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Influences of Antagonist Population Levels, Blossom Development Stage, and Canopy Temperature on the Inhibition of Sclerotinia sclerotiorum on Dry Edible Bean by Erwinia herbicola. G. Y. Yuen, Department of Plant Pathology, University of Nebraska, Lincoln 68583; M. L. Craig, E. D. Kerr, and J. R. Steadman. Department of Plant Pathology, University of Nebraska, Lincoln 68583. Phytopathology 84:495-501. Accepted for publication 15 February 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-495.

Three strains of Erwinia herbicola, applied to blossoms of dry edible bean (Phaseolus vulgaris) prior to inoculation with ascospores of Sclerotinia sclerotiorum, inhibited ascospore germination and subsequent development of white mold lesions in a bioassay. Although the strains exhibited similar multiplication rates on blossoms, increasing from initial levels of <102 cfu per blossom to stationary-phase populations of >107 cfu per blossom within 16 h at 25 C, they were effective in inhibiting S. sclerotiorum after different periods of multiplication and at different population levels. Strain B1 required incubation for at least 24 h, whereas strains B346 and B367 were inhibitory after only 6 h of incubation when population levels were <105 cfu per blossom. Multiplication of all strains was restricted to bean blossoms at the fully expanded, mature stage, which lasts for 1 day under field conditions. The strains differed in the duration of protection they provided to individual blossoms from colonization by S. sclerotiorum in the field. Following application of B346 and B367 to closed buds, antagonism against the pathogen first occurred when blossoms became fully expanded and continued as blossoms senesced and finally deteriorated. Pathogen inhibition by strain B1 did not occur, however, until blossoms had begun to senesce. In experiments conducted in western Nebraska, none of the strains was effective in reducing white mold disease severity by the end of the season; this was attributed to blossoms harboring insufficient population levels of applied bacteria. Multiplication and antagonism by E. herbicola strains on blossoms in the laboratory were highest at 2830 C and were greatly reduced at 20 C. Leaf temperatures measured within the canopy of dry edible bean were unfavorable to E. herbicola growth (<20 C) an average of more than 16 h per day and favorable 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, fully expanded stage and thereby reduced the potential for protecting individual blossoms from infection by S. sclerotiorum.