Response of Alnus fruticosa to inoculation with Valsa melanodiscus in roadside and forested environments

Valsa melanodiscus (anamorph Cytospora umbrina) has been associated with disease-related mortality of a riparian shrub, thinleaf alder (Alnus tenuifolia), across Alaska. The disease is characterized by diffuse, longitudinal stem cankers. Similar disease symptoms have also been observed on Siberian alder (Alnus fruticosa) in upland and alpine habitats. However, canker-related mortality on A. fruticosa appears to be much lower than on A. tenuifolia. We conducted a field inoculation study in upland, mixed stands to test whether the same pathogen species causing mortality in A. tenuifolia can incite disease on A. fruticosa. Because A. fruticosa can be subjected to water stress along roadsides, we also compared disease predisposition and susceptibility in roadside versus forested habitats. In July 2008, we inoculated A. fruticosa by wounding the stem and placing a colonized agar plug over the exposed inner bark and sapwood. After one year, V. melanodiscus was cultured from 91.5% of the inoculated stems. Fifty-five per cent of the inoculated stems developed sunken elliptical cankers, while wounded control stems produced callus that closed the wound. Percentage stem girdle was typically < 10%, and canker length was limited to 5 mm or less in the majority of inoculated stems. However, A. fruticosa stems inoculated along a south-facing roadside had more severe symptoms (cankers 10 to 13 mm in length). Our study suggests that disease-related mortality in upland, mixed stands is unlikely for A. fruticosa, but that site conditions associated with increased vulnerability to water stress may be related to disease susceptibility.