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black-throated green warb James W. Wolford wrote: > Also the burls on conifer trunks were caused by a rust fungus? (or > perhaps a virus or bacterium??). > > > Cheers from Jim in Wolfville, 542-9204 > Hi Jim & All, May 20, 2008 I think the cause of these burls is unknown. In addition to large sometimes frequent burls on trunks they also may be abundant on branches and especially on roots. Root burls can be at least 20 times the diameter of the root on which they develop. Early stages of burl development are readily detected on small roots & branches (<5 mm diam) as elongated (2-3 times as long as wide) low swellings and I have never seen by eye or hand lens any discolored or distorted nucleus that might have been a starting point. Burls are very rare on fir (I can recall having seen only one small one), absent on pine and hemlock (?). Above ground burls are usually on white spruce and both white and red spruce have root burls. There seems to be a soil effect, burls being most common on poorly drained soils, less common on excessively drained soils and least common or absent (?) on well drained soils with ample water holding capacity. So I suspect some localized water inbalance that leads to localized excessive cambial activity. Once initiated this localized growth inbalance would tend to be self perpetuating due to the greater phloem tissue area and length. The growth rings of burls, like the rings where two trees are growing together to become one trunk, can be ~10 times wider than the adjacent normal tissue. Yt. Dave Webster, Kentville
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