OAK POWDERY MILDEW

Oak powdery mildew is a fungus that grows on young leaves of common oak trees.  There has been a lot of it this year, probably because of favourable weather.

It shows up from a distance but close to you can see the granular nature of the fungus.

It affects the tree in three ways:

1. it stops sunlight getting to the leaves so that the plant can't photosynthesise so well - the leaves cannot feed the rest of the shoot and the tree as a whole
2. the fungus continues to transpire in hot weather when an unaffected leaf would close up its pores to retain moisture
3. without viable leaves the whole shoot can die-back

Heavily affected leaves can lead to shoot die-back

Although young leaves are colonised by the fungus, especially the second flush of leaves round about June through July and August, by September some oaks are trying again to produce new leaves which the fungus doesn't seem to be attacking.  

See the young leaves opening at the end of the shoot in the photo below.

Photos: Alan Watts