Graduate Thesis Or Dissertation
 

Chemical applications for control of armillaria root rot of Ponderosa pine

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  • Root rot caused by Armillariella mellea creates roughly circular disease centers of infected dead and dying trees in the younggrowth ponderosa pine forest under study. Research objectives concerning chemical control of the disease were: 1) to test the effectiveness of certain chemicals in protecting living pines adjacent to disease centers from lethal attack at the root collar by A. mellea and 2) to test the effectiveness of chemical fumigants in eradicating A. mellea from infected stubs and snags of trees killed by the fungus within disease centers. The literature revealed the inadequacy of trenching, ringbarking, uprooting, and biological control involving antagonistic organisms in effectively controlling Armillaria root rot in the field, but control with persistent and highly volatile fungicides used as protectants and eradicants holds promise for controlling this disease. Among seven root collar protectant chemicals tested in vitro, benomyl and copper sulfate were most toxic to A. mellea while captan, copper metal, and iron sulfate were less toxic. Vorlex and chloropicrin were also highly effective in reducing the growth of A. mellea at the concentration tested. One of three isolates of A. mellea tested was more tolerant of the chemicals than the other isolates. Protectants tested on potted seedlings showed that captan significantly promoted A. mellea infection. Chloropicrin, Vortex, and high dosages of copper sulfate were phytotoxic to the pine seedlings. Viability of A. mellea within alder inoculum segments and production of rhizomorphs was also differentially affected by the applied chemicals. The A. mellea isolate least inhibited in vitro was significantly more pathogenic than the other isolates. The seven chemicals under test as root collar protectants in the forest showed no conclusive differences because disease severity was too low for evaluation of the treatments at this time. In a preliminary study of A. mellea distribution in unfumigated stumps, the fungus was most frequently isolated from yellow stringy decay and from sound appearing wood primarily within the taproot (lower portion) of the stump. Mycelial fans beneath the bark also frequently yielded A. mellea. A. mellea was most frequently isolated from stumps lacking visible decay in fresh transverse cuts at the ground line. Isolation frequency declined as the proportion of decay to sound wood on these cuts increased. Recovery was poorest in stumps which were barely intact. Five chemical fumigants were tested on stumps of ponderosa pine killed by A. mellea in typical disease centers. Methyl bromide, Vorlex, chloropicrin, carbon disulfide, and Vapam apparently eradicated A. mellea from the treated stumps. The hyphomycete, Trichoderma, appeared to be unaffected by fumigants other than methyl bromide and Vorlex. With these, Trichoderma incidence increased over the controls. Differences in the effectiveness of fumigation method and the influence of stump size or degree of decay could not be determined at the concentrations of fumigants used. Because of the apparent effectiveness of all fumigants tested, reductions in amounts and simplification of application should reduce costs of both materials and treatment, bringing costs into the realm of economic feasibility, at least in special cases. Fumigation of A. mellea in the infected root residues of disease centers should eliminate disease development at the epidemic level by reducing the inoculum potential. This change should enable further pine production in reasonable safety.
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Déclaration de droits
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