Rhizopogon yakushimensis is an endemic hypogeous ectomycorrhizal fungus confined to and associated with Amami Pine (Pinus amamiana) that only grows on two islands in Japan. Amami Pine is globally red-listed as Endangered and estimated to consist of about 2300 trees. Although exploitation of the tree has ceased, infestations with pine wilt nematodes continue to cause serious population decline. The status and trend for the population of Amami Pine set that for R. yakushimensis. Hence, R, yakushimensis is assessed as Endangered (EN) based on the restricted geographic range (EOO 600 km2 and AOO 50 km2) together with few locations, and the declining population size consisting of less than 2500 individuals.

Rhizopogon yakushimensis was described in 2018 by Sugiyama et al.

Rhizopogon yakushimensis is an endemic truffle obligately associated with the rare Amami Pine confined to Yakushima and Tanegashima islands in Kagoshima Prefecture, Japan. The extent of occurrence is estimated to be 600 km2, with an area of occupancy estimated at 50 km2.

Rhizopogon yakushimensis is confined to the distribution of P. amamiana. The population of P. amamiana approximately consists of 2300 trees; 2000 at Yakushima and 300 at Tanegashima (Katsuki and Farjon 2013). The exploitation of the tree has ceased, but infestations with pine nematodes continue to cause serious population decline (Katsuki and Farjon 2013). The population size of R. yakushimensis is estimated to be in the same range, or more likely, smaller than that of P. amamiana and to have the same population trend, i.e., declining. A study of mycorrhizal roots and the spore bank in the remaining four stands of Amami Pine revealed R. yakushimensis to be present and one of the dominating species in all stands (Murata et al 2017).

Rhizopogon yakushimensis is a hypogeous ectomycorrhizal fungus obligately associated with the five-needle Pinus amamiana. Based on molecular phylogenetic characteristics and poor compatibility to form mycorrhiza with other pine species, it is considered that R. yakushimensis may be solely associated with P. amamiana on Yakushima and Tanegashima Islands (Sugiyama et al 2019). An analysis of mycorrhizal roots and the spore bank showed R. yakushimensis to be present and among the dominant ectomycorrhizal fungi in all four stands with P. amamiana (Murata et al 2017).

The threat of Rhizopogon yakushimensis is the same as the threat of the endangered tree Pinus amamiana. Pinus amamiana is globally redlisted as Endangered (EN)  as it is a rare and declining species, having an area of occupancy (AOO) of less than 100 km², the total population size amounts to fewer than 3,000 trees (ca. 2,000 on Yakushima) and is declining (Katsuki and Farjon 2013). These trees were formerly exploited for timber, and regeneration is slow probably due to inbreeding depression. Pine wood nematodes accidentally introduced from North America have caused increased mortality on Tanegashima island (pers.comm. Tetsukazu Yahara of the IUCN SSC Japanese Plant Specialist Group).

Hence R. yakushimensis meets the thresholds for Endangered under both criteria B (AOO and EOO together with few locations and declining population) and C (total population with continuing decline and consisting of fewer than 2500 individuals).

A few stands with Pinus amamiana in Yakushima and Tanegashima are protected, and actions are ongoing with the aim of conserving P. amamiana. This also benefits the conservation of Rhizopogon yakushimensis. A study analyzing the presence of mycorrhizal fungi in the soil of P. amamiana populations established ex situ on Yakushima island could not detect any R. yakushimensis (Sugiyama et al 2019).

None.