Growth Kinetics and Interactions ofPseudomonas syringaewith Susceptible and Resistant Bean Tissues

Abstract

Differences were found in symptom expression and in multiplication rates of P. syringae isolate Y-30 in leaves and pods of bean plants susceptible (cultivar Tenderwhite) and resistant (WBR 133) to bacterial brown spot of bean. Symptom expression in the 2 hosts was different at all inoculum concentrations tested, but there were almost no differences in bacterial growth rates and final bacterial populations in the 2 hosts at high inoculum levels. In pods that received low inoculum levels, P. syringae multiplied more slowly in ''WBR 133'' than in ''Tenderwhite.'' In leaves, rates of P. syringae multiplication were the same during the exponential growth phase (0-12 h after inoculation), but were much slower in ''WBR 133'' than in ''Tenderwhite'' during the transition stage between exponential growth and the stationary phase. Doubling times of P. syringae in ''Tenderwhite'' were not affected by inoculum concentration. Infiltration of ''Tenderwhite'' leaves and pods with the incompatible pathogen P. coronafaciens resulted in rapid necrosis of the inoculated area, a symptom characteristic of a hypersensitive response. Cessation of growth of P. coronafaciens was not correlated with the development of visible necrosis. In leaves, P. coronafaciens showed an 8-12 h lag phase before starting to multiply. Doubling times of P. coronafaciens in both leaves and pods were longer than those of P. syringae in leaves and pods of either host, unless high inoculum concentrations were used. Doubling times of P. coronafaciens decreased with increasing inoculum concentration. EM of pod tissue in the 3 host-pathogen combinations showed changes associated with an apparent defense reaction by the host in the incompatible interactions. A fibrillar material, which apparently arose from the host cell wall, enveloped bacteria in the intercellular spaces. This defense reaction did not appear effective, and, within 8 h after inoculation, bacteria were multiplying and filling the intercellular spaces. In leaves, envelopment of bacteria by fibrillar material was seen only rarely and could be found in all 3 host-pathogen combinations.