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A Hypersensitive Reaction Induced in Tobacco Leaves by a Compatible (Race 1) Isolate of Pseudomonas solanacearum. G. A. Granada, Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; Luis Sequeira, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 65:731-733. DOI: 10.1094/Phyto-65-731.

A hypersensitive response (HR) is obtained when tobacco leaves are infiltrated with suspensions of incompatible (race 2) or avirulent strains of Pseudomonas solanacearum. In contrast, most compatible (race 1) strains induce a necrotic lesion that develops slowly, allowing spread of bacteria to adjoining tissues. We now report that certain race 1 isolates can cause a typical HR when infiltrated at approximately 10⁸ cells/ml into leaves of the tobacco cultivar Cuba 12. Although a low level of pathogenicity on tobacco was correlated with induction of the HR by most of these race 1 isolates, one (S-123) was highly pathogenic. This isolate multiplied rapidly in tobacco leaves until the population reached 2 × 10⁷ cells/1 cm² leaf disk; at this point, the HR was induced and populations declined precipitously thereafter. A comparison of the physiological characteristics (such as growth and acid production with different carbohydrate substrates, production of polygalacturonase, etc.) of isolates capable or incapable of inducing the HR did not reveal any specific features that could be associated with this property.