Powdery mildew on apple
Powdery mildew diseases are very common, affecting plants of many kinds. For most plants, there is a form of powdery mildew, caused by fungi that are specific to the host plant. The symptoms are obvious – a chalky, dust-like coating on the leaves, flower buds or stems, and sometimes the fruit as well.
The most seriously affected garden plants are apples, rhododendron, roses, clematis, asters, forget-me-nots, sweet pea, foxglove, courgettes, swede turnips, strawberries and grapevines. These plants can be seriously weakened by the feeding of the fungus; leaves destroyed; flowering and fruiting stopped or spoiled.
Plants growing in greenhouses, and on light, dry soils in a sunny position, are more prone to mildew attack. It is common in dry seasons, but can also make a strong attack in a dull season when plants have grown soft. Young trees such as oak and maple are often whitened by powdery mildew but this appears to have little effect on growth.
Apple mildew can be controlled by pruning out the affected shoots in early summer. Rhododendron mildew is very severe on some varieties and affected varieties are best avoided, even rooted out – it may not be practical to spray repeatedly.
Swede turnip mildew is not controlled by these but late sowings usually escape damage. Courgette mildew looks more serious than it is but can stop growth. Over-feeding should be avoided. Grape mildew is very severe and difficult to control. Improve ventilation and do not over-feed the plant.
If mildew is severe, such as grape mildew, spraying with suitable chemicals such as Systemic Fungicide with myclobutanil can be effective. This level of spraying may be considered too onerous.