Integration of genotype and age-related resistances to reduce fungicide use in management of alternaria diseases of cotton and potato

Concepts for the integration of genotype resistance, age-related resistance, and fungicide for the suppression of Alternaria diseases were developed and evaluated by a computer simulation model and in the field. The model reflects the effects of environment, genotype resistance, and fungicide efficacy on Alternaria solani in potatoes. We found that changes in host resistance, with age and among genotypes, could be compensated for by adjusting the intensity of fungicide applications, i.e., by increasing the frequency of sprays toward the end of the season and spraying moderately resistant cultivars at longer intervals than susceptible cultivars. The time during the growing season when application of a systemic spray within a routine application of protectant sprays would most effectively suppress Alternaria spp. was examined in simulation experiments and in the field. The efficacy of a single systemic spray was highest when applied toward the end of the season when host susceptibility increased. The concepts for integration were evaluated in six field trials, three involving A. macrospora in cotton and three involving A. solani in potatoes. In most cases, the disease suppression achieved by the integration treatment did not differ significantly from that achieved by application of either protectant fungicides (maneb, mancozeb, or chlorothalonil) on a 7-day schedule or systemic fungicides (tebuconazole or difenoconazole) on a 14-day schedule, although up to five fewer sprays were applied in the integration treatment.