Commercial Sphagnum peat moss is a vector for exotic ectomycorrhizal mushrooms

Sphagnum peat moss is one of the most commonly used substrates for forest plant and houseplant production. It is extracted from peat bogs in the circumboreal region and exported worldwide. Commercial peat moss is pasteurized, and is therefore believed to be free of viable ectomycorrhizal propagules. We used a bioassay with Pinus montezumae to demonstrate that commercial peat moss carries viable ectomycorrhizal spores, able to form mycorrhizae. Ectomycorrhizal fungi on seedling root-tips were sequenced for phylogenetic analyses using the ITS rDNA barcode region. We found three species: Suillus brevipes, Sphaerosporella brunnea, and Thelephora terrestris. S. brevipes and T. terrestris were found as viable inoculum transported in the peat moss, while S. brunnea was a greenhouse contaminant. S. brevipes and T. terrestris have biological characteristics (such as heat resistant and long living spores) that facilitate their survival to the extraction, transport, and storage processes of peat moss. This allows them to colonize nursery seedlings and to become potential invasive species in plantation areas. S. brevipes and T. terrestris are two of the most introduced fungi by anthropic activities; it has been argued that the vehicle for the introductions are their pine symbionts. This is the first time it has been demonstrated that peat moss is an important vehicle for the introduction of these fungi; a fact potentially related to the pattern of introduction of these ectomycorrhizal species from the northern hemisphere to elsewhere in the world.