Powdery Mildew

A white or grayish powdery coating on leaves and stems is your first sign, starting on younger, developing foliage in summer and reaching peak severity in late summer. Unlike most fungal diseases, powdery mildew thrives in warm, dry conditions with high humidity (60-80% RH) but no free moisture on leaf surfaces, making it a problem even during dry spells. Young tissue is susceptible; as leaves mature, they become resistant. Look for tiny black pepper-like fruiting bodies embedded in the white mycelium by late season.

Powdery mildew weakens plants through reduced photosynthesis and can cause premature leaf drop, setting back flowering the next year. The disease spreads wind-blown, so dense plantings and poor air circulation are your biggest risk factors. Your primary defense is spacing plants widely for air movement and avoiding afternoon shade in the landscape. Resistant cultivars exist for many hosts like apples and crabapples. Early pruning of infected terminals and removing too many leaves damages the tree more than the disease itself, so resist the urge to aggressively prune.