Integrated management strategies of bean root rot with Bacillus subtilis and Rhizobium in Minnesota

Abstract

Dry bean root rot, caused by Fusarium solani f. sp. phaseoli, in association with Rhizoctonia solani and Fusarium oxysporum is an increasing problem in Minnesota. A standard seed treatment (SST) ( Captan 400+ Streptomycin+ Lorsban 50) has failed to effectively control the disease and resistant cultivars are unknown. The objectives of this study were to determine the effect of the biocontrol agents Bacillus subtilis strain GBO3 (Kodiak), MBI600 (Epic) and Trichoderma harzianum (T-22) on the incidence and severity of bean root rot; to compare conventional and granular formulations of B. subtilis, and to determine effectiveness of root rot control with co-inoculations of B. subtilis and Rhizobium. Greenhouse studies evaluated the effect of B. subtilis GBO3 and T. harzianum T-22 alone or in combination with Captan 400 and Vitavax 200 as biocontrol treatments in contrast to the SST. Seed application of both B. subtilis GBO3 and T. harzianum T-22 ( P>0.05) increased plant biomass and decreased disease severity (DS) under greenhouse conditions. Field experiments were conducted in 1997, 1998 and 1999 in naturally root rot infested fields at Staples and Verndale, MN. In 1997, seed treated with B. subtilis GBO3 reduced bean root rot and increased yield 31%, when compared to untreated plants. Bean seed inoculated with Rhizobium tropici UMR 1899 also increased yield 13% over the untreated control, though neither treatment eliminated disease. In 1998 in a grower’s field with favorable conditions for disease, three of the four treatments that included B. subtilis GBO3 outyielded the untreated control, and SST by 300–800 kg/ha, but in only one treatment were these results statistically significant ( P<0.05). In 1999, under severe disease pressure only co-inoculation with B. subtilis MBI600 (Epic) and R. tropici significantly reduced DS and enhanced yield relative to the control and SST treatments. Application of B. subtilis with Rhizobium is a promising approach for the improvement of bean root rot control.