Conidia of Heterobasidion annosum from Tsuga heterophylla forests in western Washington

Abstract

Heterobasidion annosum produces conidia abundantly in culture; however, since conidiophores are rare in nature, conidia are usually considered to have little or no role in dispersal. Heterokaryotic mycelia of H. annosum produce both heterokaryotic and homokaryotic conidia, whereas basidiospores are homokaryotic. This difference was exploited to assess the relative prevalence of these spore types in western hemlock forests of western Washington state. Two out of 10 spores trapped on selective media were found to give rise to heterokaryotic mycelia identified by the presence of clamp connections. However, homokaryotic conidia could not be distinguished from basidiospores by this method, so two approaches were taken in the laboratory: examining conidia for number of nuclei and determining frequency of clamp connections in conidial cultures. Both methods indicated that from a single heterokaryotic mycelium, half of the conidial progeny were homokaryotic and the other half heterokaryotic. Thus the presence of two heterokaryotic conidia in 10 spores implied that conidia may make up a third to a half of the aerial spore load of H. annosum in western hemlock forests of western Washington state.