EVALUATION OF COMPONENTS OF PARTIAL RESISTANCE TO BLACK ROOT-ROT IN BURLEY TOBACCO

Cultivars with moderate (KY 14), low (Burley 21), and no resistance (J udy's pride) to black root rot, and the F-1 of gy 14 x Burley 21 were evaluated for components of partial resistance to Thielaviopsis basico la. In addition, transgressive segregants from a previous generation m ean analysis were intercrossed and progeny of the following crosses we re evaluated: (KY 14 x Burley 21)F-2 x (KY 14 x (KY 14 x Burley 21)F-1 ), (KY 14 x Burley 21)F-2 x (Burley 21 x (KY 14 x Burley 21)F-1), and (KY 14 x (KY 14 x Burley 21)F-1) x (Burley 21 x (KY 14 x Burley 21)F-1 ). Five-week-old seedlings were transplanted into soil infested with 1 00 chlamydospores of T. basicola per gram of soil mixture and grown in the greenhouse at an average air temperature of 21 degrees C. 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 genot ypes for each component of partial resistance measured. Significant po sitive correlations were observed between lesion number and disease se verity and between lesion number and lesion length. However, a signifi cant negative correlation was observed between lesion number and popul ation density. Selection for components of partial resistance in burle y tobacco should lead to increased levels of resistance to black root rot.