Influence of Inoculum Type and Moisture on Development of Rhizoctonia solani on Foliage of Table Beets. GILBERTO OLAYA, Graduate Research Assistant; Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462. GEORGE S. ABAWI, Professor, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462. Plant Dis. 78:805-810. Accepted for publication 2 May 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0805.

Thanatephdrus cucumeris, the teleomorph of Rhizoctonia solani, was observed for the first time on table beets under field conditions in New York State during the rainy summers of 1990 and 1992 but not during the dry summer of 1991. Hymenia appeared as a thin, dusty, membranous growth on the crowns and lower part of the petioles. Isolations from small circular lesions (3-6 mm in diameter) on leaves of plants with hymenia present, as well as on adjacent plants lacking hymenia, always yielded R. solani. The efficiency of different sources of inocula of R. solani (mycelial suspension, colonized beet seeds, and pasteurized soil infested with R. solani) in initiating foliar blight (pocket rot) in experimental field plots was tested. The most effective inoculum preparation for consistent and severe disease development was the colonized table beet seeds. With all the inocula sources tested, covering the inoculated beet tissues with field soil to simulate cultivation conditions always increased disease incidence and severity. A leaf wetness period of 72 hr was necessary for infection by R. solani on table beets. Incubating inoculated plants in a mist chamber resulted in more severe disease development than placing plants in plastic bags and maintaining them in the greenhouse. Infections by R. solani were restricted on inoculated plants that were left on the bench without a cover. We concluded that the primary source of inoculum for development of foliar blight on table beets is infested soil thrown on petioles and crowns of the plants during cultivation. The disease is also exacerbated by high moisture environment and the production and spread of basidiospores.

Keyword(s): Beta vulgaris, inoculum sources, root rot