Powdery mildew is a common fungal problem that can be found on the leaves of both fruiting and flowering apple trees. It presents as a white to grey mycellium on both sides of the leaves. It is most prevalent when temperatures are warm and humidity is above 70 per cent. If unchecked, the diseases can spread to twigs, branches and fruits.
Powdery mildew is caused by either the Sphaerotheca or Podosphaera species of fungi. A white to grey powdery coating will appear either in patches or over entire leaves. Accompanying the white, powdery appearance, new growth often appears to be wilted, stunted or distorted. Fruit infected with powdery mildew can assume a netlike and russet-coloured hue that causes the apples to develop poorly.
Powdery mildew is able to survive in the previous year's infected buds over the winter. The fungus becomes active when the trees break bud and leaves begin to form. The spores that cause the white appearance, called conidia, are easily spread by wind and splashing water. Once the temperatures and humidity rise, the conidia easily germinate. A new generation of the conidia spores can be created every 5 days, ensuring a quick spread of the disease.
Where possible, prune out infected leaves and branches. Do not allow leaves with powdery mildew to fall and decompose in the soil around apple trees. When chemical treatment is warranted, apply a pesticide containing sulphur, such as lime sulphur, wettable sulphur or copper sulphate. Organic controls for powdery mildew include neem oil, horticultural oils or Serenade. Start the applications when buds first develop, and continue applying every 2 weeks, until the trees start to develop their apples.
Prevention for powdery mildew on apples consists of selecting disease-resistant varieties. For fruiting apples, Red Delicious and Stayman Winesap are two resistant cultivars. Braeburn, Golden Delicious, Granny Smith and McIntosh apples are all moderately resistant varieties. Gravenstein, Jonathan and Rome Beauty are particularly susceptible and should not be planted in areas where powdery mildew has been a problem in the past. Most species of flowering crabapple commercially available are resistant to powdery mildew.