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Sclerotinia stem rot of soybean: the South African approach

Neal McLaren: University of the Free State

<div>Average incidences of Sclerotinia stem rot for the period 2006-2015 exceeded 20% in key South African production areas. AFLP analysis indicated a relatively uniform geographic distribution of isolates, with evidence of genetic diversity. The significance of the genetic diversity in host response and locality interactions still requires elucidation. Disease responses in multi-location germplasm trials indicated highly significant genotype x environment interactions. Nonlinear regression analysis was used to quantify genotype responses and their stability to changing disease potentials. Genotypes with varying levels of tolerance to disease pressure were identified which could reduce the risk of crop losses. Regression parameters indicated disease potential required to initiate disease and subsequent rates of disease development. Ten year weather data were bulked into periodic running means and multivariate analyses were used to estimate critical growth stages and the length of infection and colonization periods. Analyses identified 21 day periods associated with R3-R5 as critical growth stages and stepwise variable selection identified rainfall, minimum humidity and minimum temperature as key driving variables. An intervention threshold model which integrates genetic resistance and weather variables is being developed with the aim of optimizing fungicide applications.</div>