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Fungicide Resistance Management Strategies

Resistance of diseases, insects, and mites to pesticides is a major concern for Pennsylvania fruit growers. Intensive and nearly exclusive use of the same pesticide kills more susceptible than resistant individuals. Thus more resistant individuals reproduce and their offspring will inherit the capacity for resistance and take over the niches previously occupied by susceptible individuals. Over time the whole population may become resistant.

Resistance has sometimes resulted in pest-management program failures. As we lose more pesticides to resistance, our pest management options decrease as the risk increases of having no effective pesticide treatment for a serious pest. In many cases resistance is inevitable, and our main strategy is to manage pests and pesticide use to delay the onset of resistance as long as possible. Below are presented tactics to help delay resistance to fungicides.

Fungicide Resistance Issues for Specific Diseases

Apple scab
Vangard, Scala, Flint, Sovran, Rubigan, Procure, and Nova are highly effective against scab infection. However, apple scab can become resistant to these fungicides, especially if any of them is continually applied alone. Growers using one of these fungicides to control apple scab must be certain to alternate it with an unrelated fungicide or to use it in combination with another fungicide, like captan, metiram (Polyram), mancozeb, Ziram, thiram, sulfur, or ferbam. These highly effective materials should not be used once the major threat of scab infection is over. Another strategy to prevent resistance is to alternate the use of these materials throughout the season. The less any one of them is used in an orchard during a given season, the lower the chances that resistance will develop.

Powdery mildew
Frequent applications of fungicide may be required for mildew control. Flint, Sovran, Bayleton, Rubigan, and Nova are effective in controlling powdery mildew. There are presently no documented cases of apple powdery mildew resistance to these materials.

Cedar apple rust
Only a brief part of the life cycle of the cedar apple rust fungus is spent on apple trees. Infection of apple leaves or fruit occurs between the pink and first cover spray periods. The cedar apple rust fungus survives 19 months or longer on red cedar. The contact between the fungus and the fungicide applied to apples is relatively short, reducing the potential for resistance to develop. If a resistant cedar apple rust fungus does develop, it must also survive on red cedar. Therefore, resistance of the cedar apple rust fungus to any fungicide is not likely.

The type of fungicide used affects the potential for a fungus to develop fungicide resistance.
Broad-spectrum fungicides like copper, captan, and sulfur act by interfering with several of the fungus's vital life functions. The multiple action of these fungicides allows little chance for resistance, since the fungus must undergo multiple changes to counteract the fungicide.

Systemic fungicides like Vangard, Scala, Flint, Sovran, Rubigan, and Nova are highly effective against many apple diseases. They interfere with one vital life function, so one change is needed for the fungus to become resistant. Thus the potential for resistance to these fungicides is much greater than to broadspectrum fungicides.

How do fungi develop resistance to a fungicide in an orchard?
As previously discussed, resistance is more likely to develop against fungicides that have a single mode of action, especially if they are used alone for a long time. In the orchard, resistant fungi may occur naturally, in very small numbers, even before the fungicide is first used. When a fungicide is applied, it reduces the number of susceptible apple scab fungi. The few scab fungi that are resistant to the fungicide are able to increase in number. As the fungicide is repeatedly used, the number of resistant fungi increases. The fungicide becomes less effective as the apple scab fungus becomes more resistant to it.

Resistant apple scab fungi are not "super" fungi
Apple scab fungi that are resistant to certain fungicides are still susceptible to others that have a different toxic action against the fungi. Benlate-resistant scab fungi are susceptible to the toxic action of captan, Rubigan, Nova, Syllit, Flint, Sovran, and other fungicides unrelated to Benlate. Using fungicide mixtures will delay the buildup of resistant scab fungi. Mixtures are most effective when used before resistance becomes a problem. Alternating chemicals that have different modes of action is another strategy to prevent resistance from developing.

The Future of Apple Disease Control
Growers can prevent resistance by using fungicide mixtures or alternating chemicals. All possible control tactics, including cultural controls, must be used against diseases like apple scab.