First report of binucleate Rhizoctonia causing damping off in tobacco seedlings in Cuba
ResumenDamping off on tobacco seedlings in floating trays were observed in the provinces of Pinar del Río and La Habana, main Cuban areas for tobacco production. In order to know the causes of that problem, thirteen samples of seedlings and substrate were collected and processed using mycological methods. All the samples showed a typical mycelium described for Rhizoctonia complex [Sneh et al., 1991]. Four isolates had multinucleates hyphae and were classified as Rhizoctonia solani anastomosis group 4 (AG-4). The nine remaining isolates showed binucleates somatics cells and had more than 90% of homology in ITS region with the genus Ceratobasidium Roger (anamorph: Ceratorhiza R. T. Moore). Four binucleates isolates were associated with AG-R and the other five with AG-G by sequences comparison. Pathogenicity test were carried out using 21 days old seedlings of tobacco (var. Criollo 98). The inoculum consisted in a mixture of corn meal and sand colonized with each binucleate isolates separately. Each inoculum was mixed with sterile soil, dampened and the seedlings were planted on it and incubated at 25°C. Control seedlings were treated in a similar way in sterilized soil. About two weeks later damping off was observed. All the isolates were able to cause the disease including those obtained from substrate. Stems of noninoculated seedlings were free of lesions. Cultures of each Rhizoctonia group were re-isolated from inoculated seedlings. So far, the only report of a binucleate Rhizoctonia affecting tobacco was made in Zimbabwe but the author do not specified the anastomosis groups involving in the diseases [Mazuca, 1998]. The AG-G and AG-R of binucleate Rhizoctonia have been previously reported in strawberry, peanut pods and cucumber [Ogoshi et al., 1983; Sneh et al., 1991, Hyakumachi et al., 2005, Matsumoto and Yoshida, 2006]. This is the first report of binucleate Rhizoctonia (AG-G and AG-R) causing damping off on tobacco seedlings in Cuba.