The occurrence and frequency of outcrossing in homothallic fungal species in nature is an unresolved question. Here we report detection of frequent outcrossing in the homothallic fungus Sclerotinia sclerotiorum. In using multilocus linkage disequilibrium (LD) to infer recombination among microsatellite alleles, high mutation rates confound the estimates of recombination. To distinguish high mutation rates from recombination to infer outcrossing, 8 population samples comprising 268 S. sclerotiorum isolates from widely distributed agricultural fields were genotyped for...