INTERACTION BETWEEN ERWINIA AMYLOVORA AND ERWINIA HERBICOLA IN VITRO, IN IMMATURE PEAR FRUITS AND IN APPLE BLOSSOMS
Studies in vitro were done by incubating equal numbers of cells of each strain together in broth with agitation. During the initial period of growth in weakly buffered broth, both species grew at similar rates. Later, E. herbicola populations increased and then stabilized while the rate of growth of E. amylovora decreased and death occurred. The change in E. amylovora populations appeared to be correlated with decreases in the pH of the growth medium. At pH values of 4.2 or less, E. amylovora failed to grow and death of cells was evident. In contrast, E. herbicola populations remained stable over the pH range of 4.2 to 3.4. In more strongly buffered medium, both species attained higher populations than in weakly buffered media. However, the stationary-phase population of E. herbicola was about 5-fold greater than the population of E. amylovora. Addition of organic nitrogen to neutralized stationary-phase culture filtrates from dual cultures resulted in renewed growth of E. amylovora. Apparently, the ability of E. herbicola to utilize inorganic nitrogen, which E. amylovora cannot use, enabled it to attain higher populations than E. amylovora. Thus, in culture the inhibition of E. amylovora by E. herbicola appears to be caused by acid production or depletion of organic nitrogen.
When immature pear fruits (2–3 cm in diameter) were treated with E. herbicola and then inoculated with E. amylovora, disease development was delayed or suppressed in direct proportion to the dose of E. herbicola used. Strains of E. herbicola differed in their effectiveness but attained similar populations in treated pears. While E. amylovora colonized