Relation of Application Timing to Efficacy of Triadimefon in Controlling Apple Powdery Mildew. L. P. Berkett, Former Graduate Assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. K. D. Hickey and H. Cole, Jr., Professors, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 72:310-313. Accepted for publication 6 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0310.

Apple (cv. Rome Beauty) trees were sprayed to runoff with triadimefon at 37.5 mg a.i./L during specific periods of each of three growing seasons. Applications were made at the tight-cluster (TC) phenological stage through the fourth-cover (4C) in 1980 and from TC through the second-cover (2C) in 1981. In 1982, the program evaluated the effectiveness of single and paired consecutive sprays at the bloom (B) and petal-fall (PF) stages and at the first-cover (1C) and second-cover (2C) sprays. Certain applications were more effective than others in reducing the incidence of secondary mildew. Applications at TC and the pink (P) stage could be eliminated without loss in disease control. The period from bloom through the second-cover spray was found to be the most critical time in which to control mildew with triadimefon. Earlier sprays during this period were more effective in controlling mildew on vegetative shoots, whereas the later sprays were more effective on bourse shoots. In 1982, two applications of triadimefon (PF+1C) were as effective as four applications (B+PF+1C+2C) on trees with low primary inoculum. The importance of shoot type on which mildew was assessed and the level of primary inoculum were important factors in determining fungicide efficacy.