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Evaluation of Virulence in Isolates of Sclerotium oryzae. S. A. Ferreira, Department of Plant Pathology, University of California, Davis 95616, Current address of senior author: Department of Botany and Plant Pathology, Colorado State University, Ft. Collins 80523; R. K. Webster, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 66:1151-1154. Accepted for publication 29 March 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1151.

A test to measure the disease reaction of individual Sclerotium oryzae isolate-host combinations was developed by estimating the ability of isolates to incite lesions or kill 25-day-old Colusa rice seedlings. Since the ability of isolates to incite lesions was correlated with the ability to kill seedlings, virulence of individual isolates can be determined by measuring their ability to incite lesions. When Colusa and Earlirose rice were compared by the seedling test, the initiation of lesions was delayed more on Colusa, indicating that Colusa is more resistant to S. oryzae than Earlirose. Therefore, the seedling test can be used to evaluate rice cultivars for stem rot resistance as well as virulence in S. oryzae.