ROLE OF FLAGELLAR MOTILITY IN APPLE BLOSSOM INVASION AND TACTIC RESPONSE TO VARIOUS PLANT NECTAR EXTRACTS BY ERWINIA AMYLOVORA

A nonmotile isolate (Nm22) and its motile revertant (MR22) of Erwinia amylovora were obtained after treating cells of the wild type (Isolate 110) with 0.3 M ethyl methanesulfonate. The pathogenicity of both isolates was similar with the wild type when inoculated to young shoots of apple seedlings. Significantly higher infection was recorded in blossoms inoculated with MR22 than with Nm22 isolate at inoculum concentrations of 5 x 10('5) and 1 x 10('7) cells per ml. The chemotactic response of E. amylovora to nectar extracts from 24 plant species with varying degrees of resistance or susceptibility to the pathogen was determined by capillary assay. Plants that were immune to E. amylovora infection had nectar extracts that were weakly attractive while susceptible plants had nectar extracts that were strongly attractive to the pathogen. Nectar extracts were fractionated using anion and cation exchange resins and the organic acid fraction was silylated and chromatographed on 3% SE-52 column. The organic acids attracting E. amylovora in nectar extracts appear to be either fumaric or malic acid or a mixture of both. Concentrations of either acid varied in different nectar extracts. The possibility of using attractant and repellent compounds to minimize apple blossom infection was explored. Negative chemotaxis of E. amylovora for benzoate and salicylate was constitutive and appears to be inducible for L-isoleucine, L-leucine and L-phenylalanine. Benzoate and salicylate were used as repellents while malate and tartarate were used as attractants in field trials. None of these compounds applied to apple blossoms as 1 or 10 nM solutions provided consistent protection from fire blight infection at an inoculum concentration of approximately 5 x 10('5) cells per ml. A 0.1 M solution of either benzoate and salicylate, when sprayed to apple blossoms, was highly phytotoxic.