Environmental and biological factors affecting resistance and susceptibility to Sclerotinia sclerotiorum in soybean

Sclerotinia stem rot (SSR), caused by the oxalic acid producing fungus Sclerotinia sclerotiorum (Ss), is an economically important disease of soybean in the north central United States and other temperate regions throughout the world. The occurrence and severity of SSR in the field is highly dependant upon prevailing weather conditions, which can prove problematic when evaluating soybean accessions for resistance. Interaction phenotypes observed in controlled environment experiments often do not correspond to those observed in the field. Soybean accessions resistant and susceptible to SSR were selected for study. These accessions were grown in a field SSR disease nursery in 2004 and 2006 and were also used in a series of experiments in which (1) the relationship between preformed soybean stem lignin concentration and resistance to Ss was examined, (2) the role of light in the soybean-Ss interaction was studied and (3) oxalic acid was used to in an attempt to indirectly screen for SSR resistance. For the first experiment in which soybean stem lignin concentration was quantified, accessions differed in lignin concentration at the early pod growth stage (P > 0.0001 to 0.0993). Positively ranked relationships were observed between accession SSR disease severity and lignin concentration for each plant part in both growing years. Accession lignin concentration may provide information about relative susceptibility or resistance to Ss in years not conducive to disease. For the second experiment in which the relationship between light intensity and the soybean-Ss interaction was observed, light intensity affected the ability to both distinguish accessions and relate field and controlled environment interactions. Controlled environments with 337 mol m-2 s-1 of photosynthetic active radiation were most predictive of field interactions (rs = 0.83 to 0.94; P = 0.05 to < 0.005). Light intensity to which plants are exposed in controlled environments should be controlled to obtain results that relate to field performance and to identify SSR resistance. For the third experiment in which detached soybean leaves were incubated in oxalic acid, resistant and susceptible accessions were distinguished and accession ranks related to disease severity observed in the field for two of four experiments. Oxalic acid is not an appropriate method to evaluate soybean for response to Ss.