Inoculation methods for selecting Populus tremuloides resistant to Hypoxylon canker

Canker expansion and the amount of callus tissue formed were measured month ly on 60 ramets from each of five trembling aspen (Populus tremuloides Mich x.) clones that had been inoculated in wounds: with Entoleuca mammata (= Hy poxylon mammatum (Wahl.) Mill.) over a 12-month period. At the clone level, the prevalence of nonlethal cankers within clones prior to the study had n o correlation with canker expansion with three of the five clones. Greenhou se inoculation of ramets derived from the same five clones resulted in the same resistance rankings as main-stem inoculations in the field. One isolat e, Hm-27, produced longer cankers, and less callus developed on all clones compared with trees inoculated with the less aggressive strain, Hm-24. Cank ers developed only on clones that were inoculated during the months of Apri l through July with April inoculations resulting in the largest cankers. Th ese results indicate that there is a limited time frame when wounds on aspe n are susceptible to infection by artificial inoculation with the pathogen as many wound inoculations neither produced cankers nor wound callus. Compa ring inoculation methods, either main-stem inoculations in the field or gre enhouse inoculations coupled with the natural canker prevalence could be us ed to include or exclude clones for use in an aspen breeding program.