A sample of poinsettia with conspicuous leaf spots was received by the Cornell Long Island Horticultural Research and Extension Center (LIHREC) Diagnostic Lab, sent from a greenhouse located in another region. The leaf spots resembled scab, caused by the fungus Sphaceloma poinsettiae. However, sporulation of the fungus Corynespora cassiicola was found. C. cassiicola is not a common problem on poinsettia, particularly in the northeast; fungal leaf spots caused by Alternaria, Colletotrichum, and Botrytis are more usually seen.
C. cassiicola can cause brown leaf spots on leaves and bracts in conditions of high moisture and high humidity. C. cassiicola is listed as occurring on a very wide host range of over 500 plants from more than 350 genera including various ornamental plants as well as numerous vegetable, fruit, herb, and field crops. Ornamental plant hosts include poinsettia, hydrangea, begonia, African violet, salvia, annual vinca, and lipstick vine. Cucurbit crops, eggplant, pepper, tomato, and basil are a few of the edible crops affected.
C. cassiicola causes irregular, often large, brown lesions on leaves and bracts, often seen at the tips or edges. Leaf spots are usually very difficult to discern from one another and assistance of a diagnostic lab is necessary.
Infested plant material or debris can harbor inoculum of C. cassiicola, and good sanitation practices will help prevent the spread of this leaf spot disease. Reducing leaf wetness and humidity will also help to manage. Regular applications of a labeled fungicide may be needed if an outbreak occurs.
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