New Taxon-Specific Heterobasidion PCR Primers Detect and Differentiate North American Heterobasidion spp. in Various Substrates and Led to the Discovery of Heterobasidion irregulare in British Columbia, Canada.

Heterobasidion annosum sensu lato is a species complex of pathogenic white-rot wood decay fungi which cause root and butt rot in conifer and hardwood species across the Northern hemisphere. Annual losses to forest managers are valued in the billions of dollars, due to tree mortality, reduction in timber yield, and wood decay. In North America, H. irregulare and H. occidentale have a partially overlapping host and geographic range, cause similar disease symptoms and produce similar fruiting bodies, making discrimination between the two of them often difficult. We developed two sets of primers that bind specifically to conserved, but species-specific portions of glyceraldehyde 3-phosphate dehydrogenase and elongation factor 1α alleles. The method is sensitive enough to detect either species from infected wood. Analysis of North American isolates has further clarified the distribution of both species on this continent, including the detection of H. irregulare for the first time on ponderosa pine (Pinus ponderosa) and eastern white pine (Pinus strobus) in British Columbia. This method has the potential to be a valuable tool for the detection of the pathogen in exported/imported wood products, as well as for the further identification and assessment of the distribution of North American Heterobasidion species.

Characterization of Heterobasidion occidentale transcriptomes reveals candidate genes and DNA polymorphisms for virulence variations.

Characterization of genes involved in differentiation of pathogen species and isolates with variations of virulence traits provides valuable information to control tree diseases for meeting the challenges of sustainable forest health and phytosanitary trade issues. Lack of genetic knowledge and genomic resources hinders novel gene discovery, molecular mechanism studies and development of diagnostic tools in the management of forest pathogens. Here, we report on transcriptome profiling of Heterobasidion occidentale isolates with contrasting virulence levels. Comparative transcriptomic analysis identified orthologous groups exclusive to H. occidentale and its isolates, revealing biological processes involved in the differentiation of isolates. Further bioinformatics analyses identified an H. occidentale secretome, CYPome and other candidate effectors, from which genes with species- and isolate-specific expression were characterized. A large proportion of differentially expressed genes were revealed to have putative activities as cell wall modification enzymes and transcription factors, suggesting their potential roles in virulence and fungal pathogenesis. Next, large numbers of simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs) were detected, including more than 14 000 interisolate non-synonymous SNPs. These polymorphic loci and species/isolate-specific genes may contribute to virulence variations and provide ideal DNA markers for development of diagnostic tools and investigation of genetic diversity.