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 (Doidge) Dye], was crossed to the susceptible tomato inbred, FIa, 7060, and subsequently F-2 and backcross seed were obtained. These generations were planted in the field, inoculated with the race T3 pathogen and evaluated for disease 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 confers hypersensitivity. The F-1 was intermediate in disease severity to the parents for both seasons. When data were combined over both seasons, the backcrosses fit the expected I:I ratios although each deviated from the expected 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 to a deficiency of resistant plants. Thirty-three F-2 plants representing an array of disease severities and hypersensitivity reactions were selected in 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 Hawaii 7981 and subsequently, F-2 and backcross generations were obtained. These generations were field inoculated with the race T3 pathogen and evaluated for disease severity. Hawaii 7981 was significantly more resistant than the F-3 parent as in the previous year. The data did not fit an additive-dominance model and epistatic interactions were significant. Thus, it appears that field resistance to race T3 of bacterial spot found in Hawaii 7981 is conferred quantitatively by Xv3 and other resistance genes. Breeding implications are discussed.