What has caused this white colouration on these leaves of pedunculate oak, Quercus robur? There seems to be a lot of it around this year and I have even seen whole trees 'white washed' with it - find it on oaks on the public footpath by the woodland. Answer to follow soon....
Has anyone else noticed an abundance of 'white washed' oaks this year - the cause of which is a fungus, with the disease being known as powdery mildew. The scientific name of the causal fungus is Erysiphe alphitoides, also known as Microsphaera alphitoides.
The white colouration that you see in the photo is the fungal mycelium growing across the surface of the leaf. The mycelium is a dense mass of thread like structures called hyphae, which are the main vegetative part of the fungus. Generally the younger leaves and shoots are attacked by the fungus, however older leaves may also become affected. It is likely that the damp and overcast conditions experienced this summer have caused an increase in the disease.
Powdery mildew was first discovered in the UK in 1908, now being common and widespread throughout the UK and indeed Europe.