This autumn my young oak sapling was covered in a white coating, which I think might be mildew.
What would you recommend to cure it, and will the disease spread to other garden plants?
Yes, oak powdery mildew is responsible for the white coating.
When the mildew appears prune out the worst affected shoot tips to reduce the amount of infection on the tree.
The following spring, as new growth starts, apply a fungicide spray to ensure a clean start.
Oak mildew can sometimes attack Beech and Sweet Chestnut, but not other garden plants.
We have had a terrible amount of mildew on our apples, this summer, what we can do to reduce attacks.
During the winter months, examine the shoots to see if they carrying the mildew infection.
These should be pruned out, cutting back to a few buds below the silvery area.
Also look for, and prune out any spurs with thin pointed buds, rather than the normal rounded healthy ones.
These actions should lessen the severity of mildew attacks in late to early summer.