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Western white pine historically dominated northern Idaho’s forested landscape and was the Inland Empire’s most economically important tree. White pine blister rust, caused by the exotic fungus Cronartium ribicola, played a principal role in the decline of western white pine. The pathogen causes branch and bole cankers, which usually girdle and kill their host. Efforts to restore western white pine populations are underway. Despite gains in resistance during over 50 years of tree improvement efforts, high infection rates still challenge growth and management of rust-resistant western white pine in northern Idaho. One rust- resistance expression of particular promise in tree breeding orchards is abnormal or slow stem-canker growth. Abnormal or slow stem-canker growth also appears to occur in pole-sized (15-20 years old) plantations of rust resistant western white pine in northern Idaho. The extent to which outward stem canker appearance is related to internal tree tissue damage has not been previously addressed.
In this study, between 2004 and 2006, blister rust-caused stem cankers were grouped into three classes of expected severity based on field assessment of exterior canker characteristics. Cankers with the most aggressive looking characteristics (those with bright orange diamond-shaped margins, heavy resinosous, and no apparent defensive reaction of the host tree) were classified as Class I cankers. Cankers that had limited or no orange margins and had mild to moderate resinosous were classified as Class II cankers. Cankers with the least aggressive looking characteristics (those with no orange margins, tree tissue swelling to the extent that the swelling made a vertical depression on the tree bole, no resinosous, and an inactive appearance) were classified as Class III cankers.
Stem cankers were examined on pole-sized, rust-resistant trees in three western white pine plantations in northern Idaho. After field classification, the cankers were harvested and cross-sectioned to determine: (1) the extent to which exterior canker appearance is related to internal tree tissue damage, (2) if tree circumference just prior to canker initiation is related to tree tissue damage during the first year following canker initiation, and (3) the relative impact of different class cankers on tree growth.
Measurements of cross-sectioned cankers indicate statistically significant differences in tree circumferential tissue damage (percent tree girdle) during years 1 through 5 following canker initiation between canker Classes I and III, and between canker Classes II and III during years 1 through 8. Difference in median percent girdle was not significant between Classes I and II. Additionally, tree circumference just prior to canker initiation and tree tissue damage during the first year following canker initiation were positively correlated; as tree size increased, first year percent girdle of host trees also increased. Finally, the ratio of circumferential tree growth post-canker initiation to growth pre-canker initiation differed significantly among trees infected with different class cankers. Differences were significant between canker Classes I and III and between Classes II and III, but not between Classes I and II. Growth ratios were smaller for trees with more aggressive appearing cankers.
These research results will help foresters field-classify stem cankers and predict performance of infected rust-resistant western white pine. Knowing the relative tree damage and impact on tree growth of cankers with different exterior characteristics will help foresters develop silvicultural prescriptions, including prescriptions for pruning and thinning to encourage development of target stand composition and structure. Abnormal or slow-growing cankers may indicate that their host will survive longer than expected. In addition, observed differences in canker morphology and growth of infected trees may enhance tree breeders’ and geneticists’ investigations of slow canker growth mechanisms. |
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