Читать книгу Plant Pathology and Plant Pathogens - John A. Lucas - Страница 22

In agricultural ecosystems, disease is one of the factors influencing crop yield and quality. Farmers, foresters, and horticulturalists are, by and large, interested only in those changes in crop performance which influence cash return per hectare. The ideal situation, in which pathogens are avoided, excluded or eliminated, is therefore a theoretical rather than practical goal. On the farm, other priorities may prevail; choice of crop or cultivar is usually based on likely profitability, rather than resistance to pests or pathogens. Unless a financial return is guaranteed, control measures may be ignored or reduced in scale. As a consequence, the significance of disease, as perceived by the grower, will depend to a large extent on the market value of the crop. Inputs of chemicals or other actions designed to reduce disease are only justified when the likely impact on yield or quality will outweigh the cost of the measure. Even a possible bonus, such as restricted carry‐over of the pathogen to the following season, may not provide sufficient incentive for any financial outlay.

Other parties with an interest in crop diseases are government advisory or extension pathologists, consultants, and representatives of the agrochemical industry. The relative resistance of new crop varieties to pathogens and the efficacy of commercial formulations of pesticides are assessed by advisory scientists under field conditions; recommendations for use may be based on these field trials. Independent consultants offer growers an overall package of advice for crop management, part of which concerns disease. Agrochemical companies provide information on the performance of their crop protection products, whilst government agencies regulating use of chemicals in the field issue guidelines and can impose restrictions on pesticide application. Nowadays, such advice includes strategies to reduce the risk of resistance developing to different pesticide classes. In recent years, more stringent legislation on the registration and use of agrochemicals, especially in Europe, has led to withdrawal of many crop protection products, and also affected the availability of new pesticides. Hence decisions on disease and pest control are influenced by many factors and often involve compromises driven by economic considerations or the regulatory system.