FIRE BLIGHT PROTECTION WITH AVIRULENT MUTANTS OF ERWINIA-AMYLOVORA

Fire blight is a necrotic disease caused by the bacterium Erwinia amyl ovora, which affects pears, apples and ornamentals including Crataegus , Pyracantha, and Cotoneaster. The disease can be only partially contr olled, through the use of resistant genotypes, cultural measures and a ntibacterial compounds, thus other methods must be investigated. It ha s long been established that avirulent isolates of the pathogen can co ntrol the disease, under experimental conditions. However, field use o f avirulent isolates is not acceptable because 3 of their unknown gene tic stability. The protective ability under controlled conditions of g enetically characterized avirulent insertion mutants of E. amylovora w as examined. A bioassay on apple seedlings was used for the determinat ion of the protective ability of 34 insertion mutants (hrp, dsp, ams). Some protective effect could be observed with most of the mutants tes ted and was dependent on the avirulent/virulent inoculum ratio as well as on the level of virulence of the pathogen; a minimal concentration of the avirulent mutant was necessary to give a significant level of protection. An early competition between avirulent and virulent strain s for putative infection sites might be involved. For six of the mutan ts tested, the protective ability was particularly high and might be r elated to the alteration of regulatory functions of hrp genes. Results obtained with Ams(-) and Ams(-) Hrp(-) mutants suggested that the bac terial exopolysaccharide might play a role in the protection.