Lactarius fuscomarginatus should be listed as Endangered under the criteria B2ab(ii,iii). This is a species restricted to Fagus grandifolia var. mexicana forests, which has been extremely reduced due to anthropogenic impacts. The known area of occupancy is less than 1.275 km2, with a maximum potential area of occupancy 4.25 km2, corresponding to the actual remaining patches of its habitat. There are only three known subpopulations and the maximum number of subpopulations is less than ten. The remaining forests of the Mexican Fagus are isolated and fragmented, covering areas smaller than 42 ha each. All the surrounding areas of these forests have been changed entirely to cropping lands and L. fuscomarginatus populations will decline due to deforestation, pollution, and edge effects.

Lactarius fuscomarginatus has a plano-convex to depressed or somewhat infundibuliform pileus, with central papilla, and margin more or less inflexed; surface rugose to venose, dry, dark chocolate brown to blackish. Distant and decurrent lamellae, cream-colored with blackish brown margin, and abundant lamelullae. In comparison with L. gerardii var. gerardii basidiomes of L. fuscomarginatus show darker tinges, mostly chocolate to blackish. Lactarius fuscomarginatus do not stain brown or red as in L. gerardii var. fagicola or in L. gerardii var. subrubescens (Montoya et al., 2012).

Lactarius fuscomarginatus is only known to develop in three subpopulations in Veracruz state (Southeastern Mexico). This species grows in relicts of subtropical cloud forests dominated by Fagus grandifolia var. mexicana, an endemic and highly threatened tree of Mexico due to agriculture, cattle raising, deforestation and other changes in land use. This species should be protected because of its limited distribution to Fagus grandifolia var. mexicana forests, the pressures of its habitat and the limited distribution of its potential host.

The known distribution of L. fuscomarginatus is limited to Veracruz state in Southeast Mexico. There are only three known subpopulations: one subpopulation in Acatlan Volcano, the second one in Mesa de la Yerba, Acajete municipality (Montoya et al., 2012), and another one near Zongolica municipality (GBIF, 2018). The three of them in Veracruz state, Mexico. Lactarius fuscomarginatus is restricted to Fagus grandifolia var. mexicana forests, whose populations have been dramatically reduced due to land use changes to introduce cropping and cattle. According to Rodríguez-Ramírez et al. (2013) of 14 original stands dominated by F. grandifolia var. mexicana over the states of Nuevo Leon, Tamaulipas, Hidalgo and Veracruz in the East of Mexico, only 11 currently persist.

The known subpopulations of L. fuscomarginatus are limited to Veracruz, Mexico. There is only one population in Acatlan Volcano, one more in Mesa de la Yerba (Montoya et al., 2012) and another one near Zongolica municipality (GBIF, 2018). Lactarius fuscomarginatus is restricted to Fagus grandifolia var. mexicana forests, whose populations have been dramatically reduced due to land use changes to introduce cropping and cattle. According to Rodríguez-Ramírez et al. (2013) of 14 original stands dominated by F. grandifolia var. mexicana over the states of Nuevo Leon, Tamaulipas, Hidalgo and Veracruz in the East of Mexico, only 11 currently persist, with a total area of 144 ha. Lactarius fuscomarginatus has only been found in two of these sites (Acatlan Volcano and Mesa de la Yerba). The third record of this species comes from small (half and hectare) cloud forest patch in Zongolica (GBIF, 2018), so is almost impossible that viable populations could further develop in this site.
The known distribution of L. fuscomargonatus is robust since we have tested the DNA barcode of the type specimen against a soil fungi database from Mexico (unpublished data). Thanks to this, we discarded the prescence of L. acatlanensis in other Fagaceae forests in Mexico (Quercus spp).

Lactarius fuscomarginatus is gregarious and it has been collected only in Fagus grandifolia var. mexicana forests (Montoya et al., 2012), which distributes in an elevation gradient from 1500–1950 m (Rodríguez-Ramirez et al., 2013). It has been demonstrated the association between F. grandifolia var. mexicana and other Lactarius species from the Acatlan volcano like L. acatlanensis (Montoya et al., 2017). Testing against a soil fungi data base from Mexico (unpublished data) we have discarded the association of L. fuscomarginatus with other Fagaceae in Mexico (Quercus spp); so its highly probable that L. fuscomarginatus, do form specific mycorrhizal associations with F. grandifolia var. mexicana.

The subtropical cloud forest, particularly those of Fagus grandifolia var. mexicana, has suffered fragmentation and is being greatly reduced due to different types of disturbances including climatic change, timber illegal extraction, and land use changes. Nowadays, according to Rodríguez-Ramirez et al. (2013) populations of the Mexican Fagus persists only in small patches. For example, all lands that surround Acatlan Volcano has been dramatically changed to cropping and cattle, increasing the isolation of F. grandifolia var. mexicana forests, which is the only habitat of Lactarius fuscomarginatus. Also, local residents collect the seeds of the Mexican Fagus to eat and/or sell locally, which could decrease the natural regeneration of these trees (Rodríguez-Ramirez et al., 2013).

By now Acatlan Volcano territory is not protected by any law. It will be necessary to be formally protected in order to preserve one of the scarce patches of relict stands of Fagus grandifolia var. mexicana at this latitude (Bandala et al., 2016), which represents a refuge for L. fuscomarginatus. Fagus grandifolia var. mexicana is currently classified as in risk of extinction by “Norma Oficial Mexicana NOM-059-Semarnat-2010”, which lists the Mexican species of animals, plants and fungi under protection, due to mainly uncontrolled human activity such as changes in land use and illegal logging. According to Rodríguez-Ramírez et al. (2013) there is only one population of Fagus grandifolia var. mexicana considered in National Park territories (in El Cielo, Biosphere Reserve, Tamaulipas, Mexico).

One of the main needs is to increase the sampling effort, particularly in other patches of Fagus grandifolia var. mexicana to increase L. fuscomarginatus our knowledege on its distribution. There has also not been direct proof the association between F. grandifolia var. mexicana and L. fuscomarginatus, so this research is pending. Some studies regarding the actual distribution of Fagus grandifolia var. mexicana populations indicates that most of them are currently isolated from each other, and some others already have disappeared (Rodríguez-Ramírez et al., 2013), so to investigate the genetic structure of Lactarius fuscomarginatus populations in those areas is needed, to detect any geographical isolation due to habitat fragmentation.

There are not reports about edibility or use of this species.

Bandala, V. M., Montoya, L., & Ramos, A. (2016). Two new Lactarius species from a subtropical cloud forest in eastern Mexico. Mycologia, 108(5), 967-980.

GBIF.org (23rd February 2018) GBIF Occurrence Download https://doi.org/10.15468/dl.ntf0yc

Montoya, L., Bandala, V. M., Haug, I., & Stubbe, D. (2012). A new species of Lactarius (subgenus Gerardii) from two relict Fagus grandifolia var. mexicana populations in Mexican montane cloud forests. Mycologia, 104(1), 175-181.

Montoya, L., Bandala, V. M., Ramos, A., & Garay-Serrano, E. (2017). The ectomycorrhizae of Lactarius rimosellus and Lactarius acatlanensis with the endangered Fagus grandifolia var. mexicana. Symbiosis, 73(2), 135-144.

Rodríguez-Ramírez, E. C., Sánchez-González, A., & Ángeles-Pérez, G. (2013). Current distribution and coverage of Mexican beech forests Fagus grandifolia subsp. mexicana in Mexico. Endangered Species Research, 20(3), 205-216.

Williams‐Linera, G., Devall, M. S., & Alvarez‐Aquino, C. (2000). A relict population of Fagus grandifolia var. mexicana at the Acatlan Volcano, Mexico: structure, litterfall, phenology and dendroecology. Journal of Biogeography, 27(6), 1297-1309.