Interactions Among Pythium Species Affecting Root Rot of Sugarcane. Y. S. Lee, Former Graduate Assistant, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. J. W. Hoy, Associate Professor, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. Plant Dis. 76:735-739. Accepted for publication 12 December 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0735.

Several Pythium species are isolated commonly from roots of sugarcane plants (interspecific hybrids of Saccharum) grown in field soils. To compare the effects of infection by one or several Pythium species, pathogenicity tests were conducted with P. arrhenomanes, P. catenulatum, P. irregulare, P. spinosum, and two unidentified Pythium species. Isolates of P. arrhenomanes caused significant growth reductions and severe root rot. Isolates of P. irregulare and P. spinosum caused significant reductions in root weight but varying effects on shoot number and weight. Root rot symptoms induced by these species were mild or not evident. P. catenulatum and the unidentified Pythium species were nonpathogenic. Severity of root rot caused by P. arrhenomanes was unaffected by combinations with other species. Combinations of P. irregulare and P. spinosum generally did not increase disease severity over that caused by each species alone. Root rot severity, total Pythium isolation frequency, P. arrhenomanes isolation frequency, and percentage of all Pythium isolates consisting of P. arrhenomanes varied among plants grown experimentally in different field soils. The percentage of all Pythium isolates from roots consisting of P. arrhenomanes correlated positively with disease severity.