Comparative epidemiology of late blight and early blight of potato under different environmental conditions and fungicide application programs

Three experiments were carried out in an experimental field from Vicosa city, Minas Gerais, Brazil, to compare late blight (LB) (Phytophthora infestans) and early blight (EB) (Alternaria grandis) epidemics under different environmental conditions and fungicide application programs. Each experiment consisted of two side-by-side trials that were arranged in a randomized complete block design with five treatments and five replications. At 30 days after planting the potato ('Agata' cultivar) plants in plots of trials 1 and 2 were inoculated with an isolate of P. infestans (A2 mating type) and four isolates of A. grandis, respectively. Fungicide applications were initiated at seven days after inoculation and repeated at 7-or 15-day intervals. The severity of LB and EB was assessed every two days, and the area under disease progress curve (AUDPC) was calculated for both diseases. The yield was quantified by weighing the tubers of all the plants from each plot after their maturation. Yield data were converted into kg ha(-1) and the percentage of loss was calculated for each treatment. Based on the AUDPC, the LB severity was greater than the EB severity under all three environmental conditions, and this was reflected in the yield. LB and EB caused yield losses as high as of 82 and 45%, respectively. The fungicide effects on both LB and EB and the associated yield losses changed according to the environmental conditions, therefore, indicating the importance of using tools such as a forecast system to help farmers to decide the best time for fungicide application.