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The role of microorganisms in the phenomenon of hemlock brownstain

University of British Columbia

Hemlock brownstain, a discoloration which varies in type and intensity and occurs in amabilis fir (Abies amabilis (Dougl.) Forbes) and western hemlock (Tsuga heterophylla (Raf) Sarg.) was investigated because it is a serious problem in the high-value lumber markets. The objective of this study was to understand the causes of hemlock brownstain, more specifically with emphasis on the role of microorganisms, and to suggest means for its control. While hemlock brownstain can vary macroscopically, a similar microscopic distribution of the brown coloration was demonstrated to be mainly associated with parenchyma cells and to a lesser degree with longitudinal tracheids. The brown deposits which were frequently associated with hyphae and bacteria, contained catechin as shown histochemically. Inoculation of western hemlock sap and wood with fungi and bacteria produced brownstain in vitro. The brown colorations which can develop in western hemlock during seasoning, were then investigated in field studies on logs and lumber. Extensive log storage time was demonstrated to promote brownstain as was salt water storage of logs, the latter producing more brownstain than logs stored on land. Fungi were isolated from freshly felled logs and from sawn lumber and they were believed to represent an endemic wood microflora. A link was suggested between fungi and brownstain. Low solubility phenols were associated with brownstained regions when compared to non-stained areas and migration of phenols to the wood surface was indicated. Infection of western hemlock lumber with Ophiostoma piceae produced brown deposits but subsequent immunolabeling with a monoclonal antibody proved unsuccessful in linking the brownstain to the fungus. However, laboratory experiments demonstrated that sapstaining fungi can produce browning in western hemlock sap as they shifted the pH from 5 to near 7, which caused ionization and oxidation of phenols. Browning induced by fungi was inhibited when the sap was buffered in the acidic range. While browning did occur at neutral pH, it did not occur in the absence of oxygen. Browning was also demonstrated in 15 /xm sections when infected with 0. piceae, suggesting that browning can occur in the presence of very small amounts of colour precursors. Furthermore, light microscopy demonstrated a lack of pigmentation of the 0. piceae hyphae when grown on western hemlock which contrasted with the formation of pigmented hyphae (sapstain) when grown on lodgepole pine. This observation suggested that physiological factors associated with the presence of the fungus triggered brownstain in western hemlock and that hemlock brownstain was unrelated to pigment formation by the fungus itself.

Thesis/Dissertation

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2009-06-04

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http://hdl.handle.net/2429/8801

Forestry

University of British Columbia

UBCV

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