Pennsylvania Apple Growers Face Increased Disease Risks

By Blake Jackson

Pennsylvania apple growers are facing increased disease risks as warm spring temperatures accelerate apple development from tight cluster to bloom.

A forecast of warm, wet conditions on April 25-26, 2025, elevates the potential for apple scab, rust, and fire blight infections.

Fire blight, caused by Erwinia amylovora, is active. A late April rain event raised concerns, but infection risk varied by bloom timing. Blossoms open before April 19th experienced an infection event due to sufficient bacteria colonization and degree hour accumulation.

However, blossoms opening on or after April 19th did not meet these criteria. A moderate to high fire blight risk is predicted for April 25-26th.

Apple scab and rust pressure is high during bloom through petal fall. Growers are advised to use complete sprays with rainfast mancozeb for resistance management and rust control.

Recommended fungicides include FRAC Groups 7, 7+9, and 7+3, with rotation of FRAC Group 7 fungicides with FRAC Groups 3 or 9 when necessary. FRAC Group 11 products should be reserved for summer rot control.

For powdery mildew control during tight cluster through pink, options include FRAC Group 3, U6, U13 fungicides, and sulfur. In scab-favorable conditions during this stage, FRAC Groups 3 or 9 fungicides are suggested, reserving Group 7 for bloom. Organic options include sulfur, potassium bicarbonate, and EcoSwing.

Brown rot blossom blight can occur in stone fruit with pistil exposure and favorable wet, warm conditions (41-86°F, optimum in the mid-70s). Longer wetting periods increase risk even at suboptimal temperatures.

Protect blossoms with fungicides like Rovral, Topsin M, Captan, or sulfur, used through petal fall. FRAC Groups 3, 9, 7, and 11 fungicides can also be used, keeping in mind later preharvest brown rot sprays.

Tank mixing with broad-spectrum protectants is advised for resistance management during bloom. Growers should stay updated on changing forecasts and implement appropriate disease management strategies.

Photo Credit: gettyimages-richard-7