• Proposed
  • Under Assessment
  • 3Preliminary Assessed
  • 4Assessed
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Cryptomyces maximus (Fr.) Rehm

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Scientific name
Cryptomyces maximus
(Fr.) Rehm
Common names
Willow Blister
skrytohubka žltoobrúbená
IUCN Specialist Group
Cup-fungi, Truffles and Allies
Assessment status
Under Assessment
Proposed by
David Minter
David Minter
David Minter
Comments etc.
Eugene Popov, Ivona Kautmanova, Anders Dahlberg, Brian Douglas, Vladimír Kunca, Maroš Peiger, Nicolas Schwab, Inita Daniele

Assessment Notes

Taxonomic notes

Recent molecular studies have shown that Cryptomyces belongs in the family Rhytismataceae close to Rhytisma (Lantz et al., 2011). Striking morphological (for example ascospore shape) and habitat differences (inhabiting branches rather than leaves), make Cryptomyces maximus easily distinguished from the several Rhytisma species known from willows.

Why suggested for a Global Red List Assessment?

The fungus which grows on living and dying branches of willows near water is conspicuous but very infrequently recorded. There have been hundreds of unsuccessful searches, suggesting it is genuinely rare. At known sites in the UK, fruitbodies are few and only on some trees. Records tend to be from unpolluted and undisturbed locations. Its geographical distribution suggests a preference for cooler climates and more northerly latitudes, from sea-level up to 600 m, in northern Europe and North America. This ecological preference may be threatened by climate change.

Geographic range

[date of most recent known record shown in square brackets] Canada (Alberta, British Columbia [1961], Manitoba, Saskatchewan [1938]), Czech Republic [pre-1896], Denmark [1903], Finland [1913], France [pre-1865], Germany [1917], Iceland [1971], Ireland, Lithuania [1907], Norway [2011], Portugal [pre-1889], Russia (Arkhangel [1920], Kursk [1915], Leningrad [1949], Vologodskaya [1925]), Sweden [2009], Ukraine [pre-1969], UK [2013], USA (Colorado [1899], New Mexico [1917], Oregon [1931], Utah [1918], Wyoming [1971])

Population and Trends

Since discovery in 1801, this species has been observed probably fewer than 100 times, and records have tended to occur in small groups, with long intervals between them. Thus, for example, Sowerby, the first to describe this species, received three separate collections from different parts of southeast England in 1801, there being then no more records from England for about 70 years. There has been a similar spurt of records between about 1988 and 2012, although the higher incidence on this occasion is partly due to intensive searching. The pattern thus seems to be of small highly dispersed or fragmented populations with very small numbers of fruitbodies produced at infrequent and unpredictable intervals. Provisionally listed as critically endangered (Anon., 2011). No records from North America since 1971. Records since 2000 only from a very few scattered sites in the UK and Scandinavia. Based on those post 2000 records, known extent of occurrence is about 800,000 km squared, and known area of occupancy is less than 60 km squared.

Population Trend: Uncertain

Habitat and Ecology

Cryptomyces maximus grows on dead portions of otherwise living willow twigs and branches, and is clearly at least partly responsible for killing them. The fungus is strongly associated with humid localities, preferring bushes of Salix close to water, such as rivers, brooks and wet ditches, or the edges of wet mires. At one site in west Wales wisited in 1987, the only trees infected were small scrubby bushes in a ditch next to an area with several horses. The horses had access to the bushes, and there was evidence that the trees had received significant damage from attempted grazing by the horses. Earlier observers in the UK have also noticed an association with damage by horses, although Granmo et al. (2012) in Scandinavia found the fungus less frequent on trees damaged by elks. The fungus produces a strong scent which Granmo et al. (2012) suggested might be attractive to insects involved in its dispersal. Ascospores of the fungus have unusual mucous appendages similar to some aquatic fungi, and these too may have some dispersal function.

Associated plants. Salix alba, Salix arbuscula, Salix atrocinerea, Salix caprea, Salix caprea x viminalis, Salix cinerea, Salix elaeagnos, Salix fragilis, Salix herbacea, Salix incana, Salix lanceolata, Salix myrsinifolia, Salix pellita, Salix pentandra, Salix phylicifolia, Salix repens, Salix viminalis, Salix sp.


The fungus is conspicuous and large by the standards of most tree-inhabiting ascomycetes, and the infrequency with which it has been recorded, coupled with hundreds of unsuccessful searches suggests that it is genuinely rare. Sites from which it has been recorded tend to be isolated and distant from large centres of populations, and in unpolluted or otherwise protected areas. At sites where it has been observed in the UK, fruitbodies are low in numbers. The geographical distribution of this fungus suggests that it prefers cooler and more northerly latitudes, having been observed at altitudes from sea-level up to 600 m in northern Europe and North America. This ecological preference may be threatened by climate change.

Conservation Actions

A. Monitoring and Surveying. In Pembrokeshire (UK), the species has been monitored since 2008, with surveys to establish its geographical distribution, including recording of unsuccessful searches of apparently suitable habitats (Harries et al., 2013).

B. Conservation Advice. Landowners and site managers in Pembrokeshire (UK) have been alerted to the presence of this species, and provided with all of the limited current information about its habitat needs (Harries et al., 2013).

C. Ex situ. Four DNA and RNA sequences of this fungus are stored in Genbank [www.ncbi.nlm.nih.gov/gquery/?term=“Cryptomyces+maximus” accessed 22 February 2014].

Research needed

Use and Trade


ANON. Species on the Edge of Survival. IUCN & Harper Collins. 400 pp. (2011). EVANS, S., HENRICI, A. & ING, B. Red Data List of Threatened British Fungi. British Mycological Society. 13 pp. (2006). FRIES, E.M. Systema Mycologicum 2 (2): 566 (1823). GRANMO, A., RÄMÄ, T. & MATHIASSEN, G. The secrets of Cryptomyces maximus (Rhytismataceae). Ecology and distribution in the Nordic countries (Norden), and a morphological and ontogenetic update. Karstenia 52: 59-72 (2012). GREVILLE, R.K. Scottish Cryptogamic Flora 4 (37-38): plate 206 (1825) [publ. 1826]. HARRIES, D., HODGES, J. & THEOBALD, T. The role of a local fungus recording network in conserving an internationally threatened species, Cryptomyces maximus (willow blister). Fungal Conservation 3: 7-11 (2013) [www.fungal-conservation.org/newsletter/issue_3_2013_09_15_low_resolution.pdf]. LANTZ, H., JOHNSTON, P.R., PARK, D. & MINTER, D.W. Molecular phylogeny reveals a core clade of Rhytismatales. Mycologia 103 (1): 57-74 (2011). MINTER, D.W. Cryptomyces maximus. IMI Descriptions of Fungi and Bacteria No. 1473 (2002). MINTER, D.W. & DUDKA, I.O. Fungi of Ukraine, a Preliminary Checklist. Egham, UK, International Mycological Institute & Kiev, Ukraine, M.G. Kholodny Institute of Botany; 361 pp. (1996). SOWERBY, J. Coloured Figures of English Fungi or Mushrooms 3 (24): tab. 356 (1802).

Country occurrence

Regional Population and Trends

Country Trend Redlisted