Apple Powdery Mildew in Wisconsin Orchards: What You Should Know
As we move into June, some Wisconsin apple growers are beginning to notice symptoms of powdery mildew showing up in their orchards. If your orchard has a history of powdery mildew—and especially if your trees experienced a relatively mild winter—conditions may now be favorable for infections. Understanding the biology of this disease and how weather patterns influence its development is critical for managing its impact on fruit yield and quality.
What Is Apple Powdery Mildew?
Apple powdery mildew is caused by the fungus Podosphaera leucotricha. It survives the winter in infected buds, which—if not killed by cold temperatures—can open in spring and initiate the first (primary) infections. These early infections release spores that are then spread by wind, leading to secondary infections throughout the growing season.
Once established, powdery mildew can cause distorted or stunted shoots, leaf discoloration, and in more severe cases, defoliation and reduced fruit set. Infected buds often break later than healthy buds, creating a cycle of continued susceptibility.
Weather Conditions Matter
This fungus doesn’t require rainfall for infection—unlike apple scab—and thrives in warm, humid weather (66–72°F with >90% relative humidity). Free water (such as rainfall or leaf wetness) can actually suppress powdery mildew germination, so dry, warm stretches in June and July can increase disease pressure. Conversely, cooler temperatures or extended leaf wetness periods tend to reduce the risk.
Given the dry conditions some regions of Wisconsin have experienced recently, growers may see more powdery mildew this season than usual—especially in orchards where the disease has been problematic in the past.
Management
Some apple cultivars are more susceptible than others. Knowing where those cultivars are planted in your orchard can help you prioritize scouting and treatment. A useful resource on cultivar susceptibility can be found through Cornell’s apple variety disease database:
Fungicide applications should begin at tight cluster and continue through midsummer, particularly while shoot growth is active. Early in the season, 7–10 day spray intervals are recommended; post-bloom, intervals can be extended to 12–14 days, depending on weather conditions and disease pressure.
Fungicides in FRAC group 3 (DMI fungicides) are highly effective and should be rotated with products from other groups to prevent resistance development. Sulfur, a protectant fungicide, is another effective option—especially early in the season—but caution is needed in temperatures above 80°F due to potential phytotoxicity.
If you’re using FRAC 7 or 11 fungicides for managing summer rots and sooty blotch later in the season, it’s wise to save those for later applications. Avoid back-to-back applications of the same FRAC group.
Organic orchards should prioritize resistant cultivars and sulfur applications. Products such as Double Nickel LC and Serenade Opti, which contain beneficial Bacillus species, are OMRI-listed alternatives, although they tend to perform best in low disease pressure environments and may require more frequent reapplication.
While dormant pruning to remove overwintering inoculum is challenging (since infected buds aren’t easily visible), some success can be achieved with the removal of visibly infected shoots in spring, particularly in younger trees with low infection rates.
As June brings warm temperatures and, in some areas, abnormally dry conditions, powdery mildew infections may begin to increase. Incorporate effective fungicide rotations, monitor high-risk cultivars closely, and be proactive with your spray program.
As always, refer to product labels for application guidelines and restrictions. The label is the law.
This article was posted in WFN, Vol. 1-4 and tagged apple powdery mildew, Apples, disease, Leslie Holland, Powdery mildew.