Evaluation of soybean cultivars for resistance to sclerotinia stalk rot in South Africa

Soybean cultivars were evaluated for resistance to sclerotinia stem rot from 2003/04 to 2005/06 in inoculated, sequentially planted field experiments. Ranking of cultivars according to disease incidence was not correlated over planting dates, seasons or localities, but differential responses of cultivars to the pathogen under different environmental conditions were evident. Nonlinear regression analysis quantified cultivar responses to the pathogen under changing environmental conditions. Regression parameters placed cultivars into three categories, i.e., those linearly related to disease potential, those highly susceptible even at low disease potentials and those with various degrees of resistance despite increasing disease potentials. Regression parameters were used to calculate the disease potential required to initiate disease and the subsequent rate of change in susceptibility with increasing disease potential. Area under the disease potential curve was used to quantify cultivar response to changing environments, but it could not differentiate between the time of disease onset and the rate of disease response to changing environment.