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I have just bought a new property that has a range of oak trees. Unfortunately, I noticed that some of the young trees in one area have a white discolouration of the leaves.

Affected oak leaves

After some searching online, I think this is oak powdery mildew (erysiphe alphitoides).

In this question, the only answer discusses using a sulphur fungicide.

I would like to ask:

  1. Is this powdery mildew?
  2. Are there any other options for managing it and can it be completely cured?
  3. Are the nearby mature oaks at risk?
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This looks indeed like powdery mildew, but in a pretty far state. Oaks are very susceptible to mildew, also in the wild. I have an oak in a pot (bonsai) and mildew seems to come back every year. I treat them with a spray which I buy from a local garden center (meant for mildew in roses). I don't know what the spray contains, but I spray it when first signs of mildew (white spots) are present. In your case it would not make sense to use the spray, the mildew is in a too far stage. Also the leafs will fall of the plant soon anyways (autumn). My advice is to leave it for now as it is, and next year try to treat it in time (when first spots appear).

So to answer your questions.

  1. Yes, I think it is.
  2. Yes there are effective sprays available, but try to use it in time next year.
  3. All oaks are susceptible to mildew, however the larger trees are stronger (usually small plants are susceptible or new leaves).

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