Canadian Forest Service Publications
Relative value of slow-canker growth and bark reactions as resistance responses to white pine blister rust. 1997. Hunt, R.S. Canadian Journal of Plant Pathology 19: 352-357.
Issued by: Pacific Forestry Centre
Catalog ID: 5025
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In a selection program of western white pine (Pinus monticola) for resistance to blister rust (Cronartium ribicola), some seedlings were classified as expressing one of two resistance responses, "bark reaction" or "slow-canker growth", following inoculation with C. ribicola. Some of the seedlings that expressed slow-canker growth later displayed normal canker growth and others healed, but most cankers continued to grow slowly. Slow-canker growth was classified as one of three types: 1) small, without swelling. 2) fusiform with swelling, or 3) large and globose. Most slow-canker-growth types were also latent, i.e. they appeared more than 22 months post-inoculation; most normal cankers appeared about 16 months post-inoculation, with only a few appearing more than 22 months post-inoculation. Latency appeared to be a host trait separate from slow-canker growth. Each slow-canker-growth type tended to be associated with particular families, whereas the bark-reacion phenotype was not. When ramets and ortets were re-inoculated, the slow-canker-growth phenotype was usually repeatable, while 41 of 45 clones possessing the bark-reaction phenotype became cankered. Fungi were isolated from the bark-reaction phenotype, and Phoma herbarum, the organism isolated most frequently. produced lesions identical to bark reactions when inoculated into healthy seedlings. Many bark-reaction symptoms can be attributed to fungi other than C. ribicola. Based on this study, bark-reaction resistance to C. ribicola is rare, and seedlings with this trait frequently are killed by C. ribicola when it grows out from tissues showing this reaction, or from separate infections. Selection seedlings with the slow-canker growth trait is desirable for seed orchards, particularly if the canker has healed or if the cankers are small and latent.