Fight Fungicide Resistance in Cherry Powdery Mildew
July 6, 2021 
Growing resistance to fungicides poses a problem for many cherry growers dealing with powdery mildew.
Photo by WSU Tree Fruit Extension/ Dr. Gary Grove
Dr. Gary Grove’s research at Washington State University (WSU) has served cherry growers for more than 35 years. He’s studied powdery mildew from the start.
“When I began my research, sulfur was our only fungicide for cherry mildew management,” he says. “Today we have many more, but we’re in a bit of a bind with our current fungicide toolbox, a threat to our ability to produce healthy, marketable fruit.”
In a recent WSU Tree Fruit Extension webinar series, Grove discussed issues around fungicide resistance. Growers may notice that managing mildew in their orchards has become more difficult, and that some fungicides are less effective seasonally or compared to previous years.
“Cherry powdery mildew is sensitive to fungicide use frequency, pattern, and application conditions,” Grove says. “Even though we haven’t been hammered industry-wide with mildew since 2017, it’s a problem for growers somewhere each season.”
Groups 11 and 3 synthetic fungicides have demonstrated significant resistance in the studies Grove performed in conjunction with Dr. Prashant Swamy. But a concept called the “fitness penalty” may lead to a possible solution.
Grove explains that when a population of powdery mildew is first exposed to a certain fungicide, many fungi are susceptible, but a few may be resistant. Over time the resistant proportion increases. The fitness penalty refers to the theory that the resist-ant population will revert back to its original susceptibility if the fungicide type is avoided for several growing seasons.
One Step Back, Two Steps Forward
Based on his research, Grove advises growers to refrain from using Groups 11 and 3 compounds in the next few growing seasons.
“It’s a short-term plan for a long-term gain,” he says. “But, hopefully, this will help restore the effectiveness of these fungicides so we can continue to use them in the future.”
If growers have already purchased resistant compounds, Grove advises mixing them with sulfur and applying only once during the season. “If you have a mildew situation getting out of control, don’t try to knock it back with a resistance-prone compound,” Grove says. “That’s just asking for trouble.”
Fortunately, there are multiple newer fungicides for use on cherries. Incorporating these chemistries is best, but refrain from applying more than twice per season or in sequence.
In any fungicide program, Grove says three best practices will also help ensure optimal grower success:
1. Thoroughly apply fungicides. “Ensure your fungicide dollars actually reach the tree and fruit. Calibrate your sprayer and apply during good weather conditions to adequately cover and protect young foliage and fruit.”
2. Don’t skip suckering. “Suckering is an important step in disease management because plant sucker foliage is extremely susceptible to mildew. When they become infected, they become spore machines.”
3. Perform cultural practices. “Prune to increase light penetration and air movement. These practices help reduce the humid conditions in which mildew thrive.”
Grove encourages growers to plan ahead and get on the same page with neighbors because air currents move spores significant distances, regardless of fences or boundaries.
“If your neighbor has a bad case of mildew, those spores will not stop at your property line,” Grove says. “Their ease of movement is part of the reason we’re in this situation.”
To find a longer-term solution, growers will need to come together with universities, distributors, packing houses, and consultants to avoid losing the use of many fungicides.
“We need to approach this problem as an industry,” Grove says. “We need to produce high-quality fruit. That’s why we’re in business.”
To learn more, watch Grove’s full presentations on WSU’s YouTube channel.
Subscribe Today For
