An optimized method for mycelial compatibility testing in Sclerotinia sclerotiorum

Classification of isolates into mycelial compatibility groups (MCGs) is used routinely in many laboratories as a quick marker for genotyping Sclerotinia sclerotiorum within populations. Scoring each new sample requires optimization of standardized conditions to support adequate growth of all paired isolates. Appropriate conditions for growth are especially important because diverse compatibility reactions are difficult to categorize and score (e.g., in samples from populations with high genetic diversity, such as those that receive immigration from genetically diverse sources or those that deviate from strict clonality). The current standard medium for MCG testing can be inhibitory to isolates from some samples, confounding scoring of compatibility. We identified two foci for optimization: (i) choice of medium, in this experiment, Patterson's medium amended with red food coloring (termed modified Patterson's medium, MPM, the current standard medium) versus potato dextrose agar (PDA) and (ii) amount of McCormick's red food coloring amended to the growth medium. The red food coloring often yields a red reaction line in incompatible interactions; alternative incompatible reactions are a line of thick or thin hyphae. Based on results to date, self-self pairings of S. sclerotiorum are compatible and are a reliable standard for scoring compatible self-nonself mycelial interactions. PDA amended with 75 microl/L of McCormick's red food coloring was identified as optimal for isolates inhibited by MPM from a highly diverse, recombining population sample. This precisely amended PDA was also suitable for isolates from highly clonal populations that were not inhibited by MPM or by higher concentrations of red food coloring. Under the optimized, standardized conditions all paired isolates grew together and produced interactions that could be scored in repeatedly identifiable categories, compatible or incompatible. Workers are advised to optimize conditions before screening a new population sample.