Plant Pathology and Plant Pathogens. John A. Lucas

Plant Pathology and Plant Pathogens - John A. Lucas


Скачать книгу
a natural diversity for disease resistance. Modern, intensive agricultural systems and the deployment of crops in new areas have disturbed this balance.

Natural Agricultural
Genotypes Diverse Uniform
Age structure Mixed Uniform
Distribution Dispersed Crowded
Nutrient status Often low Usually high
Co‐evolution span Long Short
Left: Graph of plants surviving vs. time after inoculation (weeks) displaying 2 descending curves with circle markers. Right: Graph displaying ascending curves for control population and rust-infected population.

      Source: After Paul and Ayres (1986a,b).

      One explanation for the severity of such invasive diseases is that the pathogen responsible was almost undoubtedly introduced from another continent, and hence spread through a host population not previously exposed to infection. The term new‐encounter disease has been proposed to describe such an epidemic arising from contact between a previously separate host and pathogen.

      Disease in Agriculture, Horticulture, and Forestry

      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.

      Crop Yield and Quality

Plot illustrating the relationship between yield levels and crop loss, indicating economic benefits of control with 4 horizontal dashed lines for “Theoretical maximum,” <hr><noindex><a href=Скачать книгу