Decay in Standing Trees: Two Commonly Occurring Facultative Decay Pathogens in Western Washington Trees
Two common fungi which cause failure in living healthy-looking trees, such as the case study elm, are Kretzschmaria deusta (previously referred to as Ustulina deusta) and Ganoderma applanatum. Both of these have been the subject of study to understand the mechanism by which they cause wood degradation, and are regarded as facultative fungi - that is, they cause decay in both sapwood and heartwood - and can lead to structural failure with little indication of health issue in the tree’s crown (Luley, 2022).
Diagram from Fungal Strategies of Wood Decay in Trees showing the main mode of decay for various fungi. (Schwarze, Engels, and Mattheck, 2000, pg. 39.)
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Photo from Common Fungi Affecting Pacific Northwest Trees, Implications for Tree Risk Assessment showing both Ganoderma and Kretzschmaria on a bigleaf maple. (Dunster and Edmonds, 2023)
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Photo from Common Fungi Affecting Pacific Northwest Trees, Implications for Tree Risk Assessment showing Kretzschmaria deusta. (Dunster and Edmonds, 2023)
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Kretzschmaria is a soft rot fungus mainly affecting the low trunk and roots (Luley, 2022), and it can damage both sapwood and dead/heartwood.
Kretzschmaria is suggested to be one of the more destructive decay fungi because it damages both sapwood and dead/heartwood and can cause significant loss of strength with minimal indication of decline in the crown of a tree (Dunster and Edmonds, 2023). Its fruiting bodies tend to be relatively flat and dry to hard black chunky material that looks burned - hence the common name “brittle cinder fungus” - but this makes them hard to pick out and identify reliably from a distance, especially on city trees which often have paint, burn marks, or other debris on the bark as well (personal experience). Anecdotally, however, a large percentage of bigleaf maples (Acer macrophyllum) in the Seattle area have evidence of Kretzschmaria when inspected (personal experience; various local arborists, personal communication), and the number of failures relative to the perceived frequency of infection does not seem to fit with the assertion that it causes the level of damage claimed. While this does not necessarily inform management of the case study elm, it leads to a question of whether Kretzschmaria is destructive in all cases, if its affect species to species can vary, or if there is a lookalike fungus or condition that most field arborists are misidentifying. |
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Ganoderma is another fungus which can damage both sapwood and heartwood, and typically affects the roots/butt and low trunk of a tree (Schwarze, Engels, and Mattheck, 2000, pg. 39) Ganoderma is also present all around Seattle, but is not as frequently visible as Kretzschmaria (personal experience). This may be due to its reproduction strategy, or it may be because passersby collect the conks - commonly known as “Artist’s Conks” - for their visual interest (Arora, 1991). One of the challenges of managing urban trees is that many people pass by and may cause damage to indications of decay that arborists look for. Ideally when doing a project such as a tree inventory or a one-off assessment, one would collect information about obvious conditions of the tree, but it’s possible that some of these clues have been obscured through interaction with the community which uses the site. |
Photo from Common Fungi Affecting Pacific Northwest Trees, Implications for Tree Risk Assessment showing Ganoderma applanatum on a bigleaf maple. (Dunster and Edmonds, 2023)
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This project was built as an assignment for the Tree Risk Management module within the MSc Arboriculture and Urban Forestry course at Myerscough College. All photos and content are my own unless otherwise attributed or cited. All names, addresses, and dates are made up, but accurately reflect season and site conditions at the time of assessment.
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