2019 ASHS Annual Conference

Snap beans Phaseolus vulgaris L. often referred to as ‘French’ beans, ‘string’ beans or ‘green’ beans is the most cultivated vegetable in home gardens and the third most valuable vegetable in the US following tomatoes and sweet corn. The majority of snap bean production across United States occurs in the southern states (Florida, Georgia and Tennessee). In Tennessee, snap beans ranked second in economic importance with a value of $18,368,000 in 2015. However, root rot pathogens including Pythium, Rhizoctonia, Fusarium, Phytophthora, Sclerotium rolfsii and Macrophomina phaseolina are among major constraints in snap bean production. Although fungicides provide some protection against most root rot pathogens, fungicides are detrimental to the environment and non-target beneficial microorganisms; repeated use has also been linked to fungicide resistance leaving limited options for growers. The objective of this study was to evaluate the ability of previously selected microorganisms as biological control agents (BCAs) against M. phaseolina root rot in snap beans. Endophytic bacteria that colonize plants internally are likely to be effective in field environment al where fluctuationsin temperature, moisture and UV light affect the efficacy of biological control agents. Six bacteria were evaluated in-vitro using dual culture technique and in vivo in greenhouse and two were evaluated in field environment. The BCAs inhibited pathogen colony growth by 43-75%. Seed treatment with the BCAs showed IMC8 and Y (B. thuringiensis), Prt (B. subtilis, Psl (B. amyloliquefaciens), Ps (B. vallismortis) were more effective in suppressing M. phaseolina than the fungicide tetramethylthiuram disulfide (thiram) in greenhouse studies. Only two BCAs were tested in field environment and both Enterobacter sp. and B. subtilis were as effective as the fungicide in reducing disease incidence and improving seed germination in field experiments.