Cercospora Leaf Spot Caused by Cercospora dendrobii on Dendrobium antennatoum Lindl. and Its Control

Cercospora leaf spot disease of Dendrobium spp. has been reported in Florida, Thailand, and most of the tropical areas of the world where dendrobiums are grown. It most commonly occurs in southern Florida and has been significant in dendrobium production. Other species of cercospora leaf spot pathogens are occasionally found on other genera of orchids, including Angraceum, Cattleya, Odontoglossum, Brasavola, Broughtonia, Epidendrum, and Schomburgkia. Leaf lesions on the Dendrobium are at first noted on the undersurface of the leaf as pale yellow sunken spots, 1 to 3 mm in diameter. With time, the spots continue to enlarge in a circular or irregular pattern and eventually may cover the whole underside of the leaf. Later the spots become slightly sunken and purple-black with the developing margin remaining yellow. Following the appearance of the spots on the lower leaf surface, a corresponding yellow-pale green area can be seen on the upper leaf surface. Eventually the spots turn purplish-black or black. Heavily infected leaves abscise. Prolonged periods of leaf wetness should be avoided. Chlorothalonil and thiophanate-methyl are labeled for control of cercospora leaf spot on orchid in the United States. Results from cercospora fungicide trials conducted in 2005 showed that the BASF 516 04 F (pycadostrobin + boscalid) 38% WG at 340.2 g per 379 L of water and pyraclostrobin 20% WG at 226.8 g per 379 L of water were significantly effective.