Pathogens and Diseases

Abstract

The term pathogen indicates an ability on the part of a micro-organism to cause disease which results from an intimate relationship with the host plant. In this process the pathogen derives energy for growth and reproduction without any benefit to the host. Disease is, in consequence, a change in the normal functioning of plants and results in poorer growth and lower yields. It is of primary consideration for those concerned with the control of plant pathogens to establish the amount and timing of crop loss associated with particular levels of pathogen infection. This permits an evaluation of the methods for pathogen control discussed in chapters 5–10 from which a balanced judgement can be achieved of their costs and benefits. This type of analysis has been done with considerable precision for some temperate cereal crops. For instance, Large and Doling (1962) established that the square root of the percentage leaf area infected with Erysiphe graminis (powdery mildew) is related to grain yield lost in tonnes per hectare. Horticultural crops, however, have a combination of yield characters which often makes such direct estimations difficult. The value of these crops frequently rests on quality characters such as visual appeal as with flowers, fruit, pot plants, trees, shrubs and vegetables. Alternatively, value may lie in an estimation of future productivity as with transplanting material sold for growing-on or forcing.