Why does my peach tree have powdery mildew?

A white or pale grey powdery coating on the young shoots, leaves, and sometimes fruit of a peach or nectarine tree is powdery mildew — a fungal infection caused by Podosphaera pannosa and related species. Unlike most fungal diseases, it thrives not in wet conditions but in warm, dry weather combined with cool nights, making late spring and summer the main risk period.

How powdery mildew infects peach

Unlike most fungal pathogens, powdery mildew does not need free water on the leaf surface to germinate. Its spores can germinate in high relative humidity without rain, and infection is actually inhibited by direct wetting of the foliage. The fungus grows as a superficial mycelium on the leaf and shoot surface, extracting nutrients through penetration pegs into the epidermal cells. Infected shoots are often stunted and distorted, and the white coating can spread rapidly in warm, settled weather.

Conditions that favour outbreaks

Powdery mildew is worst on soft, young growth — particularly on trees receiving high nitrogen feeds that produce lush, tender shoots. Stagnant air in a dense canopy or against a sheltered wall also promotes it. Trees under mild drought stress, where growth is soft but transpiration is reduced, are particularly vulnerable. Improving airflow through regular summer pruning is one of the most effective preventive measures.

Cultural controls

Remove infected shoot tips as soon as you see the white powder, cutting back to a healthy bud below the affected area. Bin the material — do not compost it. Reduce nitrogen feeds from midsummer onwards and switch to a high-potash fertiliser to encourage the ripening of shoots rather than soft extension growth. Ensure the tree is adequately watered to prevent the mild drought stress that makes tissues susceptible.

Organic and low-impact treatments

A spray of sodium bicarbonate solution (1 teaspoon per litre of water plus a few drops of vegetable oil as a spreader) applied to infected surfaces changes the pH and can reduce the spread. Diluted neem oil also has some efficacy against powdery mildew. Reapply after rain. These treatments are more useful as preventives or at the early infection stage than for heavy established infections.

Fungicide treatment

Sulphur-based fungicides are effective against powdery mildew when applied before infection is severe. Apply as a protectant from when conditions favour the disease in late spring, and repeat every ten to fourteen days. Do not apply sulphur when temperatures exceed 30°C, as leaf scorch may result. Triazole fungicides (myclobutanil) provide both protective and curative action against established mildew.