Southern blight caused by Sclerotium rolfsii is known to occur on several economically important orchid hosts, including Vanda species and hybrids (1--3). In the summer and fall of 2008, an outbreak of southern blight on Vanda orchids was seen in several commercial nurseries and landscapes throughout South Florida. More than a dozen orchids were affected at one of the locations, and symptoms of S. rolfsii were observed on Ascocentrum and Ascocenda orchids, which are also common in the trade and demand a resale value ranging from $20 to $150 for specimens in bloom. Affected Ascocentrum and Ascocenda orchids were found severely wilted at the apex, while around the base of the plants, tan, soft, water-soaked lesions were present. As the lesions progressed, leaves around the base of the plants began to fall off, leaving the stems bare. After 2 days, white, flabellate mycelium was seen progressing up the stem and numerous, tan-to-brown sclerotia were present. Leaves and portions of the stems were plated on acidified potato dextrose agar (APDA) and grown at 25°C. White, flabellate mycelium and tan sclerotia approximately 2 mm in diameter were produced in culture and microscopic examination revealed the presence of clamp connections. The fungus was identified as S. rolfsii and a voucher specimen was deposited with the ATCC. A PCR was performed on the ITS1, 5.8S rDNA, and ITS2 and the sequence was deposited in GenBank (Accession No. GQ358518). Pathogenicity of an isolate was tested by placing 6-mm plugs taken from APDA plates directly against the stem of five different Ascocentrum and Ascocenda orchids. Five Ascocentrum and Ascocenda orchids were inoculated with 6-mm plugs of plain APDA and five were untreated controls. Plants were housed under 50% shade, 60 to 95% humidity, and temperatures ranging from 75 to 88°F. Within 7 days, all inoculated plants developed symptoms that were identical to those observed on original plants and S. rolfsii was consistently reisolated from symptomatic tissue. Ascocentrum and Ascocenda were previously reported under miscellaneous orchid species and hybrids as hosts for S. rolfsii (1). However, this report was highly ambiguous and the most current edition does not report the host fungus combination (2). To our knowledge, this is the first report of S. rolfsii affecting Ascocentrum and Ascocenda orchids.
References: (1) S. A. Alfieri, Jr., et al. Diseases and Disorders of Plants in Florida. Bull. No. 11. Division of Plant Industry, Gainesville, FL, 1984. (2) S. A. Alfieri, Jr., et al. Diseases and Disorders of Plants in Florida. Bull. No. 14. Division of Plant Industry, Gainesville, FL, 1994. (3) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989.
