Concrete_rex said:

Is this mildew

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Beachwalker said:

Is that the only leaf with it on it? Is it possible that something spilled or splattered?

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Beachwalker said:

If it's not a splatter then yes it could be some kind of a mildew or mold, there was a similar thread with a bluish mold on here within the last couple weeks on here which I told the poster to look it up and treat it as gray mold because there's all different kinds of molds and mildew but their treatment is pretty much the same

I use Liquid copper fungicide with great results just sprayed it on the tomatoes yesterday, I know there's some people that use Liquid sulfur but I've never used that and have no experience with it

a quick stop is a level tablespoon of baking soda in a gallon of water you got to spray the plant top and bottom of the leaf, in my opinion baking soda is only a quick fix until you can get the copper fungicide, hope this helps good luck

Edit: I suppose it's possible that a flock of birds eating blueberries might have flown over and left those marks IDK suppose is possible but to my eye that looks like that same bluish-gray mold that we saw here in a thread a couple weeks ago, and I had it myself indoors in the 90s to me it looks like mold or a mildew

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