Risks of intercontinental spread
Intercontinental dispersal
Appraisal and prediction
The sessions of the Symposium at Oxford emphasized that natural and planted forests throughout the world are threatened by numerous damaging forest pathogens and insects. The hazards of intercontinental spread of these pests and the inherent problems of appraisal and prediction were the subjects of three major papers1 with many other contributions providing pertinent data.
1 HEPTING, G. H. Appraisal and prediction of international forest disease hazards.
NORDIN, V. J. The intercontinental spread of forest pathogens.
SIMMONDS, F. J. The spread of forest insects in the world, with particular reference to biological control.
This chapter reviews the information with particular reference to the risks and methods of intercontinental spread and to procedures for appraising and predicting pathogens and insects as internationally dangerous.
Previous records of intercontinental spread of forest pathogens and insects discussed at this Symposium verified the extreme damage and devastation that can result from an introduction of a pathogen or insect to another continent. Chestnut blight (Endothia parasitica) was cited as an example of an introduced disease that eliminated a commercially important tree species in the United States and the Symposium was reminded frequently that white pine blister rust (Cronartium ribicola) and Dutch elm disease (Ceratocystis ulmi) continue to cause severe annual losses in plantations, natural forests, woodlots, and urban areas in many parts of the world. Examples of the many destructive introduced insects included the larch sawfly (Pristiphora ericsonii), balsam woolly aphid (Adelges piceae), European pine shoot moth (Rhyacionia buoliana), and the fall webworm (Hyphantria cunea).
It was concluded that there are many recognized pests at present of restricted geographic distribution that constitute a great potential danger to the forests of other continents. Further, pathogens and insects of minor significance in their native habitats may become serious pests in a new environment, for example, European Adelges piceae and Asiatic Endothia parasitica in North America. In addition, "new" pests may be revealed in certain countries following adequate scientific exploration.
Both natural and artificial means of intercontinental dispersal of pests were discussed. Natural dispersal methods, although obviously largely beyond human control, were assessed because of their possible influence in weakening or nullifying exclusionary efforts by man. The natural spread of pathogens and insects can occur in many ways: through the air, water, soil, birds, and mammals. These may play vital roles in continental or more local disease and insect epidemiology, but it was concluded that they have little significance in intercontinental pest dispersal except in contiguous areas of Europe and Asia where the host tree may be uninterrupted by natural barriers such as deserts, oceans or mountains.
Human activities provide the greatest hazard to the spread of pathogens and insects through the intercontinental exchange of living plants and products, utilizing rapid methods of transportation. The most dangerous carriers for diseases and insects include living plants, soil, seeds, cuttings, and wood products such as logs, lumber, and manufactured products.
In the commerce of living plants, exotics present special hazards. A pest of an exotic may be destructive to a native tree or a pest of minor significance in its native environment may become extremely damaging to its host in a new habitat. Because of these dangers it was strongly recommended that a tree should be introduced to a new habitat only if it is superior to native species and cannot be reproduced by seed.
The exchange of seeds between countries was regarded as the safest way to provide propagative material if seeds are kept clean of foreign debris and effectively treated by fumigants and fungicides to eliminate injurious seed inhabiting insects and pathogens.
This examination of intercontinental dispersal methods led to considerations of procedures to exclude, delay, or limit the seriousness of exotic pests. While these procedures were considered in several presentations and receive more detailed attention in other chapters, the following general approaches were recommended as the most important activities requiring co-ordination at both national and international levels. An effective and continuing forest insect and disease survey should be developed by countries for detection, appraisal, and as a basis for research; programs of research need to be intensified, and in many cases, initiated, to identify the forest pests of economic consequence as a prerequisite for effective quarantines, control, or eradication; more effective quarantines are required, developed and maintained on the most recent available knowledge; and further, countries should maintain facilities for prompt control action so that introductions can be eliminated or contained. To assist in the promotion and development of these activities the Symposium recommended that FAO appoint to its forestry staff, specialists in forest entomology and pathology.
There is no comprehensive method of developing hazard ratings for potential forest pests although helpful data are provided by literature contributions from various countries that list and rate both native and introduced pests and report regularly on research activities. However, these are only starting points in the accumulation of data necessary for effective appraisal and prediction.
One approach to appraising international pest hazards is the strategic establishment of test plantations of the most important trees to expose them to the world's pests. This approach has considerable value and much can be learned by thorough inspections of existing exotic plantations. Such plantings are also useful for determining how and where a tree species grows best, and for providing sources of breeding stock for genetic studies.
However, while much useful information on the appraisal of pest hazards can be derived from plantations, there are too many weaknesses to make it the sole method of hazard appraisal. For example, disease and insect incidence fluctuates widely from place to place, and the replication required to incorporate the spectrum of variability for so many pests and host trees to obtain comprehensive and valid appraisals is so formidable as to be impractical and prohibitive.
Therefore, the need for improved means of evaluating pest hazards on an international basis was recognized.
In the field of forest diseases an interesting proposal for appraising and predicting disease hazards (abbreviated APPRE) was presented to assist any country to define which of the world's tree pathogens are of potential danger. The two main objectives of this method are to permit pathologists of any country to determine:
1. Which native tree diseases pose a threat to other countries with reference to import decisions, quarantines, control measures, and needed research.2. Which diseases in other lands pose a threat to their own forests.
Since this system would depend upon ready access to the world's accumulated knowledge on a pathogen, an information retrieval procedure was described as a basic requirement and entitled, International Tree Disease Information Register (INTREDIS). This register would accumulate published information on tree diseases on punched cards suited to electronic sorting and would rapidly provide a printed record of available literature on a pest according to host tree and other particulars.
The APPRE procedure, therefore, has its foundation in a centralized system of information retrieval, a system not yet established. An important soilborne pathogen, Phytophthora cinnamomi, was used as an example in predicting the hazard to forests in various parts of the world from a specific pathogen. A very useful detailed description was provided comparing the present distribution of this pathogen with its expected distribution as determined by the published knowledge of the organism and based on climate, soil, hosts, and other factors known to limit the pathogen or make aggressive attacks unlikely.
Another illustration showed how the threat of disease to a given species in one part of the world could be estimated. Pinus radiata in New Zealand was used as the example, along with the knowledge available of its diseases elsewhere and of differences and similarities in climate, hosts and alternate hosts, soil, and other factors between New Zealand and the areas where these diseases are active. The system would provide, in simple tabular form, the pertinent hazard data, including a list of pathogens and information for each on its presence or absence in the country, where to expect damage, and the reason why damage is to be anticipated.
One of the major weaknesses in the appraisal and prediction of internationally dangerous pests and the effective implementation of a procedure such as APPRE is the need for the essential information resulting from increased surveys and research in some countries, and even the beginnings of such programs in others.
The Symposium recognized the importance of an information retrieval system and requested the proposer of the system and a small committee to undertake further enquiry into the implementation of such a scheme and report the results to IUFRO and FAO by 1 June 1965.
