Reducing the Threat of Fungicide Resistance

This article originally appeared in "The Badger Berry Dispatch," Vol. 20, No. 3, May 1996.

In the pest management bulletins published by the University of Wisconsin and other institutions, there is always a section warning growers about plant pathogens developing resistance toward the fungicides used to control them. In this article I’ll explain in a bit more detail how fungicide resistance develops in the field and how this affects disease management for berry growers.

A population of a fungal pathogen within a single planting is variable---while most individuals will be inhibited by a fungicide, some will be tolerant, and a few may be nearly immune. The numbers and distribution of fungicide-sensitive and -resistant strains within a population will depend upon the species of fungus and the fungicide in question, as well as fungicide use history in that planting. Compared to most plants and animals, fungi have very short generation times and are extremely prolific---some pathogens can undergo more than 10 generations within a single growing season, with each leaf spot or lesion producing several thousands of spores.

Repeated exposure of the entire population to a single fungicide will kill off the sensitive strains allowing the resistant strains to flourish, free from competition. The successful resistant strains will produce resistant offspring. Disease control problems in the form of "fungicide resistance" arise when the resistant strains increase in numbers and eventually predominate in the planting.

Resistance to broad-spectrum fungicides (e.g., captan and copper) has not been reported. Resistance to fungicides with more specific ways of attacking fungi (e.g., benomyl, thiophanate-methyl, dodine, vinclozolin, and iprodione) has been reported from areas outside Wisconsin for many plant pathogens, including Botrytis cinerea, the gray mold fungus on strawberry and raspberry, and Sphaerotheca macularis f. sp. fragariae, the powdery mildew fungus on strawberry. Fungicide resistance in these pathogens has not been confirmed in Wisconsin.

If you have noticed a loss of efficacy of a fungicide in controlling a pathogen, it could be because resistant strains of the pathogen predominate in the planting. However, growers and researchers alike are quick to cry "fungicide resistance" before other factors have been ruled out. Other potential explanations for fungicide "failure" include inadequate spray coverage, using too low a rate, deteriorated product, poor cultural practices, extremely high disease pressure, and misidentified pathogens or problems.

Because the number of fungicides registered for use on strawberries and raspberries is small, we need to take steps to reduce the risk of fungicide resistance and thereby maintain the efficacy of these products for years to come. The goal is to keep total populations of pathogens low (the smaller the population, the less chance there is of having strains with high levels of natural resistance), and to minimize exposure of the pathogen to the fungicide. Total populations of pathogens can be kept relatively low by planting resistant cultivars and adopting good cultural practices. Exposure to a fungicide can be reduced by spraying only when conditions justify it and by alternating sprays of a fungicide with an unrelated fungicide. With any fungicide prone to resistance problems, it helps to tank mix with captan. Captan is not registered on brambles, however, so the only option is to keep the number of sprays to a minimum and to alternate between unrelated compounds.

Benomyl (Benlate) and thiophanate-methyl (Topsin-M) are both in the benzimidazole class of fungicides; vinclozolin (Ronilan) and iprodione (Rovral) are both in the dicarboximide class. Therefore, to control gray mold on strawberry you could alternate between benomyl + captan, and iprodione + captan, but not between benomyl + captan, and thiophante-methyl + captan. But if gray mold is not a problem, do not spray a dicarboximide just for the sake of alternating! The dicarboximides do not control leaf diseases.

Recently I surveyed strawberry growers and learned that most in Wisconsin believe they could not make a profit without including fungicides in their disease management programs. With no new chemicals on the horizon, and the efficacy of biological controls not yet widely demonstrated, the arsenal is limited. This underscores the importance of taking steps to reduce the risk of fungicide resistance.