Assessment Notes

Justification

Boletinellus merulioides is a common, well-known and widespread terrestrial fungus restricted to ash in eastern North America, west into the plains states, and with planted ash in Colorado and Oregon. It has a fascinating symbiosis with the leaf-curl aphid associated with ash. Ash (Fraxinus spp.) is rapidly declining from the invasion of the Emerald Ash Borer, (Agrilus planipennis) from Asia starting in the early 2000’s. The Emerald Ash Borer has rapidly spread across much of the native range of ash, shows no sign of stopping and is devastating for all North American ash species. It causes near 100% mortality in mature ash trees. Since 2017, White Ash (Fraxinus americana), the most common native ash of the USA, has been assessed as Critically Endangered with an estimated decline of 80% in 100 years. Boletinellus merulioides will decline in concert with the decline of ashes. The decline is estimated to exceed 30% in 30 years (corresponding to 3 generations of B. merulioides). It is listed as Vulnerable.

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Taxonomic notes

First described by Schweinitz in 1832 as Daedalea merulioides, based on a collection from Massachusetts, USA, Murrill transferred the species to the genus Boletinellus in 1909. Singer transferred it into Gyrodon (1938). Genetic studies showed it wasn’t closely related to Gyrodon, and belonged in the genus Boletinellus.

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Geographic range

This species is common and widespread in eastern North America, west into the plains states, and with planted ash in Colorado and Oregon. It is also reported from Mexico and Asia; these collections should be compared with the eastern North American species.

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Population and Trends

Common and widespread in eastern North America, west into the plains states, and with planted ash in Colorado and Oregon. Also reported from Mexico and Asia; these collections should be compared with the eastern North American species. Disappearing at a rapid pace with the invasion of the Emerald Ash Borer, (Agrilus planipennis) killing the host trees in large swaths of the current range. Jerome et al. put population decline of at least 80% over the next 100 years for F. americana. Boletinellus merulioides will decline in concert with the decline of ashes. The decline is estimated to exceed 30% in 30 years (corresponding to 3 generations of B merulioides, cf. Dahlberg & Mueller 2011).

Population Trend: decreasing

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Habitat and Ecology

This fungus is found growing alone or scattered under ash trees and has a complex symbiotic relationship with the leafcurl ash aphidan (Meliarhizophagus fraxinifolii) occurring on the roots of ash (Fraxinus) species. The fungus forms small sclerotia encapsulating the aphids, getting nutrients from the aphid’s ‘honeydew’ (a mix of sugars, amino acids and minerals).

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Threats

Ash is rapidly declining in North America due to the invasion of the Emerald Ash Borer, (Agrilus planipennis) from Asia starting in the early 2000s. The Emerald Ash Borer has rapidly spread across much of the native range of White Ash, shows no sign of stopping and is devastating for all North American ash species. It causes near 100% mortality in mature ash trees. Since 2017, White Ash (Fraxinus americana), the most common native ash of the US, has been assessed as Critically Endangered with an estimated decline of 80% in 100 years.

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Source and Citation

Siegel, N. 2019. Boletinellus merulioides. The IUCN Red List of Threatened Species 2019: e.T125434301A125435550. https://dx.doi.org/10.2305/IUCN.UK.2019-1.RLTS.T125434301A125435550.en .Accessed on 1 February 2025

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• Country occurrence