Graduate Thesis Or Dissertation


Epidemiological insights into Armillaria mellea root rot in a managed Ponderosa pine forest Public Deposited

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  • Root rot of ponderosa pine caused by Armillaria mellea was studied in a pine forest that had been under management for 30 years in Klickitat County, Washington. Information sources included disease survey, detailed observation and description of specific disease situations, and examination of roots in situ and removed. Roughly circular infection centers in stands of sapling and pole-sized pines appear to arise regularly from stumps of harvested old-growth trees, some of which have contained viable A. mellea for over 30 years. Distinctive concentric zones caused by the disease mark the infection centers. These zones differ in above-ground appearance, in degree of root decomposition, and in level of infection. Location of a specific zone depends on the interval since cutting of the old-growth tree, while zone width depends on tree sizes in the young stand. Distribution of root material in the interior zones is influenced most strongly by rooting characteristics of the former old-growth trees, and by activity of A. mellea and other decay agents over time, while in the young growth pines of the intermediate zones, time since lethal attack by A. mellea and tree size most influence root condition. Extensive examination of the outer zones disclosed that, in stands containing trees from 3 to 6 inches in diameter, infected root material extended into the healthy-appearing pines 11 ft beyond the outermost dead tree of the infection center, while in stands with trees up to 16 inches in diameter the extent reached 18 ft. A highly significant linear regression with good correlation was found between: 1) the extent of infected root material beyond the outermost dead tree; and 2) mean tree size of the affected stand. Infected root segments from beyond the outermost dead tree, within the region where infected and healthy roots intermingle, produced rhizomorphs, the assumed infection structures, when incubated in bags of moist quartz sand. The expansion rate of infection centers was calculated to be 2. 9 ft (0.88 m) /year. Cross plating techniques indicated that the 1500 acre study area was occupied by a single strain of A. mellea, suggesting that spores are unimportant to the field disease cycle. Current loss to disease in the 1500 acre tract is 35 cu ft per acre per yr, three times the loss in 1957. From 1957 to 1971, the proportion of the area in which disease was detectable remained constant. The increased loss resulted from death of fewer, but larger, trees in 1971. In inoculation tests on ponderosa pine seedlings, A. mellea isolates from old-growth pine stumps and from young pine trees were pathogenic, while isolates from living hardwoods were weakly or nonpathogenic. Reliable methods are described for isolating A. mellea from infected tissues and of testing the capacity of naturally infected root materials to produce rhizomorphs. The abundance of A. mellea over a three year interval following five root removal treatments increased in missed root residues. Resinous lesions were commonly found at the site of A. mellea attack. Rarely does mycelium spread proximally along a living root from one of these lesions, but growth frequently occurs from the lesion toward the root tip following, or concurrent with death of the extremity. Girdling resinous lesions on lateral roots show little effect on tree vigor, while such lesions high on the tap root kill the tree. Soon after tree death, mycelial fans spread under the bark throughout the root system. To obtain insight on how the resinous lesions retard the fungus, tests were conducted on the effects of resin on growth of A. mellea in culture. Autoclaved resin, acetone extracts of resin, water extracts of resin, and non-sterilized resin all stimulated, rather than restricted, growth over non-resin containing media. Removal of phenols from the water extract of resin with polyvinylpyrrolidone destroyed the growth stimulating effect of the resin. Propylene oxide was effective for sterilization of granulated resin.
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