The SBI fungicides (Bayleton, Funginex, Orbit, Nova, Rubigan, and Procure) are widely used for disease control on apples and stone fruits because of their protectant and eradicant properties, wide spectrum of disease control, and their low toxicity to man and other mammals. They have a potential Achilles heel, however, in that their narrow mode of action on target fungi make them vulnerable to the development of resistant strains of these fungi. Much effort by University researchers and chemical companies has therefore been devoted to minimizing the risk of resistance development. In a recent issue of American Fruit Grower, an article by Wayne Wilcox and Wolfram Koeller of Cornell University outlined the main points of a anti- resistance strategy for apple scab:

1. Keep the scab population low in the orchard. This reduces the possbility that a "problem" (resistant) isolate will appear. This means thorough spray coverage, timing applications properly, and tank mixing a contact fungicide (such as mancozeb, metiram, or captan) with the SBI fungicide. Do NOT cut back on rates of the SBI fungicides below label recommendations, and do NOT stretch out spray intervals beyond recommendations, because this can encourage reproduction of resistant isolates.

2. Use the SBI's only within the primary scab season. The primary scab season extends from green tip through first cover (about 10 days after the last flower petals have fallen from the tree). The target should be four, or at most five, applications per season. Many SBI products are labeled for use through the summer, but continued use past first cover only raises the risk that scab-resistant isolates will multiply and cause problems.

This article originally appeared in the May 12, 1995 issue, p. 62.