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Stem rot of Rice in California. R. A. Krause, Department of Plant Pathology, University of California, Davis 95616, Present address of senior author: Department of Plant Pathology, The Pennsylvania State University, University Park 16802; R. K. Webster, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 63:518-523. Accepted for publication 1 November 1972. DOI: 10.1094/Phyto-63-518.

A disease index based on the depth of penetration of Magnaporthe salvinii into the tiller was developed which correlated well with variations in inoculum levels, times of infection, cultivar susceptibility, and yield loss. Under field conditions a 12% yield loss and a 12% reduction in the number of tillers/plant resulted when plants were inoculated with 500 viable sclerotia/plant (vs/p) during the period of stem elongation. Inoculations with equal or lower numbers of vs/p at mid-tillering or at the boot stage did not result in yield or tiller losses. Six California cultivars varied in their degree of susceptibility to Sclerotium oryzae. ‘Earlirose’ was the most susceptible; ‘Caloro’, ‘Calrose’, ‘CSM-3’, and ‘Kokohoe Rose’ were moderately susceptible; and ‘Colusa’ was the least susceptible. Yield losses up to 22% were measured under actual field conditions.