Pathogenicity of North Carolina and French isolates of Chalara elegans and comparison of tobacco varieties resistant to black root rot
Growth chamber and field experiments on tobacco black root rot (BRR) were conducted at the Institut du Tabac, Bergerac, France in summer 1989. Pathogenicity of 14 isolates of Chalara elegans, collected from 7 fields in North Carolina (NC) and 7 fields in France (FR) were compared on Burley 21 (low resistance) in rooted leaf and seedling tests. Both groups of isolates exhibited a similar range of virulence from low to high in the rooted leaf tests and all were highly virulent in the seedling tests. In other experiments, two NC and two FR isolates were compared on varieties Judy's Pride (susceptible), Burley 21 (low) Kentucky 14 (moderate) and Kentucky 17 (high resistance). Isolate X variety interactions were not significant and disease was present on Kentucky 17 in the rooted leaf tests, but not in the seedling tests. In experiments with burley hybrids MS Burley 21 X Kentucky 10 and MS Kentucky 14 x L8, the former showed low resistance and did not differ significantly from its two parents, whereas, the latter showed a moderate level of resistance which was greater than its Kentucky 14 but equal to its L8 parent. In a field with a high C. elegans population, 20 tobacco varieties, including black, burley and flue-cured types differing in resistance to BRR, were compared. The season was very dry and little to no BRR was present on roots at the end of the season. However, plant growth and end-of-season populations of C. elegans were related to levels of BRR resistance. For example, Tennessee 86 (high resistance), KY 14 x 8 (moderate resistance) and Virginie D (susceptible), had yields of 4265, 3211 and 1201 kg/ha and end -of-season soil populations of C. elegans were reduced by 70% remained unchanged and were doubled, respectively.