First Report of Root Rot Caused by Binucleate Rhizoctonia Anastomosis Group F on Musa spp.

A research program was initiated at the University of Georgia in 2003 to identify banana cultivars suitable for production in the coastal and southern areas of the state. During a root disease survey conducted in October 2007 on bananas (Musa spp.) grown at the University of Georgia Bamboo Farm and Coastal Gardens in Savannah, GA, root lesions and root rot were observed on banana cvs. Gold Finger, Kandarian, and Manzano. Root lesions were dark brown to black and irregular in shape, with partial or entire roots affected. Lateral roots and outer layers of cord roots (roots arising from interior layers of the corm) of infected plants were blackened and rotted. Diseased root samples were collected from three plants of each cultivar, surface sterilized with 0.6% sodium hypochlorite, and placed on tannic acid benomyl agar (TABA). Pure cultures of the fungus consistently associated with diseased tissue were obtained by subculturing hyphal tips on TABA. Mycelia of the fungus on potato dextrose agar (PDA) were light to deep brown and the hyphae tended to branch at right angles. A septum was present in each hyphal branch near the point of origin and a slight constriction at the branch was observed. The hyphae of two isolates were stained with 0.6% phenosafranin and 3% KOH and binucleate hyphal cells were observed. On the basis of these morphological features, the isolates appeared to be binucleate Rhizoctonia anamorphs (teleomorph Ceratobasidium Rogers). For molecular identification, the internal transcribed spacer (ITS) regions and the 5.8S gene from rDNA of the isolates were cloned and sequenced (GenBank Accession No. HQ168370). The ITS regions (775 bp) were 100% identical between the two isolates and 99% identical to Ceratobasidium sp. AG-F strain SIR-1 isolated from sweet potato in Japan (GenBank Accession No. AF354085). The anastomosis group of the isolates was confirmed by pairing with strain SIR-1 on PDA. On the basis of morphological and molecular characteristics and the anastomosis assay, the two isolates were identified as a Ceratobasidium sp. AG-F (1-3). Pathogenicity assays were conducted by inoculating banana plants (cv. Golden pillow, synonym = Manzano) grown in pots under greenhouse conditions (25 to 27°C). Twenty wheat seeds infested with each isolate were placed uniformly around each plant at a depth of 10 cm in the soil. The plants were incubated in the greenhouse and the roots were examined 2 months after inoculation. Brown-to-black lesions and root rot, identical to symptoms associated with field banana roots, were observed on all inoculated plants but not on the noninoculated control plants. The fungus was reisolated from affected root samples and the identity was confirmed by morphological and molecular characteristics and the anastomosis assay. To our knowledge, this is the first report of banana root rot caused by binucleate Rhizoctonia anastomosis group F. With the increased interest in producing bananas for food and ornamental purposes, the occurrence of Ceratobasidium root rot on bananas needs to be considered when designing disease management programs and searching for suitable cultivars for banana production. References: (1) L. L. Burpee et al. Mycologia 70:1281, 1978. (2) D. Gonzalez et al. Mycologia 93:1138, 2001. (3) B. Sneh et al. Identification of Rhizoctonia Species. The American Phytopathological Society, St. Paul, MN. 1991.