INFLUENCE OF PHYSIOLOGICAL AGE OF PIMA COTTON ON THE NEED FOR FUNGICIDE TREATMENT TO SUPPRESS ALTERNARIA LEAF-SPOT

Field and controlled-environment trials were conducted to develop conc epts for integrating age-related resistance and fungicides in the mana gement of Alternaria macrospora, the causal agent of Alternaria leaf s pot of Pima cotton. The influence of physiological age of the crop and of the individual leaves on infection incidence and on the rate of le sion expansion was examined on leaves sampled from the field. Both com ponents increased with the physiological age of individual leaves but were not affected by the age of the crop. The effects of age-related r esistance on the need for fungicide treatment were examined in two tri als in 1990 and 1991. The efficacy of maneb applied in a 10- or 14-day schedule relative to weekly maneb applications decreased with crop ag e, while relative control efficacy of tebuconazole applied in a 10-, 1 4-, 17- or 21-day schedule did not change. It was concluded that the t ype of fungicide and the frequency of application should be determined by the age-related response of the host plant to the pathogen.