Nontarget Effect of a Fungicide Spray Program on Phytophagous and Predacious Mite Populations in a Scab-Resistant Apple Orchard

Scab–resistant cultivars can reduce the need for fungicides in apple production. However, management of powdery mildew, Pudosphaera leucotricha (Ell. & Ell) Salmon, cedar apple rust, Gymnosporangium juniperi–virginianae Schwein., and frog–eye leaf spot, Botryosphacria abtusa (Schwien.), may require limited fungicide use. Because fungicides can have mite–suppressive activity, it is important to determine the impact this reduction would have on mite populations. This study, conducted during 1988 and 1989, investigated the impact of a fungicide spray program (six applications of benomy1 and mancozeb versus no fungicide application) on phytophagous and predacious mite populations in a Vermont apple orchard. Levels of mite infestation were determined on four scab–resistant cultivars by counting motile phytophagous and predacious mites on leaf samples collected on 16 dates in each growing season. Data were evaluated separately for each cultivar, on each assessment date and over time, using an analysis of variance with a completely randomized design. Within each cultivar there were three to five single tree replicates per treatment. The impact of the fungicide spray program on predacious mite populations was clearly evident in both years. Approximately 4 wk after the last fungicide application, significantly higher predacious mite populations were detected on nontreated trees. In, 36 (97.3%) of the 37 samples for all cultivars in which there was a significant difference in predacious mites, the mean number of predacious mites was significantly higher on nonfungicide treated trees. In 1988, few significant differences in phytophagolls mite populations within cultivars were detected. However, incidences where there were significant differences in phytophagolls mite, populations increased in 1989; in the majority of the incidences, populations were significantly higher on fungicide–treated trees. This study shows that a fungicide spray program consisting of six applications can affect the mite populations on apple trees. This study also documents the potential benefit of eliminating fungicide applications.