MECHANISMS OF INHIBITION OF ERWINIA-AMYLOVORA BY ERW-HERBICOLA IN-VITRO AND IN-VIVO

The mechanisms by which Erwinia herbicola inhibits Erwinia amylovora, the fire blight pathogen, were investigated. The optimum pH for growth of Erw. amylovora strain Ea273 in nutrient-yeast extract-glucose brot h (NYGB) was 7.0 and growth was markedly reduced at pH values below 6. 0. In contrast, the growth rates of Erw. herbicola strains Eh141 and E h112Y were only slightly reduced at pH levels as low as 4.5, relative to pH 6-8. When Ea273 was grown in NYGB in the presence of Eh141 or Eh 112Y, the media became acidic and lower populations of Ea273 were reco vered, compared with populations from buffered NYGB. Acidification of plant tissue as a consequence of growth of Erw. herbicola did not occu r, however, and thus acid-based inhibition of growth in planta is unli kely. The growth rates of nine strains of Erw. herbicola and their abi lities to reduce the pH of NYGB did not correlate with their different abilities to prevent development of fire blight incited by Ea273 in a research apple orchard. When grown in mixed culture, Eh141 and Eh112Y grew to higher populations than Ea273 due to depletion of a nitrogen source needed by Ea273. The ability of 12 strains of Erw. herbicola to produce antibiotics inhibitory to Ea273 on a glucose-asparagine mediu m correlated with the effectiveness of the strains in suppressing fire blight. A crude preparation of the Eh318 antibiotic delayed developme nt of disease in immature pear fruits incited by Ea273 but not by stra in Ea273R318, which is resistant in vitro to the Eh318 antibiotic. Cel ls of Eh318 protected immature pear fruits more effectively from infec tion by Ea273 than from the resistant strain Ea273R318.