# Yellow patches on top of leaves with grey-purple fuzz underneath: likely causes and fixes

> Yellow patches on top of leaves with grey-purple fuzz underneath on cannabis is most often Downy mildew. How to tell them apart, the fix for each and what recovery looks like.

Also called: downy mildew, fuzz under leaves, yellow patches with mould underneath.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=whole&s=fuzzy-underside-yellow-top

## Likely causes, most likely first

### Downy mildew (Possible)

- Kind: Disease; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Article: [White or grey coating on cannabis: the master decision tree](https://genetixbay.com/knowledge/diagnosis/white-grey-coatings-decision-tree.md)

## Note

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.