Comparing visual inspection of trees and molecular analysis of internal wood tissues for the diagnosis of wood decay fungi

The extent to which the presence of wood decay fungi in standing trees is underestimated when diagnosis is based on the visual inspection of trees was studied and whether the rate of underestimation may vary depending on the environmental context (urban vs. forest sites) and the fungal species was tested. A total of 903 broadleaf and conifer standing trees were inspected for the presence of fruiting bodies or other signs, and sampled through a drill-based technique. Multiplex PCRs were used to analyse wood samples. Trees with emerging fruiting bodies ranged from 0% to 11.6% depending on site. However, when analysed through molecular methods, the rate of infected trees raised to 15.7% to 58.0%. On average, visual inspection of trees underestimated more than 90% of infected trees compared to molecular methods in both environmental contexts. Higher rates of underestimation were observed for Armillaria spp., Heterobasidion spp. and Phaeolus schweinitzii, while lower rates (<80%) for Ganoderma spp. and Perenniporia fraxinea. The range of variation of the underestimation rate was limited; therefore, 90% underestimation may be used for estimating trees infected by wood decay fungi based on the frequency of fruiting bodies and other signs.