Clonal variation in the incidence of Hypoxylon canker on trembling aspen

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Abstract
The incidence of Hypoxylon canker, caused by Hypoxylon mammatum (Whal.) Miller, on ramets of 80 pole-sized Populus iremuloides clones was determined on three sites near Pellston, Michigan, and two sites near Iron River, Michigan. Variation in the incidence of canker within clones was slight. No significant difference (P >.05) in canker incidence was found among NE, SE, SW, and NW quadrants of 47 clones on the Pellston Plain site. Significant differences (P <.005) were found among clones in the incidence of canker in four of the five sites studied. In some cases, nearby or adjacent pairs of clones showed significant and striking differences in the incidence of canker infection. The range in canker incidence (percentage of living and dead ramets with stem or branch cankers) for all clones was 9 to 90%; the range was wide on all sites. Low stand density and light bark color of ramets were both significantly (P <.01 and P <.05, respectively) correlated with high canker infection.Although genetic differences between clones in the incidence of canker were not proven, the marked phenotypic differences among clones and other findings suggested moderate genetic control on four of the five sites investigated. In studies relating the incidence of canker to site and biotic factors, the clonal structure of aspen stands should be recognized, and the clone should be used as the basic unit of sampling.

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