Comparison of Resistance in Tobacco toPseudomonas syringaepv.tabaciRaces 0 and 1 by Infectivity Titrations and Bacterial Multiplication

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Abstract
The responses of tobaccos susceptible to Pseudomonas syringae pv. tabaci race 0 and 1, susceptible only to race 1, and susceptible to neither race were compared by infectivity titrations. When quantal responses were recorded, values of median effective dose and slope of the log-dose/logit-response curve were used to compare resistance. Host-race combinations that were compatible or incompatible in the field behaved similarly in infectivity titration experiments, except for line 8A2S4-9 (resistant to both races), which appeared more susceptible to race 1 than to race 0. The slopes of the response curves indicated that the bacterial cells apparently acted independently in vivo to cause infection, except for race 0 on 8A2S4-9. Lesion development and time to response in incompatible combinations were typical of the hypersensitive response. When bacterial multiplication was measured by infiltrating tobacco leaves with P. s. pv. tabaci and determining the subsequent changes in population by dilution planting, multiplication was higher in compatible than in incompatible combinations.