VIEW ARTICLE

Ecology and Epidemiology

Decline in Numbers and Inoculum Potential of Sclerotium oryzae in Field Soil. W. W. Bockus, Assistant professor, Department of Plant Pathology, Kansas State University, Manhattan, KS 66506; R. K. Webster, professor, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 69:389-392. Accepted for publication 20 October 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-389.

A calculated half-life of 1.9 yr was obtained for sclerotia of Sclerotium oryzae rototilled into field soil to an 18-cm depth with or without an amendment of dried, noninfested rice residue. During the first 4 wk, living sclerotia incorporated in soil germinated 80% when recovered from soil and placed on water agar. Germination of sclerotia on water agar declined to 20% after 6 mo and thereafter stabilized between 20 and 30%. Treatment with 0.5% NaOCl indicated that loss of germinability was due to dormancy but not to complete death of sclerotia. Number of sclerotia per gram of soil that germinated when placed on water agar amended with streptomycin sulfate and penicillin G was a good measure of the inoculum potential of S. oryzae. Initially the inoculum potential of freshly-produced sclerotia incorporated in field soil declined rapidly due to the rapid loss in the ability of sclerotia to germinate, but the total numbers of sclerotia decreased only slightly. After the rapid decline, inoculum potential stabilized and thereafter is expected to be influenced largely by a decline in actual numbers of sclerotia. Because the decline in numbers of sclerotia of S. oryzae was relatively slow (half-life = 1.9 yr), alternate year cropping of rice would not be expected to control stem rot.