Inheritance of resistance in tomato to race T3 of the bacterial spot pathogen

Hawaii 7981 tomato (Lycopersicon esculentum Mill,), resistant to race T3 of the bacterial spot pathogen [Xanthomonas campestris pv. vesicatoria (Doidg e) Dye], was crossed to the susceptible tomato inbred, FIa, 7060, and subse quently F-2 and backcross seed were obtained. These generations were plante d in the field, inoculated with the race T3 pathogen and evaluated for dise ase severity over two summer seasons. Data were tested for goodness-of-fit to a model based on control by the incompletely dominant gene Xv3 that conf ers hypersensitivity. The F-1 was intermediate in disease severity to the p arents for both seasons. When data were combined over both seasons, the bac kcrosses fit the expected I:I ratios although each deviated from the expect ed ratio in one of the 2 years tested. The F-2 did not fit the expected 1:2 :1 ratio in either pear or when data from the two years were combined due t o a deficiency of resistant plants. Thirty-three F-2 plants representing an array of disease severities and hypersensitivity reactions were selected i n the second season and their F-3 progeny were inoculated and evaluated for disease severity. Hawaii 7981 was significantly more resistant than the 12 most resistant F-3 selections even though all expressed hypersensitivity. A hypersensitive F-3 With intermediate field resistance was crossed to Hawa ii 7981 and subsequently, F-2 and backcross generations were obtained. Thes e generations were field inoculated with the race T3 pathogen and evaluated for disease severity. Hawaii 7981 was significantly more resistant than th e F-3 parent as in the previous year. The data did not fit an additive-domi nance model and epistatic interactions were significant. Thus, it appears t hat field resistance to race T3 of bacterial spot found in Hawaii 7981 is c onferred quantitatively by Xv3 and other resistance genes. Breeding implica tions are discussed.