Graduate Thesis Or Dissertation
 

Epidemiology and population biology of Inonotus tomentosus as a pathogen of forests of British Columbia

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  • The root disease fungus Inonotus tomentosus, common in the old growth boreal forests of British Columbia, poses a threat to the health of second growth forests established on sites with a previous history of root disease. Colonized stumps occur in groups of 1 to 6; the groups are clumped within a clearcut. Therefore, surveys for disease incidence need to employ wide (10 m) transects and cover at least 10% of the stand area. The fungus remains viable in stumps for at least 30 years and can infect roots of regeneration trees in contact with stump roots. Spruce stump roots cause twice as many infections as pine stump roots because more spruce roots are colonized, they are longer, and they are horizontally oriented. Trees growing within 200 cm of diseased spruce stumps have a 25% chance of infection. At 350 cm - 400 cm the chance of infection is 10%. Trees growing within 50 cm and 250 cm 300 cm of diseased pine stumps have a 25% and 10% chance of infection respectively. Within a stand, disease centres are usually small (less than 5 trees) and composed of a single genotype. Larger centres consist of several genotypes. Electrophoretic protein profiles and vegetative compatibility tests were co-supportive methods used to determine genotype similarities. The clumped distribution of disease centres and the frequency of unique genotypes suggest that spores may have an important role in disease spread. Infection of roots by I. tomentosus occurs through direct penetration of the bark of small roots, or by infection of a feeder root. In roots approximately 5 cm diameter or less, the fungus grew in or on the bark and often preceded decay in the wood. In larger roots fungal growth is in the heartwood; radial growth to the bark is limited until root death. Inoculations of mature spruce and pine trees suggest that spruce is more readily infected than pine, due in part to greater resin production and phenol accumulation by challenged pine roots. Infection of root wood by I. tomentosus caused host cell death at the hyphal front, increased phenolic deposition, peroxidase activity, and starch depletion beyond the hyphal front. Inonotus circinatus (a pathogen of pine in eastern North America) and I. tomentosus show consistent differences at the protein level as indicated by protein electrophoresis and southern hybridization of a homologous probe (random genomic clone) to total genomic DNA. Protein patterns of I. tomentosus isolates are more variable than patterns of I. circinatus which may be due to greater diversity of I. tomentosus hosts and climate.
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