Importance of local and long-distance Heterobasidion irregulare aerial basidiospore dispersal for future infection centres in thinned red pine plantations in Quebec

Abstract

Heterobasidion irregulare is a basidiomyceteous forest pathogenic fungus which causes a root disease capable of killing large trees. Infection probability is proportional to the density of aerial basidiospores infecting freshly cut stumps. The purpose of this study was to quantify for the first time H. irregulare aerial basidiospore density in southern Québec and to determine the importance of a local basidiospore load from a nearby infested plantation. Spore counts from automated rotary arm spore collectors were assayed using a ribosomal ITS TaqMan real-time PCR detection assay. Cumulative basidiospore deposits on a 30-cm stump were estimated to be highest in the infected plantation with 2.37 basidiospores per stump, with decreasing count of 0.52 basidiospores at a 500-m distance and 0.46 basidiospores at a 1.5-km distance. At 5 km from the infected plantation, the number of basidiospores deposited on a stump was 0.22, a value similar to those at distances of 25 and 140 km. Since a single Heterobasidion basidiospore may travel as far as 500 km, the level detected at 5 km may represent the background basidiospore load resulting from long-distance dispersal, and basidiospores from a nearby infected plantation are unlikely to be an added risk of infection to plantations located more than 5 km away. Long distance aerial basidiospore dispersal will remain for now the most important constant source of infectious H. irregulare in Québec. Despite the very low risk level per stump in this study, the large number of trees thinned annually warrants that stump treatment with Rotstop® C is still the best strategy, especially in valuable plantations.