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Just in the past week, just as my grapes are ripening, this powder is showing up on some of them, and some of the green stocks are showing some brown. What’s wrong and how do I fix it? The vine is about 4 years old and we are in the Pacific North West. enter image description here Edit, the leaves are not really affected yet, just the stocks.

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    Looks like some sort of mildew. Can you take pictures of both sides of the leaves? Commented Aug 18, 2018 at 4:09
  • they're going to make a very special wine, but I can't remember what it's called for some reason. Commented Aug 18, 2018 at 4:09
  • What have you used for fertilizer? Plants that are trying to make grapes and their own food via photosynthesis have to have chemistry added to the soil...judiciously. I am seeing nitrogen starved leaves and powdery mildew. Disease and insect damage become a problem on plants that are trying to survive without proper chemistry. Your plants need fertilizer, balanced, NPK where N is the lowest of the 3 numbers.
    – stormy
    Commented Aug 18, 2018 at 5:42
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    Can you post an image of an affected leaf please? You may want to compare to here and here. Did any of the grapes burst open (yet)?
    – Drux
    Commented Aug 18, 2018 at 10:02
  • Judging from how the stocks look and from a Google search (I haven't seen this pattern myself in practice yet): could it be Phomopsis viticola? Anyway, those stems don't look good at all and I'm afraid they will have to be cut back severely.
    – Drux
    Commented Aug 19, 2018 at 18:26

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I have grown the variety Fredonia which showed this exact mildew. It is powdery mildew most likely. Ontario Grape IPM has a great page on this problem, including a nice pair of images comparing powdery with downy mildew. The timing of the question coincides exactly with when this problem appears in the northern hemisphere, as we go into cooler overnight temperatures with the onset of the fall. The infection starts early in the season but is often not very evident until the berries start to mature. The seriousness of the fungus rises and falls with the varying daily changes in temperature, humidity and wind; it is exacerbated by poor air circulation so keep the vine canopy open with judicious pruning.

For spray solutions not involving organic fungicides read up on hydrogen peroxide and baking soda plus soap in a grape cultivation context which may help.

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