2026 Disease Update: Powdery Mildew Risk with Dry Weather Forecast

The recent warm weather has pushed apple trees in the southern half of Pennsylvania into tight cluster and early pink; peach trees are blooming. Powdery mildew control begins at tight cluster for apple; petal fall in peach begins rusty spot control. Currently, no rain is forecasted from April 7 through April 15. This will be an excellent stretch of decent weather for primary powdery mildew infections. As a reminder, powdery mildew infections occur on dry days when temperatures are 50-77°F, and humidity is above 90%. Folks are usually on alert during warm, rainy periods because these conditions favor many tree fruit diseases. Don't be complacent during the calm, dry days during the early season. Powdery mildew is lurking. It is important to closely monitor your location. General tips for disease management during this time are discussed.

Apple disease management for early April

Focusing on primary powdery mildew control:

• Axios (FRAC Group 52; 3 fl oz)
• Cevya (FRAC Group 3; 5 fl oz/A)
• Rally (FRAC Group 3; 10 oz/A) or Sonoma (FRAC Group 3; 4.8 fl oz/A)
• Rhyme (FRAC Group 3; 6.5 fl oz/A)
• Procure (FRAC Group 3; 16 fl oz/A)
• Torino (FRAC Group U6; 6.8 fl oz/A)
• Gatten (FRAC Group U13; 6 – 8 fl oz/A)
• Micronized sulfur (8 – 10 lb/A)

For organic control, options include:

• Micronized sulfur (8 – 10 lb/A)
• Potassium bicarbonate (3 lb/A)
• ProBlad Verde (40 fl oz/A)
• EcoSwing (2 pt/A)

Be sure to tank mix with a rainfast mancozeb (3 lb/A). Although dry weather favors low apple scab pressure, it does not favor low powdery mildew pressure. Some strong powdery mildew products are not as effective against scab; however, tank mixing with a broad-spectrum fungicide (i.e. mancozeb) will keep the scab in check when we do have rain events in the middle of dry stretches. For those using low-risk programs, sulfur and potassium bicarbonate can both control powdery mildew and scab.

Growers are highly encouraged not to use the FRAC Group 7 fungicides during this time; these fungicides are best saved for peak apple scab and Marssonina blotch pressure, which is from late pink through petal fall for apple scab, and petal fall through the second cover for Marssonina blotch.

For growers who may be a little behind and experiencing green tip through half-inch green tip, powdery mildew is not an issue for you just yet. At this stage, disease management can be conservative, focusing on controlling apple scab, and possibly getting an early jump on powdery mildew control if warm weather accelerates bud development.

Management options:

• Rainfast mancozeb alone at 3 lb/A
• Rainfast mancozeb 3 lb/A + Syllit (FRAC Group U12; 1.5 pt/A)
• Rainfast mancozeb 3 lb/A + Micronized sulfur (6 – 10 lb/A)
• Organic options: Potassium bicarbonate (3 lb/A) or sulfur

Peach management for early April

Rusty spot

Many peach orchards are in bloom, or about to bloom. The summer-like temperatures forecasted for later in the week and into next week will move these trees further along, and soon they will be in petal fall. Petal fall marks the start of rusty spot control. Rusty spot ("powdery mildew" in peach) is caused by the same fungus that causes powdery mildew in apple (Podosphaera leucotricha), and symptoms only occur on fruit. Spots on fruit are mostly circular, and sometimes initially white, then turn rust-colored from the center outward. As the fruit gets larger, the lesion surface becomes russetted, and cracking can occur during pre-harvest swell. The key timings for controlling rusty spots are petal fall, shuck split, first cover, and second cover. Symptoms are typically first observed approximately three weeks after shuck split.

Cultivars vary in susceptibility. For example, Gloria, Snow Giant, and Redhaven have low susceptibility to rusty spot, whereas Messina, Desiree, John Boy, Suncrest, and Jerseyglo are considered moderately susceptible; and Bounty, Jerseyqueen, and Autumnglo are considered highly susceptible.

To manage rusty spot effectively, a combination of cultural and chemical controls can be employed:

• Locate peach orchards away from powdery mildew susceptible apples.
• Open canopy for better air circulation and spray coverage.
• Consider some of these fungicide options for management from petal fall through second cover:
  • Rally/Sonoma, Indar, Orius, Cevya, Quash (FRAC Group 3)
  • Inspire Super (FRAC Groups 3 + 9)
  • Luna Flex, Luna Experience (FRAC Groups 3 + 7)
  • Fontelis, Miravis, Tesaris (FRAC Group 7)
  • Flint Extra (FRAC Group 11)
  • Merivon, Pristine, Luna Sensation (FRAC Groups 7 + 11)
  • Micronized sulfur (conventional and organic)
  • ProBlad Verde, potassium bicarbonate (organic options)

Additional organic options are being evaluated for efficacy against rusty spot.

Brown rot

Although the risk of brown rot blossom infections may be low this week, it is important to stay alert, especially on warm days when thunderstorms may occur. Blossom infections from the brown rot fungus can occur whenever pistils are exposed, and a favorable climate exists. Infections can occur during wetting periods when temperatures range from 41 to 86°F. However, optimum conditions for infection occur with wetting and temperatures in the mid-70s. During long wetting periods (several days or more), blossoms can be infected regardless of temperature. For example, at 50°F, tissues must remain wet for 18 hours for infection to occur, while only 5 hours of wetness are needed at 77°F. Generally, infections that occur under suboptimal conditions are less severe. Blossoms and fruitlets will remain susceptible until the pistil desiccates (sometime between petal fall and shuck split). Keep blossoms protected with fungicides for blossom blight:

• Rovral, Meteor (FRAC Group 2; 1 - 2 pt/A ) – can be used through petal fall
• Topsin M (FRAC Group 1; 1.0 – 1.5 lb/A)
• Captan (FRAC Group M4; 3 lb/A)
• Micronized sulfur (FRAC Group M2; 8 – 10 lb/A)

Some areas may experience a protracted bloom on their stone fruit trees this year. If this occurs, growers can also use fungicides in the FRAC Groups 3, 9, 7, and 11. Be mindful of anticipated preharvest brown-rot management sprays later in the season when using these FRAC Groups. Tank mix broad-spectrum protectant fungicides (captan, sulfur, chlorothalonil) for fungicide resistance management during the bloom period. For organic options, sulfur is your best choice. Of the fungicides recommended for controlling brown rot blossom blight, only sulfur is effective against rusty spot. Captan and iprodione (Rovral, Meteor) are not effective in controlling rusty spot; Topsin M is considered "fair" for rusty spot control.

Additional Resources

Dr. Tree Fruit Don Podcast

Penn State 2026 - 2027 Tree Fruit Production Guide

For commercial fruit growers, please note: When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. To assist with management decisions (especially during infection events), growers can use the NEWA website, which has weather stations all over Pennsylvania.  Before chemical products are applied, be sure to comply with the current usage regulations and examine the product label. Product information can easily be obtained from CDMS, Agrian, or Greenbook.