Interactive effects of host, pathogen and mineral nutrition on grey leaf spot epidemics in Uganda

Grey leaf spot incited by Cercospora zeae-maydis is a new devastating foliar disease of maize in East Africa. For effective control, elucidation of the most critical elements of the grey leaf spot disease pyramid is important. This study investigated the role of mineral nutrition, pathogen variability and host resistance in the epidemic. Trials were conducted under field and controlled environments. The 28 isolates used in the controlled environment varied significantly (Pless than or equal to0.05) in parasitic fitness measured indirectly as disease efficiency, but no infection pattern could be attributed to known C. zeae-maydis pathotypes. Data from field trials showed that host resistance and mineral nutrition significantly (Pless than or equal to0.05) affected disease efficiency, with highest disease development occurring in nitrogen-augmented plots. Exclusive phosphorus application had no clear effect on grey leaf spot epidemics but combined application with nitrogen significantly (Pless than or equal to0.05) reduced the predisposition effects of nitrogen to the disease. Overall, treated plots had less disease than unfertilised plots. Fertiliser application had no effect on sporulation capacity, while cultivars significantly affected it. Geographic differences in amount of disease were observed, suggesting environment influences on grey leaf spot incidence. The results suggest that the current grey leaf spot epidemics in East Africa are due to favourable cultivars, poor mineral nutrition and environmental interactions.