigants often have on soil.
Other issues discussed in the Conference were the induced suppressiveness in solarized soils due to development of populations of fungi and bacteria which serve as biological control agents for various plant pathogens and pests. The addition of aggressive biocontrol organisms to solarized soils to reinforce or increase the suppressiveness of soils to plant pathogens also was discussed.
Concerns were expressed about our limited knowledge on how solarization of soil affects insect and mite populations and the side-effects of soil solarization. This is an important matter that should not be overlooked; we could make the same mistakes we have done with the application of chemicals. Other concerns involved the major problem of disposal of plastics following solarization. The best solution appeared to be the recycling of plastics and the use of plastics that have multiple uses.
Among the highlights of the meeting was a visit to the Jordan Valley where soil solarization in both experimental and commercial plastic houses was being used. Also, new innovations such as the effective use of solar heated water to irrigate soils during solarization was demonstrated. Field trips to Jerash and Petra provided an exceptional opportunity to learn something of the history of Jordan and its peoples.
Soil solarization, as presented in this conference, is now one of the major research topics in many countries; it is moving quickly from the research and investigation phases to commercial use. Its use has extended from limited application in greenhouses and seed-beds to large-scale areas. It has been used as we have seen, even as a post-plant treatment. It has proved in many cases as efficient and as effective as chemotherapy; it also qualifies as an element of IPM programmer. Soil solarization contributes significantly to the increase in yield and vegetation of many crops, and therefore can be an ingredient in crop management programmes; moreover, it has considerable potential for water management in post-plant applications.
The significance of this Conference on soil solarization is indicated by the participation of over 100 scientists from 21 countries, with worldwide representation. Approximately 40 papers dealing with current topics on soil solarization reflect the interest in this technology, a technology which offers a non-chemical approach for the safe, non-pollulting, and effective control of plant diseases and pests.