The effects of a pesticide program on non-target epiphytic microbial populations of apple leaves.

The epiphytic microbial community was quantitatively and qualitatively altered when a standard pesticide schedule that comprised applications of an insecticide, a bactericide, and a fungicide was applied to McIntosh apple trees. Effects on non-target organisms were observed for two seasons by three indirect methods and three direct methods: plating of leaf washings, imprinting of leaves onto five different media, spore fall patterns, light microscopy, scanning electron microscopy, and isolation of propagules from leaves incubated in humidity chambers. Magnitude of reduction of bacteria, filamentous fungi, yeasts, and actinomycetes varied annually and between categories of microflora. Populations from treated leaves were reduced 10- to 1000-fold in 1976 and up to 50-fold in 1977. Qualitatively, fluorescent pseudomonads and lactic acid-type bacteria were among those depressed by pesticide. Fungal populations on treated leaves were less diverse than on control leaves. Aureobasidium was only slightly affected and incidence of Sporobolomyces was substantially higher on treated leaves than on controls. The results suggest that numbers of antagonists to foliar pathogens of apple which may occur as part of the natural epiphytic microbial community may be reduced by current pesticide programs and hence have possible implications for the development of biological approaches to integrated control strategies.