Cultivars and breeding lines submitted for the 2000 Uniform Variety Trials were inoculated with Agrobacterium tumefaciens isolate 25A, grown in the greenhouse, and observed for plant development, yield, seed quality, bronze wilt, root necrosis, and bacterial concentration in roots. Five plants of each of 128 entries were sacrificed just prior to initial bloom and 10 others were allowed to develop mature bolls. Of the 1280 mature plants, 181 (14%) had severe root necrosis, 125 (10%) had severe bronze wilt, and 20 (2%) showed severe root necrosis and bronze wilt. Among the 128 cultivars and lines, 60 showed severe root necrosis and 49 showed severe bronze wilt in 10% or more of the plants. Cultivars that had 30% or more of the plants with severe root necrosis or bronze wilt yielded 14% and 15% less seed cotton, respectively, than cultivars that showed neither symptom. Within cultivars, plants that had severe bronze wilt or root necrosis yielded 10% and 14% less, respectively, than plants with no or mild symptoms. Plants with both symptoms yielded 26% less than control plants. Losses from bronze wilt were entirely due to reductions in seed weight, which were accompanied by large increases in the percentage of light seed. In contrast, losses from root necrosis were entirely due to reduction of seed number per plant with no change in seed weight or percentage of light seed. Decreased root weight occurred with both symptoms. However, the greatest reduction in root weight occurred before flowering with root necrosis, and after fruit set with bronze wilt. Cultivars and individual plants with root necrosis consistently had greater concentrations of A. tumefaciens in roots than those without symptoms, whereas bacterial concentrations in roots of cultivars with bronze wilt were not different or slightly less than those in cultivars with no symptoms. The results are consistent with the hypothesis that root necrosis is truly a susceptible reaction to A. tumefaciens, whereas bronze wilt results from toxins produced in roots and transported to leaves during fruit development. Losses from root necrosis may be a major, unrecognized cause of yield stagnation in cotton, since A. tumefaciens strains similar to 25A now occur in all cotton seed lots.