Epiphytic colonization of dry edible bean by bacteria antagonistic to Sclerotinia sclerotiorum and potential for biological control of white mold disease

Abstract

Abstract Bacteria isolated from blossoms of dry edible bean (Phaseolus vulgaris) were evaluated for their potential as biological control agents against white mold disease of dry bean, caused by Sclerotinia sclerotiorum. Of the 10 isolates from blossoms tested, Erwinia herbicola strain B1 and Bacillus polymyxa strain B8 inhibited mycelial growth of S. sclerotiorum on agar media. When applied to detached blossoms, both strains reduced the germination of ascospores of S. sclerotiorum to approximately 40% compared to 69% in the control and inhibited the growth of germ tubes. In a growth chamber efficacy test, pretreatment of bean blossoms with either bacterial strain prevented the colonization of blossoms by the pathogen and the subsequent development of white mold lesions on the stems. The bacteria were applied to bean foliage in three field experiments to examine their population dynamics in the bean canopy and the ability to control white mold disease. E. herbicola B1 survived on inoculated bean leaves for 4 weeks after a single application at mean population levels exceeding log 4 colony-forming units (CFU) per leaf. Over 70% of the blossoms sampled from B1-treated plants during the same period were colonized by bacteria with the colony morphology of strain B 1, with mean populations exceeding log 4 CFU/blossom. Strain B1 significantly reduced the severity of white mold in one experiment in which it was applied three times and when the disease pressure was relatively low, as evident from the disease severity in the nontreated control (25%). However, B1 had no effect in two experiments in which there was a greater severity of white mold in the controls (>44%) and the bacterium was applied once. As there was little difference between single applications and three weekly applications in populations of B 1-type bacteria detected, efficacy may have been affected to a greater extent by the amounts of white mold disease than by plant colonization by the bacteria. In contrast, B. polymyxa strain B8 survived on leaves and was not detected on treated blossoms 1 week after application. As a consequence, it had no effect on white mold severity in any of the field experiments. Hence, the antagonist strains, while effective in inhibiting pathogen mycelial growth in vitro and colonization of blossoms in growth chamber tests, had limited efficacy in the field, particularly when disease pressure was moderate to high.