Control - Elimination of water on leaves is important to completely control Alternaria leaf spot of poinsettias. Annette Hegg cultivars have been reported as very resistant to Alternaria leaf spot as well. These areas enlarge to about 1 inch in diameter and are tan to dark brown. Control - Keep bracts and leaves as dry as possible to minimize conditions for pathogen spread and infection. The fungicides which are mentioned for control of Alternaria leaf spot should work equally well on Corynespora leaf and bract spot.
Necrotic areas are dark-brown and mushy.
Examination of the rotted areas reveals sporodochia which are irregularly shaped, black and have a white fringe of mycelium. Control - Using fungicides when temperatures are between 70 and 85 F and fertilizing at recommended levels contribute to minimizing severity of Myrothecium diseases. Powdery mildew Pathogen - Oidium sp. Symptoms - A white powdery coating covers the top and sometimes the bottom leaves as well as bracts of the affected plants. The covering sometimes forms in circular lesions and sometimes covers the entire surface of the leaf.