Cortinarius bulliardii is a species complex. It should be noted that the concept assessed here includes the two further cryptic 'bulliardii' from deciduous forest (I. Krisai-Greilhuber pers. comm.)

Cortinarius bulliardii, is a characteristic mycorrhizal species associating with Quercus, sometimes with Fagus and Corylus. It is a calciphilic species inhabiting mainly natural or near-natural oak and beech forests growing on the limestone soil. This habitat is undergoing an ongoing and significant decline due to loss of area and reduced ecological conditions (transformation to other types of habitats, clear-cutting, eutrophication, oak diseases, etc). The characteristic appearance of C. bulliardii and the fact that it belongs to the rich fungal element of this type of habitat, make it possible to consider this species as an indicator of valuable natural territories and “mushroom hotspots”.

There are approximately 350 known localities/sites in Europe and one in Türkiye, and the estimated total number is approximately 3,500 localities, which corresponds to c. 70,000 mature individuals. The decline of the calcareous Quercus forests in the 50 years (three generations) evaluation period is thought to be 20%, and the population is suspected to be declining at a similar rate. Based on this, the species is assessed as Near Threatened according to criterion A.

Cortinarius bulliardii is a European species, widely distributed within the temperate-Mediterranean European Quercus-Fagus range, known from at least 25 countries, including European part of Russia, east to the Tula region (Russia), and north to southern Sweden (Nezdojminogo 1996, Mahiques 2000, Consiglio et al. 2003, Peric and Peric 2005, Tănase and Pop 2005, Gyosheva et al. 2006, Tkalčec et al. 2008, Krieglsteiner and Gminder 2010, Rebriev et al. 2012, Knutsson and Fritz 2014, GBIF Secretariat 2021). It is also reported from Türkiye (Şengül and Türkekul 2021). Reports from North America and Japan are not verified by sequencing, and probably represent other species.

According to GBIF data and other listings, Cortinarius bulliardii is recorded from at least 28 sites in southern Sweden, 28 in France, 25 in Spain, 20 in Denmark, 17 in UK, 10 in Germany and seven in Slovenia. Some countries are not represented in GBIF, and some of those already mentioned above are only partially represented, but they have data on this species' distribution in their national and regional databases: there are 46 known localities in Austria, about 50 in Switzerland, 60 in Italy, 30 in Hungary, eight in Russia and one in Albania. Based on this, there are an estimated approximate 350 known localities in Europe, with one in Türkiye. The largest number of sites are in the countries where limestone soils and appropriate forest communities are widespread.

Despite the relatively large number of localities in some countries, a decline has been noted, and the species is Red Listed in Austria, Bulgaria, Denmark, Germany, Montenegro, Poland, Sweden, Switzerland (ECCF 2018), as well as in some regions of Russia (Ministry of Natural Resources and Ecology of the Tula Oblast 2020 2020). Habitat assessments at the EU biogeographical level shows that the mentioned habitats (9150, 9160, 9170) are assessed as Unfavourable-Inadequate or Unfavourable-Bad due to indicators of structure and function and poor future prospects (European Environment Agency 2013-2018). An especially alarming situation is observed in oak calcareous forests, which are have undergone a significant decline since the 1960's, due to oak diseases, area loss, abandonment of traditional management like grazing, and forestry (Grigorjev et al. 2000, Knutsson and Fritz 2014).

Taking in account all available evidence the population in Europe is predicted to occur at approximately 3,500 localities, which would equate to approximately 70,000 mature individuals, assuming two functional individuals per site, which would equate to 20 mature individuals per site (per Dahlberg and Mueller 2011). The estimated historical and ongoing habitat loss is 20% over the 50 years covering three generations of this species, which is predicted to continue into the future. Thus the population is suspected to be declining at a similar rate.

Cortinarius bulliardii is a mycorrhizal symbiont of Quercus spp., Fagus sylvatica, Corylus avellana and sometimes with Tilia cordata. It is considered to be a thermophilous species preferring warm forests and restricted to relatively old, dry oak and beech forest on limestone ground. According to Nature 2000 habitats it is confined to oak and beech calcareous forests of codes 9150 (Medio-European limestone beech forests of the Cephalanthero-Fagion), 9160 (Sub-Atlantic and medio-European oak or oak-hornbeam forests of the Carpinion betuli), and 9170 (Galio-Carpinetum oak-hornbeam forests). In Eastern Europe, including the European part of Russia, it is found mainly in oak forests (or polidominant and mixed forest with oak) growing on limestone slopes of southern exposure, while in the Mediterranean region it also grows in evergreen oak forests

The main threats to this species and its habitat are forestry, including clear-cutting of its habitat, conversion into conifer or poplar plantations, abandonment of traditional management like grazing, limestone mining and transformation of natural forest communities into parks and other recreation zones. Intensive anthropogenic impacts, including nitrogen deposition as well as tourism due to mass visiting and trampling of the leaf litter, can also be a direct threat to the population, since disturbance or destruction of litter leads to degradation of mycelia, which usually spread in the upper layer of the soil and the lower layer of the litter. The spread of Phytophthora disease throughout the region is a serious emerging threat. Drought due to changing climate in some parts of its rage are also stressing its hosts.

No use or trade is known.

The species is included in red lists of Switzerland (VU), Sweden (VU), Croatia (NT), Austria (NT), Bulgaria (EN), Denmark (EN), Germany (VU), Romania (VU) (Tănase and Pop 2005, Gyosheva et al. 2006, Tkalčec et al. 2008, Dämon and Krisai-Greilhuber 2017, ECCF 2018, AU Ecoscience 2019, SLU Artdatabanken 2020, Rote-Liste-Zentrum 2021), also in a regional Red Data Book of Russia (Ministry of Natural Resources and Ecology of the Tula Oblast 2020). The conservation actions suggested are mainly focused on area/site protection from logging and transformation of habitats, as well as limestone mining in the species' known locations and the surrounding areas, and regulation of recreational activities leading to the destruction of leaf litter.

More data on distribution and monitoring of Cortinarius bulliardii are needed as well as trends in the state of calcareous Carpinus-Quercus forests.  Taxonomic research is needed to clarify the phylogenetic relations of the European population, and collections noted from America and Japan.