Some of the leaves on one of my new tomato plants have started showing small black spots. The tomato plant is growing in a hydroponic setup (Dutch bucket) outdoors in southern California.

The plant has been bearing fruit and looks to be in great condition aside from these new spots. I have seen pictures of septoria, and from what I can tell, the septoria spots look to be brown/black/yellow, with colors kind of fading out from the center. In addition, most of the leaves infected with septoria also look like they are yellowing/curling at the edges. They just look like bad leaves in general.

The thing that has me confused is that my leaf looks A+ aside from these black polka dots everywhere! Why are my leaves polka dotted?

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  • Are the spots just coloring or a raised or textured surface?
    – Evil Elf
    Commented Jan 7, 2021 at 12:55
  • The growth pattern reminds me of moss or algae. Commented Oct 8, 2021 at 22:56

1 Answer 1


My bet is on Xanthomonas (campestris). Perhaps it isn't a 100% match, but out of all the possible options this bacterial infection comes the closest. Copper-based fungicides are the only sprays that are effective against it (avoid using pure copper sulfate though, that would burn the leaves!). The affected leaves should be immediately removed and the plant sprayed quickly, because otherwise it can infect the fruits as well and render them completely inedible.

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