Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/6/2004
Publication Date: 8/1/2004
Citation: Pusey, P.L., Curry, E.A. 2004. Temperature and pomaceous flower age related to colonization by Erwinia amylovora and antagonists. Phytopathology. 94:901-911. Interpretive Summary: Fire blight, a serious bacterial disease of apple and pear, is initiated mainly in blossoms. Under dry western conditions, bacteria colonize flower stigmas, and subsequent rain or heavy dew facilitates movement to the floral cup where infection generally occurs. Understanding bacterial colonization on stigmas is critical to an accurate assessment of disease risk and to successful management of fire blight with beneficial bacteria. In this investigation, we worked with detached crab apple flowers in the laboratory and mature Gala apple trees enclosed in polyethylene. The maximum age of stigmas supporting bacterial multiplication was shown to decrease as temperature increased, and it was reduced by pollination. The study also revealed attributes and limitations of beneficial bacteria currently being exploited for fire blight management. In addition, experiments affirmed that the efficacy of particular beneficial bacteria in preemptively excluding the fire blight organism is relatively constant throughout all stages of stigma development and senescence.
Technical Abstract: Fire blight of apple and pear is initiated mainly by epiphytic populations of Erwinia amylovora on flower stigmas. Understanding bacterial colonization on stigmas is relevant to disease risk assessment and biological management. When detached crab apple flowers were subjected to various temperatures and incubation periods before inoculation, maximum age of stigmas supporting bacterial multiplication decreased as temperature increased, and was reduced slightly by pollination. This was confirmed with mature Gala apple trees enclosed in polyethylene and maintained at two different temperature ranges. Depending on these variables and bacterial strain, stigma conduciveness to bacterial growth lasted 3 to 12 days. For E. amylovora, this period corresponded to the interval when stigma papillae were bulbous in scanning electron micrographs. The study revealed attributes and limitations of antagonist strains currently being exploited for fire blight management. Antagonist Pseudomonas fluorescens strain A506 failed to multiply under certain conditions favorable for the pathogen, i.e when temperatures were relatively high or stigmas old. Other antagonists, Pantoea agglomerans strains C9-1 and E325, however, grew under such conditions. Dual inoculations affirmed that efficacy of these antagonists in preemptively excluding E. amylovora is relatively constant throughout stigma development and senescence stages suitable for pathogen growth.