It sounds like rust, a catch-all name for a class of moulds. Avoid wetting
the leaves when watering, try to promote air circulation if possible and
treat with wettable sulphur or other fungicide.
Indeed, I am sure that's what I did wrong. And all that moisture just
attracts more slugs. I think what may have done it was this water based
insecticide I used at night. I was dosuing them with Spinosad at night so
as to not harm any bees.
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