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Assessment of Microsclerotia of Verticillium albo-atrum in Field Soils. L. J. Ashworth, Jr., Associate Plant Pathologist, Department of Plant Pathology, University of California, Berkeley 94720; J. E. Waters(2), A. G. George(3), and O. D. McCutcheon(4). (2)Research Associate, Department of Plant Pathology, University of California, Berkeley 94720; (3)(4)Farm Advisors, respectively, Tulare and Kings Counties, California. Phytopathology 62:715-719. Accepted for publication 31 January 1972. DOI: 10.1094/Phyto-62-715.

A substrate consisting of Czapek’s agar, less sugar, plus 200 µg/ml streptomycin sulfate and covered with a layer of cellophane film, trapped Verticillium albo-atrum from soils. The presence of the fungus in 32 soils was qualitatively determined by spreading 0.5-g samples of soils on this substrate and counting groups of microsclerotia that developed on the cellulose film after 6 days at 26 C. Groups of microsclerotia were observed without interference from other microorganisms after soil was washed from the surface of the substrate. About 90% of all the groups of microsclerotia detected in soils collected from June to September 1971 were found to arise from microsclerotia free in the soil. Nine of ten cultures of the fungus isolated from this substrate were pathogenic to cultivar Acala S J-1 cotton seedlings. The procedure was made quantitative by the wet-sieving of 15-g soil samples, followed by the culturing of residues of soils arrested by 37-, 53-, and 74-µ sieves, upon which about 90% of free microsclerotia were arrested. The procedure was quantitative at an inoculum density of 0.13 microsclerotia/g soil. The inoculum density in 33 soil collections, ranging from sandy loams to clay loams in texture, varied from 0.03 to about 50 microsclerotia/g soil.

Additional keywords: Gossypium hirsutum.