Sounds like a mold or fungus issue. In warm, humid conditions, any droplet of water on a leaf can be an incubator for them, especially if there is little air movement around the plants.
You might try making cinnamon "tea" - stir a heaping spoonful of powdered cinnamon into about a liter of boiling water. After it cools to room temperature, filter the sediment out, and the brown liquid is a pretty good fungicide.
Spray the plant and soak the medium and your issues should go away.
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