Reaction of a Resistant Breeding Line and Susceptible California Rice Cultivars to Sclerotium oryzae. J. J. Oster, California Cooperative Rice Research Foundation, Inc., Biggs, CA 95917. . Plant Dis. 76:740-744. Accepted for publication 2 January 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0740.

Resistance to Sclerotium oryzae (= Magnaporthe salvinii), the causal agent of stem rot of rice, was found in a wild species of Oryza (O. rufipogon) and incorporated into the high-yielding line, 87Y550. S. oryzae isolates were screened against this line and two susceptible cultivars. No isolate was able to differentially attack the resistant line. There were significant year, isolate, cultivar, and year × cultivar effects for both stem rot score and number of sclerotia per tiller (P = 0.05). Cultivars differed in stem rot score and number of sclerotia formed per tiller. There was a significant year × isolate effect for stem rot score (P = 0.05) but a slightly less significant effect for year × number of sclerotia per tiller (P = 0.1). Variability due to year effects was greater than that attributable to S. oryzae isolates. Stem rot score and number of sclerotia per tiller were highly correlated (r² = 0.74–0.78, P < 0.001). If the stem rot score was less than 3 (top four leaf sheaths healthy), no sclerotia were produced. Sclerotial production was greatest in leaf sheaths 2 and 3 (counting from the flag leaf down) and the culm.