Occurrence of Sclerotium rolfsii on Ascocentrum and Ascocenda orchids in Florida

In summer and fall of 2008, a severe outbreak of southern blight affected Vanda orchids in commercial nurseries and landscapes throughout south Florida. Several Ascocentrum and Ascocenda orchids grown in the landscape were found severely wilted at the apex, while around lie base of the plants tan, soft, water-soaked lesions were present. As the lesions progressed, basal leaves began to drop, leaving the stems bare. After two days, white, flabellate mycelium was seen progressing up the stem with numerous tan to brown sclerotia present. Leaves and stems were plated on APDA and grown at 25C. The fungus was identified as Sclerotium rolfsii and a voucher specimen deposited with the ATCC. A PCR was performed on the ITS1, 5.8S rDNA, and ITS2 and the sequence was deposited in GenBank. Pathogenicity of an isolate was tested by placing 6 mm plugs taken from APDA plates directly against the stem of 5 different Ascocentrum and Ascocenda orchids. Five Ascocentrum and Ascocenda orchids were inoculated with 6 mm plugs of plain APDA, and 5 were untreated controls. Plants were kept in a shade house under 50% shade, 60-95% humidity and temperature ranging from 75-88 F. Within 7 days, all inoculated plants developed symptoms that were identical to the symptoms observed on original plants and S. rolfsii was consistently re-isolated from symptomatic tissue. To our knowledge, this is the first report of S. rolfsii affecting Ascocentrum and Ascocenda orchids.