Bitter knight

Tricholoma acerbum


Горчлива витезовка



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Not evaluated Data deficient Least concern Near threatened VULNERABLE Endangered Critically endangered Extinct in the wild Extinct
Assessment info
Habitat and Ecology
Use and Trade
Conservation Actions


Scientific name

Tricholoma acerbum


(Bull.) Quél.


Agaricus acerbus Bull.

Assessment info

IUCN Red List Category and Criteria

VU – Vulnerable C2a(i)

Date assessed

November 2020


Tofilovska, S.


Kost, G.


Karadelev, M.

Rusevska, K.

Gjorgonoski, M.


Miskovic, M.


Tricholoma acerbum is a mycorrhizal species mainly associated with Quercus spp. and Castanea sativa in North Macedonia. It is a rare species, recorded at only 13 sites in well-preserved forests. The population size is estimated to be up to 2,600 mature individuals based on the currently known and predicted sites and a few fungal individuals per site. The major threat is loss and dwindled habitat quality of oak and sweet chestnut forests. In practice, the silvicultural system has clear cutting in oak forest; as a result, bare lands and even-aged forests are created while old growth forests are in decline. The species is assessed as vulnerable, under criteria C2a(i) based on an estimated population of 2,600 mature individuals, ongoing decline and small subpopulations.


Current population trend



The low number of recorded sites (13), with only one mature individual observed per site, substantiates low frequency and a small population size. According to the instructions for estimation of mature individuals provided by Dahlberg and Mueller (2011), based on the currently known and predicted sites, the population size is estimated to be up to 2,600 mature individuals (130 guesstimated total number of sites x 2 functional individuals at each locality x 10 expected mature individuals each reproducing genotype may give rise to). Each subpopulation at one location holds only a few mature individuals.

Based on forest management and the effects of habitat destruction, mainly by logging activities, the quality of the appropriate habitat is continuously declining, leading to a population decline of this ectomycorrhizal species; as well as in the global assessment of the species, where it is categorised as vulnerable (Brandrud 2015a).

Habitat and Ecology



Habitat and Ecology

Tricholoma acerbum is a species mycorrhizal with Quercus spp. Castanea, Tilia and Corylus, possibly also with Fagus. It is registered at only 13 sites in the mountains of Bistra, Dobra Voda, Galicica, Kitka, Pelister, Skopska Crna Gora and Vodno, in broadleaf forests. It is mostly found in well-preserved oak forests (Quercus frainetto, Q. cerris and Q. trojana) and also under sweet chestnut and hornbeam trees (Castanea sativa and Carpinus spp.), at elevation ranges between 600 and 1.100 m a.s.l.. Three of the locations are in Galicica NP and one in Pelister NP: all locations are in the zone for sustainable use. The species is easily identifiable but it has not been frequently recorded in the past, and it is found in habitat that is widespread in the country; thus, an estimation of probable sites has been made (Dahlberg and Mueller 2011). The total number of sites is guesstimated to be up to 10 times higher than the current number, resulting in ca. 130 guesstimated sites. It grows in the summer – autumn period.

Use and trade

Use and trade

The species is not edible; it has bitter taste, and it is known to even be slightly toxic. It does not possess pharmaceutical relevance, and it is not a subject of trade.



The species is threatened by loss and degradation of Quercus forest habitats due to logging, deforestation and diseases. In practice, the silvicultural system has clear-cutting in oak forest; as a result bare lands and even-aged forests are created while old growth forests are in decline (Trajkov et al. 2016), posing a major threat to maintenance of a stable population. In North Macedonia, ca. 1% of forests are logged annually (Kolevska et al. 2017, State Statistics Office 2018), with additional 30% illegal logging to legal wood extraction. Albeit the species is found in national parks, forests in NPs are also managed, only with restriction in regard to the logging system (Trajkov et al. 2016). According to the National Action Plan for Fight against Desertification in the Republic of Macedonia (2017), in the 2000–2012 period, a decrease of 35,000 ha (4%) of forest area had occurred based on analysis of CORINE land cover. The severely reduced appropriate quality of the habitat, creates unfavourable conditions for upholding the population.

Furthermore, it is threatened by planting of nonnative species instead of oak species in the lower oak belt (Kolevska et al. 2017).

Diseases of the ectomycorrhizal partner Quercus spp. and Castanea sativa, with varying degrees of intensity, have been reported (DPRS 2019). Diseases of oak stands are primarily caused by Euproctis chrysorrhoe L., Lymantria dispar L. and species of the family Tortricidae while diseases of sweet chestnut trees by Cryphonectria parasitica (Murrill) Barr (DPRS 2019).

Conservation actions

Conservation actions

Conservation needed: The already known locations must be protected against utilisation of the ectomycorrhizal partner and they should be preserved in their existing condition. Due to the type of silvicultural management of oak forest (Trajkov et al. 2016), in order to sustain a good habitat quality, it is vital to constitute forest reserves with maintenance of semi-open conditions at places and oak recruitment. Reforestation in the lower oak belt with alien tree species rather than oak should not be applied.

Research needed: Regular monitoring on an annual basis at the established site, coupled with field research at the noted potential sites in order to explore the distribution and dynamic of occurrence of Tricholoma acerbum. Assessment of forest health and, if required, undertaking essential measures to preserve the health of forest stands. It is advisable to digitize and regularly update forest inventories and forestry plans. There is a necessity of elaboration of a habitat map.



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