Integrated management of edaphic and biotic factors limiting yield of irrigated soybean and dry bean in Minnesota

Root rot of dry bean (Phaseolus vulgaris L.) caused by Fusarium solani f. sp. phaseoli in a complex with Rhizoctonia solani and Fusarium oxysporum is an increasingly important problem for dry bean and soybean producers in central Minnesota. Shortened rotation intervals, reduced tillage, use of susceptible cultivars, and soil compaction are all factors that may be contributing to a more favorable environment for the development of root rots caused by these pathogens. A Bt horizon, characteristic of sandy soils in central Minnesota is exacerbated by reduced tillage, impairs root development and contributes to plant stress. The objective of this research was to determine if reduced tillage or moldboard plowing and seed treatment with a peat formulation of Bacillus subtilis and either Rhizobium leguminosarum (dry bean) or Bradyrhizobium japonicum (soybean) would reduce root rot severity and allow increased yield of dry bean and soybean (Glycine max L.). The study consisted of experiments planted at Staples in 2000, and Staples and Verndale in 2001. Dry bean and soybean were planted in adjacent experiments arranged in a split-plot design with tillage as the main plot and seed treatment as the subplot. The soil type at both sites is a Verndale sandy loam. The impedance layer was located at a shallower depth at Staples than at Verndale. Mean cone index values in reduced tillage plots exceeded 2500Pa from 60 to 300mm in the soil profile at Staples in 2000, and from 105-300 to 165-300mm at Staples and at Verndale, respectively in 2001. Moldboard plowing reduced cone index to less than 2000Pa in the upper 300mm of the soil profile at Staples in 2000 and 2001 and the upper 285mm of the soil profile at Verndale in 2001. Gravimetric soil moisture at 15-22cm depth was 4 and 6% less in moldboard plowing than reduced tillage in 2000 and 2001, respectively. Dry bean yield was greater in moldboard plowing than reduced tillage at Staples in 2000 and 2001 (1751 vs. 1148 and 653 vs. 495kgha^-1, respectively) and at Verndale in 2001 (2003 vs. 1395kgha^-1, respectively). Seed inoculation with R. leguminosarum and B. subtilis increased dry bean yield by 11% at Staples in 2000 and 16% in 2001. Soybean yields were not increased significantly by moldboard plowing. At Verndale in 2001, seed inoculation with B. subtilis and B. japonicum increased soybean yields from 2203 to 3021kgha^-1. Seed treatment did not produce a consistent effect on disease severity in dry bean at Staples in 2000 or at Verndale in 2001. However, seed treatment decreased disease severity in dry bean at Staples in 2001 and in soybean at Staples and Verndale in 2001. The effect of the treatment factors, inoculation with B. subtilis and Rhizobium spp. and tillage, differed among crops, sites, and environments. A combination of management practices such as deep tillage, and seed inoculation with B. subtilis and Rhizobium spp. will be necessary to restore and maintain optimum dry bean and soybean productivity.