Svstematics, efficacy and population dynamics of the biocontrol fungus, Chondrostereum purpureum
Date
2018-11-14
Authors
Becker, Elisa Madeleine
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Abstract
Biological control shows great potential as a non-chemical alternative for forestry vegetation management. Current methods of deciduous weed control include mechanical and manual removal, combined with chemical herbicide application. Manual cutting is labour-intensive and ineffective due to the rapid re-sprouting of most deciduous species from cut stumps. The basidiomycete fungus Chondrostereum purpureum was identified as a promising candidate for development as a stump treatment to suppress re-sprouting. Phylogenetic relationships were estimated by comparison of chitin synthase gene fragments among C. purpureum and other fungi thought to be closely related, or of the same ecological niche. The removal of C. purpureum from the genus Stereum was supported by these analyses. This study provided an independent confirmation of evolutionary hypotheses based on ribosomal DNA sequences. PCR-based genetic markers were developed to confirm the identity and source of C. purpureum individuals in infected trees, wood samples and mycelial cultures, allowing hundreds of field trial samples to be assayed for C. purpureum. Field-inoculated C. purpureum was re-isolated and identified, satisfying Koch's postulates for plant pathogens. The extent of infection in different hosts by C. purpureum was related to the relative success of biocontrol in these treatments. A lower rate of C. purpureum infection of treated aspen stumps, as compared to Sitka alder, was correlated with less effective suppression of this species. The same diagnostic markers were also applied to assess the distribution of genetic variation among natural populations of C. purpureum and estimate the extent of gene flow and other evolutionary forces. Genetic variation within the species revealed little evidence of substructuring that could be attributed to evolutionary processes such as genetic drift or selection. Accordingly, no geographic or host specialization was evident in C. purpureum within B.C. Spore trapping experiments were designed to assess the persistence of individual genotypes of C. purpureum following a biocontrol application. The genotypes of C. purpureum isolated from the field site before the trial, and those isolated from spore traps, were compared with the released isolate. No increase in band sharing, which would be evidence for persistence of the genotype, was apparent among the post-trial C. purpureum cultures. The PCR primers used to identify and differentiate C. purpureum amplified a number of polymorphic fragments, hypothesised to be repetitive DNA. These fragments were further characterized by comparison with published sequences and Southern hybridization. Based on sequence alignments, the repetitive DNA fragments amplified by the C. purpureum primers were hypothesised to be inactive retrotransposons, which is supported by the presence of methylation within the amplified fragments. Preliminary experiments showed that these primers can also be used to amplify polymorphic repetitive DNA from other basidiomycetes. The results of experiments summarized in this dissertation have provided an estimation of the evolutionary history of the genus by phylogenetic analysis, an assessment of the natural population structure of the species and an investigation of the dynamics of this fungus after field application. This research has expanded our understanding of fungal evolution, while concurrently supporting the development of a native fungus as a biological control agent for use in our forests.
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Keywords
Phytopathogenic fungi, Trees, Weed control