Root and Stem Rot of Parrotfeather (Myriophyllum brasiliense) Caused by Pythium carolinianum. E. A. Bernhardt, Department of Plant Pathology, University of California, Davis 95616. J. M. Duniway, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 68:999-1003. Accepted for publication 14 June 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-999.

A disease of the emersed aquatic weed parrotfeather was observed during 1980 and 1981 in a drainage ditch in northern California. Diseased plants showed a severe rot of underwater roots and stems and frequently wilted and collapsed. An isolate of Pythium carolinianum from diseased plants was pathogenic to parrotfeather cuttings. Mycelium inserted into parrotfeather stems girdled the stems and caused collapse of the shoots. Zoospore inoculations of plants in solution culture resulted in root rot and suppressed growth. Parrotfeather grown in basins in the field at Davis, CA, and inoculated with the pathogenic isolate produced significantly less biomass and fewer shoots than did uninoculated controls.