There is a wide range of fungicides to control powdery mildew. That’s good, because it’s important to use different classes of chemistry when treating for the disease to ward off resistance to a particular pesticide class.
“Most growers were aware of the possibility of developing fungicide resistance in their grapes,” says Paul Verdegaal, University of California Cooperative Extension farm advisor in San Joaquin County.
“But, until we had several years of wet, cool weather, they weren’t paying much attention to it. They were using materials that worked really well and staying with them. When problems occurred, they started calling their consultants, chemical dealers and me, wondering why they weren’t getting the control of powdery mildew the way they had been, even though they had been following label instructions.
“Now PCAs and growers have started more closely following University of California guidelines for managing disease resistance, and their control has improved. Even though last year was another wet, cool year, I didn’t get many of those calls.”
Sulfur continues to be the standard of grape fungicides, Verdegaal says. Although its mode of action is still not completely understood, resistance has not occurred. Today’s highly-effective commercial fungicides target a specific point in the metabolic pathway. It’s easier for the few fungi that are naturally resistant to just one particular mode of action to survive and multiply through repeated applications of a fungicide class as the number of susceptible fungi declines. Before long the few with resistance become a large pathogen population immune to the material.
Proper planning and management in selecting and using these products can prevent this from happening, and growers can continue to benefit from the broad array of fungicides offering different modes of action.
University of California plant pathologists report that, as a general rule, the risk of resistance development is highest when fungicides are applied after the disease has appeared or when spraying at reduced rates.
The Fungicide Resistance Action Committee (FRAC), a group of scientists organized through CropLife International, identifies fungicides with similar modes of action by a group number. Usually, you’ll find it on the front of the label near the top or just below the trade name. If the product contains more than one active ingredient, all of the FRAC numbers will be shown.
Alternating fungicides with different modes of action is essential to prevent pathogen populations from developing resistance to fungicides, Verdegaal notes. When treating powdery mildew in grapes, UC plant pathologists recommend making no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different group number. None should be applied more than two consecutive times before rotating to a fungicide with a different mode of action group number. Also, it may be best not to tank mix a fungicide with products to which resistance has already developed. And it is best to avoid using one of the DMIs (FRAC number 3) for the first application of the season, if powdery mildew has been a problem in past years.
“Generally, it makes no difference which FRAC number you use during the season,” Verdegaal says. “Also, you may want to consider applying fungicides with multiple modes of action, or using pre-mixes or tank mixes of two chemical classes.”
Using sulfur, a multi-mode of action material, as part of a season long strategy when rotating between a medium or high-risk material is another way to prevent fungicide resistance from developing.
“In all the years sulfur has been used, there hasn’t been any documentation of disease resistance to it, whether applied in a dust, wettable powder or micronized form,” Verdegaal says. “If you aren’t under a lot of powdery mildew pressure early in the season or just before harvest, you could use a biological or some of the new environmentally-friendly fungicides to add another mode of action to your control program.”
Cultural practices that lower disease pressure offer still another option to reduce the likelihood of fungicide resistance. They include irrigation and fertilizer management to prevent excessive vine vigor, shoot removal and positioning, or leaf removal. In addition, these practices can improve fungicide effectiveness by improving spray penetration.
Plant pathologists have developed the following RULES for reducing development of fungicide resistance when treating grapes and other crops for fungal diseases
- Rotate or mix fungicides of different mode of actions.
- Use labeled rates — for strobilurins, use upper label rates.
- Limit the total number of applications of any single-site mode of action fungicide class to, ideally, one and no more than two per orchard per season in a rotation program.
- Educate yourself about fungicide activity, mode of action, and class — as well as resistance management practices.
- Start a fungicide program with multi-site mode of action materials.
More information on managing fungicide resistance is available at www.ipm.ucdavis.edu