Respirometric Testing of Decay Resistance of Discolored Root Wood

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Abstract
Oxygen uptake by decay fungi on milled wood samples was used as a measure of decay resistance of healthy versus discolored wood in roots of Picea ubens and Abies balsamea. Healthy and discolored zones were excised, milled, sterilized, placed on water agar, and inoculated with a mycelial homogenate of the test fungus (usually Resinicium bicolor). Oxygen uptake rose rapidly on healthy samples and reached a peak at about day 10, when starch was depleted and degradation of structural polymers had begun. Discolored samples varied but generally supported much lower levels of oxygen uptake than did healthy samples. This supports the concept that these discolored tissues are best described as reaction zone rather than zones of fungal colonization or incipient decay. Low starch levels as well as wood extractives appear to play important roles in controlling fungal colonization. Bioassay of discolored zones in roots of spruce and fir produced in response to field inoculations with R. bicolor and Scytinostroma galactinum showed that the host response was maximally effective within 4 wk after inoculation. Large roots, but not small roots, of A. balsamea have a weaker, inconsistent wound response compared with roots of P. rubens. This may account for the greater susceptibility of A. balsamea to root disease.