Watch out for diseases! by Dr. Steve Nameth OSU Department of Plant Pathology Be on the lookout for a couple of late season diseases that have the potential to cause problems on poinsettia at this time of year. First off, Powdery Mildew of Poinsettia has been identified in the Midwest and the Eastern United States. This disease can be very difficult to control, particularly once the bracts have turned. The fungus is relatively easy to identify. It will appear as small white spots on the leaves but can eventually cover the entire leaf with a white fluffy growth. Infected plants should be immediately removed from the greenhouse. This will help eliminate the source of inoculum. Fungicide applications should follow. Once color has developed, caution must be exercised as some chemicals could burn bracts or leave an unsightly residue. Systane and Phyton-27 are both labeled for safe application at this late stage. Another common fungal disease that can be a problem at this time of year is Botrytis Gray Mold. As the plant canopy expands and the temperature's cool Botrytis can bractsbecome a problem. The disease first appears on the leaf margins and spreads inward. It is important to keep the relative humidity in the greenhouse below 85% and maintain good air circulation. If chemical treatment is warranted Phyton-27 is the only product that can be applied without the possibility of burn.