Chemical Control of Alternaria Blotch of Apples Caused by Alternaria mali. Nenad Filajdi?, Graduate Research Assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. T. B. Sutton, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 76:126-130. Accepted for publication 15 July 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0126.

Four fungicides alone or in combination were tested in the field for activity on Alternaria mali. No satisfactory control was achieved except with iprodione at 0.30 g a.i./L. Captan and mancozeb alone and in combination with benomyl failed to control Alternaria blotch after artificial inoculation, whereas up to 65.1% disease reduction was achieved under conditions of natural infection compared with the nontreated control. Iprodione provided 69.1–79.8% and 71.6–75.1% disease reduction for the higher (0.30 g/L) and lower rate (0.15 g/L), respectively, under conditions of natural infection. After artificial inoculation, disease reduction ranged from 53.7 to 68.9% and 34.1 to 59.8% at the higher and lower rates of iprodione, respectively. The EC₅₀ of iprodione for three isolates of A. mali was 1.04 ?g/ml. No resistance of A. mali to iprodione was detected in the field. The incidence and severity relationship of Alternaria blotch was described by the equation: S^1/2 = b + cI, where S = disease severity, b = the intercept coefficient, c = the slope coefficient, and I = disease incidence.