Yellow patches on top of leaves with grey-purple fuzz underneath
Angular yellow patches bounded by veins on the upper surface, fuzzy growth below, in cool, wet, humid conditions..
- Also called
- downy mildew
- fuzz under leaves
- yellow patches with mould underneath
Likely causes, most likely first
Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.
PossibleDowny mildew
Fix: Rarely reported on cannabis. Remove infected leaves, drop RH and avoid cool wet nights; confirm with a lab before treating.
Recovery: No new patches within 2 weeks.
How to confirm it
- Send a sample to a plant diagnostic lab. For rots, wilts, leaf spots and viruses you can't separate by eye, send a fresh sample (whole plant with roots or a wilting branch with healthy and diseased tissue) in a sealed bag to a university or commercial plant diagnostic clinic. Results in days to 2 weeks.
Look for
Upper side: vein-bounded yellow patches. Underside: grey to purplish downy growth directly beneath..
The full guide: White or grey coating on cannabis: the master decision tree
Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.
