Clonal Spread of <i>Sclerotium cepivorum</i> in Onion Production with Evidence of Past Recombination Events

Three field populations of Sclerotium cepivorum from Ontario and Que-bec in Canada, one field sample from the North Island of New Zealand, as well as isolates from other onion- and garlic-growing areas were classified in mycelial compatibility groups (MCGs) and further characterized by DNA fingerprints and DNA sequence polymorphisms at six genomic regions. MCGs in the population samples were genetically distinct clones. Members of a clone belonged to a unique MCG with an associated multi-locus genotype and DNA fingerprint. The structure in each field population was predominantly clonal based on the repeated sampling of genotypes over a wide geographic area. Only three clones were identified within each of the population samples. The hypothesis that these clones were introduced to the three agricultural production areas is supported by their recovery from additional locations. Some evidence for past recombination was detected when genotypes from the three population samples and from other locations were pooled.