Bacterial wilt caused by Ralstonia solanacearum is an extremely damaging disease of flue-cured tobacco and can be spread mechanically on farm machinery. Tobacco roots contain naturally occurring resistance genes to the tomato strain of R. solanacearum. Tomato strains of R. solanacearum contain an avirulence gene avrA, which elicits a defense reaction in tobacco. Pathogenic tobacco strains of R. solanacearum contain the avrA gene, which has been mutated by a random insertion, blocking gene function and subsequent host recognition. The present study evaluated the ability of tomato strains of R. solanacearum with a functional avrA gene to turn on the resistance gene in tobacco stem tissue and block infection by a pathogenic tobacco strain of R. solanacearum. Pathogenic strains of tobacco isolate SC10 and NC 132 (2009) and SC 10, SC 06, and Y3 (2010) were applied (1 x 10⁶ cfu) in combination with tomato strains SC11 or SC12 (2009) and SC 11, SC 12, SC 14, or AW1 (2010), at 1 x 10⁸ cfu, in all possible combinations to tobacco stems during flower removal. The tomato strains elicited a defense reaction in tobacco stem tissue and reduced stem necrosis averaged over strains from 4.5 to 1.6 and from 4.4 to 2.1 (2009, 2010, P < 0.001), respectively, on a 0-5 scale, effectively blocking disease development. Stem necrosis could be observed following inoculation with a pathogenic tomato strain, but was limited to tissue adjacent to the inoculation point. (Reprinted with permission)