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I have an oak (Quercus robur) sapling I've grown from seed in a pot (it is 2 years old) and its leaves are exhibiting a powdery white mildew.

After some searching online I think it's a case of erysiphe alphitoides (it looks exactly like the photographs I've seen online). The information I've seen online states that it can be treated with fungicides that contain sulphur, but the information contains the caveat

this can damage some plants

Would a fungicide that contains sulphur harm a quercus robur oak? If so, what alternative fungicide would be appropriate?

I am in the UK, in the South of England if that's useful information.

  • The context is not other than the one being treated - the site seems to recommend a sulphur based fungicide for any case of powdery mildew with that caveat because it's pretty vague. My question was asking whether a sulphur based fungicide is safe specifically for quercus robur and while your suggestions of a baking soda spray is interesting it doesn't answer the specific question about oak and sulphur. However, please add your baking soda suggestion as an answer and I will upvote it as I haven't heard about this use before. – Matt Jones Aug 1 '17 at 13:41
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    It gives the same advice for cucurbit powdery mildew which is why I thought it was a generic advice with a caveat to be careful for some plants, but without specifying which plants! – Matt Jones Aug 1 '17 at 13:52
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Sulphur is a contact fungicide. Really there are two forms of sulphur fungicide: powder or soluble. I assume that you need only the soluble one. "Contact fungicide" mean that you can test just few parts of the plant, and you can see if there are some problems with the plant.

As pnut wrote in comment, I think the interpretation is that it can damage some other plants. In any case it is very widely used, for plants, orchards, vineyards, vegetables and garden. So it is not common to have problems with sulphur. But as far I know, some fruits (as plant part) doesn't like it (e.g. some apple's varieties).

So check to buy just sulphur (the kind that dissolve in water). It is cheap. Many other fungicides includes it: because it is cheap, and because it prevent resistance, but better to avoid them, if there are no needed.

But you should read carefully the instructions. There are temperature limits (minimum and maximum). Outside such temperatures, the sulphur could burn the leaves.

EDIT:

I find only the following sensitivity on Sulphur:

  • high temperature: all plants
  • Apple varieties: Golden Delicious, Imperatore, Jonathan, Rome Beauty, Stayman
  • Pears: Williams
  • some peaches and most apricots.

If I remember correctly the problems are just in fruits (and just fruits that are subject to "rust")

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  • Do you specifically know if sulphur fungicide is harmful to an oak? Other than actually just try it out on my tree? – Matt Jones Aug 1 '17 at 13:43
  • For oaks, no. I leave the mildew on the leaves, and it seems it is under control. But Really I wrote to test it on few branches. Worst case the leaves on such branches will have few burn spots. – Giacomo Catenazzi Aug 1 '17 at 17:00
  • Did you just say that it promotes resistance? – stormy Aug 1 '17 at 21:15
  • @stormy: no, contact fungicide are very basic (chemically, physiologically), so usually they doesn't cause resistance. Other fungicides, which combat fungi from inside, have a more complex biology, and they are usually prone to make resistance (note: many of them must not be used alone, and most of them have a maximum use per year). Mixing contact fungicide with other fungicides really help: the first will strongly reduce/weak existing infection (but short lifetime), the second will combat inside with much less fungi, and prevent new infections. Less fungi, less diversity, less resistance. – Giacomo Catenazzi Aug 2 '17 at 8:03
  • sorry it took me so long to accept this answer - life comes at you pretty fast sometimes! Many thanks though, this is very helpful. – Matt Jones Sep 7 '17 at 10:42

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