Examination of species of wood decay fungi present on hazard street trees in Thunder Bay, Ontario

Abstract

Urban forests are an integral component in modern day urban infrastructure. Forests systematically planted in unnatural growing conditions are predisposed to injury, deficiency and disease, therefore requiring regular maintenance. Species of wood decaying fungi are the most significant threat to the structural integrity of urban trees. These organisms are capable of colonizing suitable host trees, most commonly through wounds on the stem, branches or roots of trees in distress. Once established, wood decaying fungi can enzymatically break down wood components into easily digestible compounds. Overtime, infected trees can become structurally weakened and hazardous, resulting in branch breakage and stem failure. In this study, urban street trees were examined in Thunder Bay, Ontario for the presence of fungal fruiting bodies, with the purpose of identifying the hazard severity and host specificity among trees colonized by wood decaying fungi. The tree species, condition, location, size and species of decay fungi were recorded. A total of 102 infected trees were recorded, with 117 different occurrences of decay fungi. There were 19 different species of decay fungi identified and 12 different tree species infected. The three most commonly infected tree species included silver maple, white birch and Manitoba maple whereas the three most frequently occurring species of decay fungi were Cerrena unicolor, Pholiota squarrosa and Pholiota aurivella. The study found that the canopy cover of silver maple in Thunder Bay will likely decline significantly in the future as a result of an excess of overly mature trees susceptible to the opportunistic pathogen, Cerrena unicolor. Street trees only colonized by Pholiota squarrosa are less likely to become a hazard tree as opposed to trees infected with multiple species of decay fungi. Street trees found with fruiting bodies of Chondrosterum purpureum likely contain large portions of progressive sapwood decay and should be viewed as being a high-risk hazard.