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DMI Fungicides on Apples: Survival of the Apple Scab Pathogen in Sprayed Leaves, Late-Season Scab, and Sustainability of DMI Use in IPM Programs 1997

Project Leaders: David M. Gadoury, Robert C. Seem, and Wayne F. Wilcox

Objectives:

1. Determine the impact of use patterns of a DMI fungicide upon survival of infections and delayed symptom expression.

2. Determine the potential impact of such survivorship upon selection for resistance to DMI fungicides.

Results:

The DMI fungicide fenarimol significantly reduced the establishment of the apple scab pathogen on sprayed trees. However, despite the shortening of the interval between applications from 14 to 7 days, lesions formed abundantly on trees first sprayed 96 hours after inoculation. As in 1996, the first wave of lesions that appeared represented only a portion of the total, and lesions continued to develop for several weeks from a single infection period. Whether or not this is due to resistance to fenarimol will be known once sensitivity assays are completed in 1998.