Assessment Notes

Justification

Biscogniauxia bartholomaei is a wood-inhabiting saprotrophic fungus forming large, black, flat stromata (fruiting structure) in massive clusters often several metres long on dead and dying Alnus, and causing large areas of bark to fall off and the trunks taking on a “leopard spot” appearance.  However, it has only been observed and reported a small number of times and at a few localities in the Pacific Northwest and the Russian far east. It is not known from any other regions in the world, and was not encountered during 45 years of general forest disease surveys in western Canada. There are only 10 known current sites, but the albeit infrequent collections have been reliably identified from two continents and from Alnus hosts with a broad geographical range, which elevates the estimated number of sites where B. bartholomaei might occur to 100. Further targeted surveys within the range of its Alnus host will be required to better estimate the status and trend of B. bartholomaei. Therefore, the present assessment is Data Deficient (DD).

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Taxonomic notes

The type collection was made in 1909 at Rolling Bay WA, on Alnus rubra.  The fungus is somewhat similar in appearance to Biscogniauxia nummularia, which is predominantly a European species, but differs in ascospore size and the robust white conidial state that develops under the bark before it is pushed off by the emerging stromata.

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Geographic range

Biscogniauxia bartholomaei is currently known from the Pacific Northwest and the Russian Far East. Found to date in coastal Alnus rubra stands on southern Vancouver Island, British Columbia, Canada. In the United States from western Washington state (Alnus rubra and Populus trichocarpa, but the latter host record should be confirmed), and northern Idaho (Alnus tenuifolia). Also collected in the Russian Far East, in the Kamtchatka Peninsula, on Alnus kamschatia. Described by Vassiljeva (2009) as having a northern amphipacific or “Bering disjunction” in its global distribution.

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Population and Trends

There are fewer than 20 collections known of Biscogniauxia bartholomaei worldwide, from approximately 10 different sites. In the United States it is known from the type collection made in 1909 at Rolling Bay WA, and another 1909 collection at nearby Bainbridge Island, WA, Kitsap County, all on Alnus rubra. There are two other collections from Washington State, one in Sequim in 1901 on Populus trichocarpa (an unusual host, which should be confirmed), and another in Snohomish County on A. rubra. It is also known from a coastal site in Oregon, Cape Perpetua, Lincoln Co. in 2015, on A. rubra.

The first Canadian record of B. bartholomaei was collected on Southern Vancouver Island in 1995 on A. rubra. Since then it has been observed in three nearby decadent A. rubra stands located in the southern tip of Vancouver Island, BC, but nowhere else in Canada. A separate host association is reported from two locations in Northern Idaho, collected in 1922, and 2004 on A. tenuifolia, at higher altitudes. It has also been collected from the Russian Far East on the Kamtchatka peninsula, on A. kamschatia.
As the clustered fruitings on exposed wood are often large and showy and especially visible in winter and early spring before leaves develop on surrounding vegetation, it seems unlikely that the species is overlooked. In the Pacific Northwest in particular, there are no other pyrenomycete species with the same combination of morphology and habit where it occurs on mature decadent A. rubra in coastal stands.

Taking into account that there are ten known sites worldwide, we estimate that 30 trees per site will bear fruiting bodies, because the tree groves bearing the fungus usually have several infected trunks due to close proximity. Adding a factor of 10 genotypes per tree due to high genetic diversity in stromata as demonstrated in other Biscogniauxia species (Henriques et al. 2014), the disjunct distribution, the host range of Alnus species contiguous with known sites, and allowing the total number of sites to be 10 times higher to allow for those yet undiscovered, we estimate the total number of mature individuals of B. bartholomaei to be 30,000.

Information on the full extent of the population is too sparse to appropriately evaluate the status and trend.

Population Trend: unknown

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Habitat and Ecology

Biscogniauxia bartholomaei is a saprotrophic fungus causing a white rot of alder wood, on branches and trunks of host trees, where the black stromata (reproductive structures) develop under the bark and force the bark off in large sheets.  In coastal regions of the Pacific Northwest, it has been found on large, over-mature trees with broken tops. Biscogniauxia species are known to be saprotrophic, i.e. causing wood decay and persisting to produce fruiting bodies for years after tree death on fallen logs, but they also opportunistic parasites, causing bark loss and cankers on standing live trees stressed by environmental factors such as drought stress or top breakage from weather events (Edwards et al. 2003). Therefore the fungus might be less prevalent in healthy stands, and more prevalent on the boundaries of natural distribution of the tree hosts where they are more likely to be physiologically stressed, for example in sites prone to drought or at high altitudes or coastal decadent stands stressed by soil erosion and winter storms.

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Threats

Threats in the coastal Pacific Northwest sites include reduction of habitat due to urbanization. As Alnus is the preferred host for Bisconiauxia bartholomaei, there is potential risk of habitat destruction by the oomycete Phytophthora alni Brasier and S.A.Kirk , a virulent pathogen of alder, if it were introduced. Alnus rubra is highly susceptible to this pathogen (Downing et al. 2010).

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Conservation Actions

On Vancouver Island, British Columbia, two of the known sites occur in parks or other controlled access sites (military base), so some degree of protection is afforded to these populations.

Research needed: 
1) Further collections to better delimit the full range of the fungus in coastal North America, determination of host tree species range within the genus Alnus. 
2) Plot based surveys to estimate number of stands affected, and number of stems in affected stand. 
3) Comparison of high altitude Idaho collections on Alnus tenuifolia to coastal collections on A. rubra; determination of genetic relatedness of these populations.

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Source and Citation

Callan, B. 2020. Biscogniauxia bartholomaei. The IUCN Red List of Threatened Species 2020: e.T95383414A185679467. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T95383414A185679467.en .Accessed on 1 February 2024

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• Country occurrence