Brassidium hybrid orchid leaves were sent to the University of Florida Plant Disease Clinic in the summer of 2002. Symptoms on leaves were a spreading, cottony, white mass of mycelium with necrotic centers 3 to 5 × 5 to 9 mm. Approximately 1% of leaves submitted were affected. Sclerotium rolfsii Sacc. was isolated from the affected leaves using acid potato dextrose agar (APDA) (1). Two plastic boxes were lined with wet paper towels and 6 uninoculated leaves were placed in each. Three leaves in each box were treated as controls and received 3 mm2 of APDA, while the other three leaves in each box received 3 mm2 of S. rolfsii culture on APDA. All leaves were misted with water and enclosed to maintain humidity. Twenty-four hours later, infection was visible on two treated leaves. Within 72 h, significant infection was visible on three of the six inoculated leaves. All control leaves remained uninfected. S. rolfsii was consistently reisolated from the inoculated leaves, fulfilling Koch's postulates. Over the past 12 months, S. rolfsii has been detected in 30% of commercial orchid nurseries as well as homeowner orchid collections in Miami-Dade County, Florida. No resistance to this disease in orchids has been reported. To our knowledge, this is the first report of S. rolfsii on this orchid hybrid.
Reference: (1) J. Tuite. Media and nutrient solutions used by plant pathologist and mycologist. Page 53 in: Plant Pathological Methods Fungi and Bacteria. Burgess Publishing Company, Minneapolis, MN, 1969.
