Alan Jones,MSU Dept. of Botany & Plant Pathology
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Resistance of diseases, insects, and mites to pesticides is a major concern for Michigan fruit growers. Failures in pest management programs have been seen due to pesticide resistance. Perhaps you have experienced this on your farm with insecticides and fungicides. This article will focus on fungicide resistance management for apple scab.
In controlling apple scab in the past, we have experienced resistance to Benlate and Dodine as well as other fungicides. Recently, Alan Jones, Michigan State University (MSU) Department of Plant Pathology, has reported data that shows that apple scab resistance to sterol inhibitors is beginning to be found in isolated areas of Michigan. In essence, we know resistance to almost any pesticide is likely to happen at some time in the future. What’s really crucial is keeping that resistance way off in the very distant future. Hopefully, this article will give you some tools to accomplish this.
In a nutshell, fungicide resistance occurs when intensive and nearly exclusive use of the same fungicide kills more susceptible scab spores than resistant scab spores. Thus the more resistant individuals continue to reproduce. These more resistant spores, in turn, produce even more spores that can inherit the capacity for resistance. Now, you have a population of apple scab spores that are less or not at all affected by the fungicide they have developed resistance to. These resistance spores grow and take over the niches previously occupied by susceptible individuals. Over time the whole population may become resistant. This is a simplified overview of how resistance occurs.
Fungicide resistance can occur for any fungus, but has a high probability to occur for apple scab for several reasons. Strategies for resistance management should be incorporated into any apple scab control program to prevent problems from occurring.
Because of favorable weather conditions, Michigan growers can experience numerous apple scab infections in one growing season and therefore, have to stay covered for these infections several times in the spring. The more often you have to use a fungicide, the more likely the populations of resistant fungi will develop. This is especially true if the fungicides chosen are applied alone.
Growers using any of the currently labeled fungicides to control apple scab must be certain to use it in combination with another fungicide of a different classification. For another strategy to prevent resistance, 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.
Growers should be honest in their assessment of the amount of overwintering apple scab lesions in their apple blocks. Unfortunately, not all of the spores produced on these leaves will be released during any one infection period in the spring. Because of the extended period during which apple scab spores are released, it is necessary to concentrate on controlling scab for about two months in the spring. In Michigan, spore release peaks from the pink through the petal fall periods, but can continue well into June in some years. Poor control of apple scab one year, often leads to increased scab problems to attend to the following year.
Using labeled rates can be an important resistance management tool. Continually cutting rates is a sure road to resistance. Reduced rates select for control of only the most susceptible members of the population, while allowing the ‘stronger’, more resistant members to survive to reproduce another day.
Poor pesticide application, due to adverse weather conditions and calibration needs, can lead to a false sense of adequate control. Being ahead of the rains is by far the more safe timing for fungicide applications than applications done after the infection period. Most fungicides have better action as protectants than as eradicants. After-infection applications should be saved for those times when you get behind with sprays, or the weather conditions warrant.
The type of fungicide affects the potential for a fungus to develop fungicide resistance. Broad-spectrum fungicides like copper, captan, and sulfur are effective by interfering with several of the vital life functions of the apple scab fungus. The multiple actions of these fungicides allows a much lessened chance for resistance, because the fungus must undergo multiple changes to counteract the fungicide’s action against it.
Systemic fungicides like Benlate, Topsin-M, Rubigan, and Nova are highly effective against many apple diseases. They control apple scab by interfering with only one vital life function. Resistance is more likely to occur with these products because the fungal organism only needs to undergo one change for it to become resistant. Thus the potential for resistance to these fungicides is much greater than to broad-spectrum 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. That particular fungicide becomes less effective as the apple scab fungus becomes more resistant to it.
Switching to a different chemical can help to eliminate the populations that are building resistance to another chemical, thereby ‘clearing the slate,’ so to speak. Resistant apple scab fungi are not “super” fungi that can no longer be controlled. 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 still susceptible to the toxic action of captan, Rubigan, Nova, Syllit, and other fungicides unrelated to Benlate.
This is why it is so important to use fungicide mixtures and to alternate products throughout the season. Fungicide combinations are an important tool that will delay the buildup of resistant scab fungi. Of course, 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.
It should be noted that all research shows that the new class of fungicides, the strobilurins, have a great potential for resistance development to apple scab and other fruit diseases. These fungicides work on only one site of the fungal organism, so resistance potential is very high. It will be very important to use the strobilurins wisely to get the most years of use from them.
If we lose important pesticides to resistance, our pest management options decrease. At the same time, the risk of having no effective pesticide treatment for serious pests increases. In many cases resistance is inevitable, so the main strategy is to manage pests and pesticide use to delay the onset of resistance as long as possible.
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