Powdery mildew can spread rapidly once the conditions are favorable, and an epidemic can be devastating to a poinsettia crop. The key is finding it early and staying on top of management.
After a long while of not seeing powdery mildew on poinsettia, several cases were seen last season. Perhaps that was a fluke, and we won’t see it again – let’s hope that is the case. But let’s use the mildew seen last year as a reminder and take the opportunity to review this disease.
Typically you see the white dusty or powdery fungal growth on the surfaces of leaves, however, the early symptoms on poinsettia are pale and subtle yellow or chlorotic spots on the leaf surface with white/light gray fungal growth on the leaf undersides. As the disease progresses, you can see the fungal growth develop on the leaf surfaces and even the bracts.
Read more at e-gro.org