Comparison of two methods to assess the virulence of the mountain pine beetle associate, Grosmannia clavigera, to Pinus contorta

Abstract Characterization of the virulence of bark beetle-vectored fungi is important for assessing potential impacts of beetle outbreaks. Massive inoculation of trees with a cork borer appears to give the most accurate estimate of fungal virulence, but cork borer inoculation is time and labor intensive. In October 2003, 18 Pinus contorta var. latifolia were inoculated with a beetle-associated fungus, Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel et al., at densities of 200 and 800 cork borer holes m−2. In July 2004 nine trees were inoculated using bark flap inoculations. The fungal-induced moisture content reduction, sapwood occlusion area and needle discoloration were similar for the 800 cork borer holes m−2 density and for bark flap inoculations, while pathogenicity symptoms induced by the 200 cork borer holes m−2 were less intense. Bark flap inoculations were three times faster to perform than high-density cork borer inoculations, but differences in incubation time and yearly weather variation highlight the need for further studies. The bark flap method may be an efficient alternative to using massive inoculation densities when testing the ability of specific fungi to kill hosts, while the cork borer method may be a better method to assess pathogenic symptoms or the aggressiveness of specific fungi.