SUMMARY: In the leaves of their natural (homologous) hosts, the bean and cherry respectively, and increased logarithmically for at least 4 days after inoculation. Transition into the stationary phase was gradual and accompanied by water congestion of the infected tissues, followed by typical field symptoms. Increases in the inoculum dose had relatively little or no effect on the generation time but growth ceased earlier at the higher doses. In the reciprocal (heterologous) combinations, logarithmic growth was abruptly terminated after 2-3 days, due apparently to a specific defensive reaction in the host. This coincided at the higher inoculum doses with the appearance of dry necrotic symptoms in the leaves. No macroscopic symptoms were observed with the lower doses, and with the lowest dose in bean there was an acceleration of leaf maturation in the presence of heterologous organisms. Generation times were lower in heterologous combinations but increased markedly with the inoculum dose. The growth of the pear strain of in bean and cherry leaves showed typical heterologous characteristics. The final yields of bacteria per unit inoculum were invariably higher in homologous combinations, but they decreased with increasing dose, whereas heterologous yields increased. The differences in net growth response were therefore greatest at the lowest doses. This suggested that host specificity in the field was associated with factors controlling growth of the organisms from small initial inocula.


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