INFLUENCES OF ANTAGONIST POPULATION-LEVELS, BLOSSOM DEVELOPMENT STAGE, AND CANOPY TEMPERATURE ON THE INHIBITION OF SCLEROTINIA-SCLEROTIORUMON DRY EDIBLE BEAN BY ERWINIA-HERBICOLA

Three strains of Erwinia herbicola, applied to blossoms of dry edible bean (Phaseolus vulgaris) prior to inoculation with ascospores of Scle rotinia sclerotiorum, inhibited ascospore germination and subsequent d evelopment of white mold lesions in a bioassay. Although the strains e xhibited similar multiplication rates on blossoms, increasing from ini tial levels of <10(2) cfu per blossom to stationary-phase populations of >10(7) cfu per blossom within 16 h at 25 C, they were effective in inhibiting S. sclerotiorum after different periods of multiplication a nd at different population levels. Strain B1 required incubation for a t least 24 h, whereas strains B346 and B367 were inhibitory after only 6 h of incubation when population levels were <10(5) cfu per blossom. Multiplication of air strains was restricted to bean blossoms at the fully expanded, mature stage, which lasts for 1 day under field condit ions. The strains differed in the duration of protection they provided to individual blossoms from colonization by S. sclerotiorum in the fi eld. Following application of B346 and B367 to closed buds, antagonism against the pathogen first occurred when blossoms became fully expand ed and continued as blossoms senesced and finally deteriorated. Pathog en inhibition by strain B1 did not occur, however, until blossoms had begun to senesce. In experiments conducted in western Nebraska, none o f the strains was effective in reducing white mold disease severity by the end of the season; this was attributed to blossoms harboring insu fficient population levels of applied bacteria. Multiplication and ant agonism by E. herbicola strains on blossoms in the laboratory were hig hest at 28-30 C and were greatly reduced at 20 C. Leaf temperatures me asured within the canopy of dry edible bean were unfavorable to E. her bicola growth (<20 C) an average of more than 16 h per day and favorab le to E. herbicola growth (>25 C) for less than 6 h per day. Therefore , low temperatures within the canopy may have limited the ability of E . herbicola strains to multiply while blossoms were in the mature, ful ly expanded stage and thereby reduced the potential for protecting ind ividual blossoms from infection by S. sclerotiorum.