Patterns of colonization and spread in the fungal spruce pathogen Onnia tomentosa

The basidiomycetous fungus Onnia tomentosa is one of the most widespread root rot pathogens in North America. Although the disease is more severe on spruce and pine trees, this pathogen can infect several coniferous species. In order to study the population structure of O. tomentosa, we harvested 180 basidiocarps in a 45-year-old white spruce plantation in western Quebec in autumn 1997, and extracted DNA directly from individual basidiocarps. Using a combination of spatial coordinates and molecular data based on the analysis of two mitochondrial and three nuclear loci, we measured the average genet size and molecular diversity and assessed the relative contribution of basidiospores and vegetative growth to the stand colonization. Most of the sampled basidiocarps that clustered spatially belonged to the same genet. A total of thirty-seven discrete multilocus genets of an average size of 3.42 m were obtained. The genet size distribution was skewed toward smaller genets (< 3 m) that di...