Comparison of Three Media for Enumeration of Sclerotia of Macrophomina phaseolina. G. L. Cloud, Former Graduate Student, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. J. C. Rupe, Associate Professor, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Plant Dis. 75:771-772. Accepted for publication 23 January 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0771.

The selective medium RB (39 g/L of potato-dextrose agar, 100 mg/L of rifampicin, 224 mg a.i./L of metalaxyl, and 1.0 ml/L of nonoxynol [Tergitol NP-10]) was compared with MP and MSK, two selective media commonly used to enumerate sclerotia of Macrophomina phaseolina. The comparison was made with 20 field soils collected from Arkansas that were naturally infested with M. phaseolina and with a sterilized soil artificially infested with various levels of sclerotia. The population of sclerotia enumerated with RB was as high or higher than that enumerated with MP or MSK. MP enumerated more sclerotia in artificially infested sterilized soil, but enumerations with all three media reflected the relative levels of M. phaseolina in the soil. RB does not contain the fungicide chloroneb (as do MP and MSK) or mercuric chloride (as does MSK). Sclerotia can be counted on RB 3–14 days after plating, compared with no more than 4 days on MP and no fewer than 7 days on MSK.

Keyword(s): charcoal rot.