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Populations of Thielaviopsis basicola and the Occurrence of Black Root Rot on Burley Tobacco in Western North Carolina. J. Meyer, Graduate Research Assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. H. D. Shew, and P. B. Shoemaker. Associate Professor, and Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 73:239-242. Accepted for publication 31 October 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0239.

Populations of the black root rot pathogen, Thielaviopsis basicola, and occurrence of black root rot on burley tobacco were determined in an extensive, random survey of 80 burley tobacco fields in the burley-growing region of the Appalachian Mountains in North Carolina. The pathogen was detected in 47 (59%) of the surveyed fields at a mean population of 40 cfu/g of soil and a range of 2219 cfu/g of soil. Black root rot was observed in 32 (68%) of the infested fields. Disease severity was positively correlated with inoculum density of T. basicola and was lower on moderately resistant cultivars than on cultivars with low resistance to black root rot. No disease was detected on the very highly resistant cultivar, Tennessee 86, in any field. Disease severity on cultivars with low resistance was positively correlated with exchangeable soil Ca, Mg, K, cation exchange capacity, and base saturation. Disease severity on moderately resistant cultivars was not correlated with soil chemical factors. Pathogen populations were positively correlated with the number of years in tobacco. T. basicola was found less frequently and in lower inoculum densities in fields in which rotation had been practiced as compared with those in continuous tobacco.