EFFECTS OF FOLIAR DISEASES ON GAS-EXCHANGE PROCESSES - A COMPARATIVE-STUDY

The effects of 10 different foliar diseases on the photosynthetic and transpirational activities of five annual and three perennial crops were examined under field conditions. The following pathosystems were investigated: leaf rust, powdery mildew, and Septoria tritici blotch of wheat; rust of corn; downy mildew of cucumber; Alternaria leaf spot of cotton; powdery mildew of pepper; Pestalotia of mango; powdery mildew of peach; and powdery mildew of grapes. Visual estimates of infection do not always indicate adequately the effect of a pathogen on these physiological processes. Three different patterns of host response were observed in diseased leaf area relative to disease-free leaf area. The decrease in rate of photosynthesis or transpiration was a) proportional, b) proportionally greater, or c) proportionally smaller than the corresponding reduction of healthy leaf area due to disease. Host responses were not always related to the systematic group of either the host or the pathogen. The pattern of responses and the amount of reduction in photosynthesis and/or transpiration rates were, however, related to the type of trophic relationships. This article provides regression models that may be used for incorporating the effects of various foliar diseases to crop growth simulators.