ABSTRACT
Soybean Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, has recently emerged from being a minor problem in areas where soybeans of maturity groups 0 to I are grown to a significant cause of soybean yield losses in the north-central region, which produces 80% of soybean in the United States. Studies were conducted in Iowa to quantify varietal response to S. sclerotiorum for cultivars of maturity groups I to III in fields that had uniform infestation histories. Over the course of the study, disease incidence was generally high at the northern Iowa sites but low in central Iowa, with disease incidence of susceptible standards >60% and <30%, respectively. Regression analysis showed that maturity class significantly affected disease incidence, with greater effects in environments where susceptible standard cultivars had high disease incidences. Consistency of varietal response among the environments was quantified using Pearson correlation analysis. When disease incidence was high, varietal responses measured by disease ratings and yield were consistent among locations, but the responses were inconsistent when disease incidence was low. Pearson correlation coefficients ranged from 0.80 to 0.94 for disease incidence and 0.58 to 0.81 for yield among the experiments having high disease incidence in susceptible standards. The relationship between disease incidence and yield was well described by linear regression models with coefficients of determination (r2) ranging from 0.59 to 0.83. Based on regression slopes (significant at P < 0.0001), yield losses are estimated to range from 170 to 335 kg/ha for each 10 percentage points of disease incidence. Regression analysis also showed that maturity groups had a linear relationship with disease incidence (r2 = 0.18 to 0.39, P < 0.01).
