How to recognise it
A white, powdery coating appears on leaves and shoots, causing them to shrivel and blacken. Leaf growth may become smaller and the new shoot growth may be stunted or stop altogether.
When apical buds are infected it will lead to the growth of many, deformed stems from the tip, which are known as ‘witches’ brooms’.
Why it’s a problem
Young plants can be severely affected. The fungus overwinters in buds ready to cause further damage the subsequent season.
In mature trees the disease will mainly affect the younger stems, but the overall impact is rarely serious.
Where you are likely to find it
A common disease of oak (Quercus) plants, which also occasionally affects beech and sweet chestnut trees. It appears from mid-May and affects plants throughout the summer.
In very wet seasons it is less likely to be a problem as it prefers dry and warm conditions.
How to prevent it
If you are planting young oak trees ensure they are well spread out to maintain the air flow between them. Ensure that plants are well watered and mulched (though don’t allow the mulch to touch the stems of young plants) to prevent the dry conditions which this disease prefers. Water plants from ground level, as using overhead watering can splash the fungal spores between leaves and plants.
Thoroughly clean your hands/gloves and equipment you’ve used when dealing with infected plants so you don’t spread the disease to other plants.
How to get rid of it
Fungicides, including sulphur (although this can damage some plants), can be effective against powdery mildew on young trees and should be applied at least fortnightly in dry, summer periods.
You can remove the infected plant parts (particularly leaves) however you must be careful when doing this, so you don’t shower neighbouring leaves with fungal spores. An easy way around this is to carefully place a bag over the leaf to be removed before pruning it, so any spores fall into the bag. Where stem tips have formed ‘witches brooms’, prune them out in autumn or winter if it’s practical to do so. Any diseased cuttings should be destroyed (eg burned) and not put on the garden compost heap.
Is it good for anything?!
Other useful information
The disease was first found in Europe in 1907 and soon spread around the world.
Powdery mildew in various forms can be found on a large number of plants. It is distinguishable from downy mildew as it is usually found on the upper surface of leaves (downy mildew is usually on the lower surfaces) and being in hot, dry conditions (downy mildew prefers a humid environment).