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Delaying the Onset of Fungicide Programs for Control of Apple Scab in Orchards with Low Potential Ascospore Dose of Venturia inaequalis. William E. MacHardy, Professor, Department of Plant Biology, University of New Hampshire, Durham 03824. David M. Gadoury, and David A. Rosenberger. Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Plant Dis. 77:372-375. Accepted for publication 24 November 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0372.

Conventional fungicide spray programs for the control of apple scab have long involved the application of fungicides prior to the pink stage of fruit-bud development. Our earlier studies, which showed a relationship between the relatively low levels of overwintering inoculum found in modern commercial orchards and the onset of scab epidemics, indicate that early-season fungicide sprays are not necessary in many commercial orchards in the northeastern United States. To test this hypothesis, experimental plots were established in several commmercial orchards in New Hampshire and New York, and tests were run from 1985 to 1991. The minimum delay of apple scab epidemics from the time of budbreak was forecast based upon estimates of potential ascospore dose (PAD) in these orchards by using a model developed in earlier research. PAD ranged from six to 45,450 ascospores per square meter, and the forecast minimum delay of the epidemics (Δt) based upon PAD ranged from 2 to 24 days after budbreak. Early protectant-spray programs were initiated either before Δt or between Δt and the first infection period following. Delayed protectant-spray programs were initiated after one to five infection periods had occurred following the initiation of the early program. Delayed protectant-spray programs provided control of fruit and foliar scab that was equal to that provided by earlier treatment in 27 of 30 orchards, despite the occurrence of one to five infection periods between the early and delayed fungicide treatments. At two of the remaining three orchards, the increased level of fruit scab, while statistically significant (P ? 0.05), was less than 1.0%. Fruit scab at one of the 30 sites reached 10.7% when treatment was delayed beyond the time recommended by Δt, and PAD at this site was 19,700 ascospores per square meter, among the highest levels included in this study. At sites where PAD was low, the first fungicide application could be safely delayed to coincide with the control of arthropod pests at the tight-cluster to pink stage of fruit-bud development.