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Evaluation of Components of Partial Resistance to Black Root Rot in Burley Tobacco. C. A. Wilkinson, Associate Professor, Southern Piedmont Agricultural Research and Extension Center, Virginial Polytechnic Institute and State University, Blackstone 23824 . H. D. Shew, Professor, Department of Plant Pathology, Box 7616, and R. C. Rufty, Associate Professor, Department of Crop Science, Box 7620, North Carolina State University, Raleigh 27695. Plant Dis. 79:738-741. Accepted for publication 6 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0738.

Cultivars with moderate (KY 14), low (Burley 21), and no resistance (Judy's Pride) to black root rot, and the F1 of KY 14 x Burley 21 were evaluated for components of partial resistance to Thielaviopsis basicola. In addition, transgressive segregants from a previous generation mean analysis were intercrossed and progeny of the following crosses were evaluated: (KY 14 x Burley 21)F2 ? (KY 14 x (KY 14 ? Burley 21)F1,), (KY 14 x Burley 21)F2 x (Burley 21 x (KY 14 ? Burley 21)F1,), and (KY 14 x (KY 14 x Burley 21)F1) x (Burley 21 x (KY 14 x Burley 21)F1). Five-week-old seedlings were transplanted into soil infested with 100 chlamydospores of T. basicola per gram of soil mixture and grown in the greenhouse at an average air temperature of 21C. Disease severity (percent root necrosis), number of lesions per root, lesion length, and population density of the pathogen were estimated 3 weeks after transplanting. Significant differences were observed among genotypes for each component of partial resistance measured. Significant positive correlations were observed between lesion number and disease severity and between lesion number and lesion length. However, a significant negative correlation was observed between lesion number and population density. Selection for components of partial resistance in burley tobacco should lead to increased levels of resistance to black root rot.

Keyword(s): Chlara elegans, Nicotiana tabacum