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ABSTRACT:White mold caused by the fungus Sclerotinia sclerotiorum is an important disease in relation to soybean. The use of less susceptible genotypes can be a productive strategy in the management of this disease, and the development of an appropriate methodology for soybean inoculation is useful for the differentiation of disease-resistant genotypes. The present study aimed to assess the susceptibility of 77 soybean genotypes based on their reaction to oxalic acid, as well as to determine correlations between three traditional disease assay methods (detached leaf, non-wounded stem and straw tests) and the results of the oxalic acid assay. Oxalic acid susceptibility was assessed by using a wilting score scale. For the other methods, the severity of disease symptoms was assessed. To compare methodologies, the values obtained for the genotypes using each method were categorized into classes, and a severity index was used to represent individuals within each class. All the methods used were efficient for the differentiation of soybean genotypes in terms of susceptibility to S. sclerotiorum; however, the behavior of the genotypes depended on the inoculation method adopted. Even though no significant relationship was identified between the severities of the damage resulting from the methodologies, the rankings acquired from the methods strongly agreed. The oxalic acid method was the most rapid, the least laborious, and was the cheapest compared with the other methods that were used.

Caused by Sclerotinia sclerotiorum, white mold is an important seed-transmitted disease of soybean (Glycine max). Incubation-based methods available for the detection and quantification of seed-borne inoculum such as the blotter test, paper roll and Neon-S assay are time-consuming, laborious, and not always sensitive. In this study, we developed and evaluated a molecular assay for the detection of S. sclerotiorum in soybean seeds using a species-specific PCR (polymerase chain reaction) primer set and seed soaking (without DNA extraction) for up to 72 h. The PCR products were amplified in all the samples infected with the pathogen, but not in the other samples of plant material or the other seed-borne fungi DNA.The minimum amount of DNA detected was 10 pg, or one artificially infested seed in a 400-seed sample (0.25 % fungal incidence) and one naturally infected seed in a 300-seed sample (0.33 % incidence). The PCR-based assay was rapid (< 9 h), did not require DNA extraction and was very sensitive.