We have had a report of powdery mildew in poinsettia in NY already. It hasn’t been confirmed but this is a good time to remind you to check your plants when you are scouting for white fly (or even if you aren’t scouting for whitefly – do both!). Look for yellow spots on the leaves, then turn them over to look for white fungal colonies on the underside of the leaf.
This is another disease with a temperature connection. Once greenhouse daytime temperatures are staying below 86°F, it can start to spread if it is there.
No need to treat if you don’t see colonies but if you do, use a fungicide labeled for powdery mildew. (And always check the label for phytotoxicity.)
For more information, please see The American Phytopathological Society's Fact Sheet: Poinsettia: The Christmas Flower. Scroll down for photos.