Interrelationship of Temperature, Flower Development and Biological Control of Fire Blight

BlightBan (a.i., Pseudomonas fluorescens strain A506) has been available in recent years and other biological agents (e.g., Pantoea agglomerans strains C9-1 and E325) are being developed for fire blight control. Advances will partly depend on an understanding of interrelationships involving environment, flower development and senescence, and microorganisms. Laboratory experiments were performed with detached blossoms of crab apple (Malus sp. 'Manchurian'). When inoculated stigmas were held at various temperatures, strain A506 failed to grow at the upper temperature range of Erwinia amylovora, but strains C9-1 and E325 had ranges extending beyond that of the pathogen. When flowers were subject to various temperatures and incubation periods before inoculation, the duration of stigma conduciveness to bacterial multiplication decreased as temperature increased, and it was shorter when flowers were pollinated versus non-pollinated. These interactions were confirmed with mature apple trees (Malus x domestica Borkh.'Gala') surrounded by plastic enclosures with heating and cooling to maintain different temperature ranges. In trials in 2000 and 2001, temperatures during bloom averaged 13.4° and 14.1°C at the low range and 21.5° and 21.9°C at the high range, respectively. The longest period stigmas retained the capacity to support bacterial growth when inoculated was about 14 days, occurring with non-pollinated flowers at low temperatures. The shortest period was 3 or 4 days, occurring with pollinated flowers at high temperatures. Results were similar with different bacteria. However, strain A506 failed to multiply at late stages of stigma senescence, which did allow growth of the pathogen and strain E325. On crab apple flowers, antagonist strains differed in their capacity to reduce pathogen populations and these differences were consistent during the period when stigmas were conducive to pathogen growth.