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Influence of Soil Inoculum Concentrations on Host Range and Disease Reactions Caused by Isolates of Thielaviopsis basicola and Comparison of Soil Assay Methods. M. Tabachnik, Graduate research student, Department of Plant Pathology, University of California, Davis 95616, Present address of senior author: Department of Ornamental Horticulture, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76-100, Israel; J. E. DeVay(2), R. H. Garber(3), and R. J. Wakeman(4). (2)(4)Professor, and staff research associate, respectively, Department of Plant Pathology, University of California, Davis 95616; (3)Research plant pathologist, USDA-SEA/AR U.S. Cotton Research Station, Shafter, CA 93263. Phytopathology 69:974-977. Accepted for publication 25 March 1979. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1979. DOI: 10.1094/Phyto-69-974.

Nine isolates of Thielaviopsis basicola were tested for host specificity at 102, 103, and 104 endoconidia per gram of soil and for disease reactions caused on cotton, bean, pea, peanut, and soybean. Sunflower, a nonhost, served as control. The isolates of various geographic origin were from cotton, bean, soybean, orange, sesame, and field soils cropped to soybean, tobacco, peanut, or guar. Cotton was the most susceptible at each of the inoculum concentrations, and sunflower apparently was not affected by any of the inoculum concentrations. For all isolates, host specificity was most apparent at an inoculum concentration of 104 endoconidia per gram of soil, except for cotton and bean; isolate 415 was nonpathogenic on bean but caused severe black root rot on cotton at 104 endoconidia per gram of soil. Although isolates of T. basicola were from various hosts, they were not specific for these hosts. In assays for T. basicola in soils from nine cotton fields in the San Joaquin Valley in California, each with a history of seedling disease problems, eight fields had 10 or fewer propagules per gram of soil and one field had 70 propagules per gram of soil. Of various selective media compared for sensitivity in detecting low concentrations of propagules of T. basicola from naturally infested soils, the carrot disk technique was the most sensitive.

Additional keywords: black root rot, soil assays.