Risk of False Positives during Sampling for Heterobasidion annosum s.l

A standard method to detect infection by Heterobasidion annosum sensu lato (s.l.) in stumps or stems is to cut a disc and examine it under a microscope. Concerns have been raised that spores can be transferred from the bark to the cut surface, thus contaminating the sample. The aims of this study were to test whether viable basidiospores of H. annosum s.l. can be transferred from the bark onto disc surfaces by a chainsaw and to investigate the impacts of different sampling procedures on the extent of contaminations. Logs were cut with or without adding basidiospores to the bark prior to the cut. Infection measurements were significantly greater for discs with treated bark (100% infected, infection coverage 40 cm² dm^-2 of disc area) compared with control discs (47% infected, infection coverage 0.2 to 0.3 cm² dm^-2). In addition, trees were sampled under authentic field conditions using different procedures. The infection measurements differed significantly depending on the procedure; sampling involving debarking or disinfection of the bark with 70% ethanol prior to cutting had lower measurements (6 to 19% and 13% infected, respectively) compared with leaving the bark on untreated (63 to 75% infected). Consideration of the contamination risk is warranted when evaluating the results of earlier studies and when planning new experiments.