Why Does My Apple Tree Have Powdery Mildew on the Shoots?

Apple powdery mildew (Podosphaera leucotricha) is one of the most persistent apple fungal diseases and is easily identified by the characteristic white, powdery coating it produces on the surface of young shoots, leaves and flower buds. Unlike many other diseases that cause spots or lesions, mildew coats the entire surface of affected tissue with a grey-white felt. Severely infected growing tips become stunted, the leaves are small and crinkled, and flower buds from mildewed spurs may fail to develop normally, reducing fruit set.

The mildew lifecycle

Apple mildew overwinters in the dormant buds of infected shoots. As buds open in spring, the infected primary shoots emerge already coated in white mildew — these are "primary" or "flag" shoots and are the most important early-season source. The fungus produces airborne spores from these primary infections throughout the growing season, spreading to new healthy shoots and producing secondary infections throughout summer. Unlike scab, mildew does not require leaf wetness — it spreads in dry conditions with high humidity.

Identifying and removing primary (flag) shoots

In early spring, before leaf expansion, walk around the tree and identify shoots that opened from infected buds — these will show the distinctive grey-white felt on the leaf surfaces before other shoots have had time to become infected. Remove these primary infected shoots to a point below the visible infection plus 5 cm. This single action, done early, reduces secondary spread significantly. Dispose of removed material in household waste.

Fungicide treatment

Sulphur-based fungicide spray applied at bud burst and then every ten to fourteen days until midsummer provides effective control. Sulphur is approved for organic use. Avoid applying during hot weather (above 25°C) as sulphur can cause phytotoxic leaf scorch. Potassium bicarbonate spray is an alternative with good mildew efficacy. Do not use the same fungicide class repeatedly — alternate to avoid resistance development.

Resistant varieties

Cox is highly susceptible; Fiesta, Jonagold and many modern varieties show much better mildew resistance. If mildew is a chronic problem, variety choice at replanting is a significant factor.