Rhododendron adenosum was categorized as Extinct in the Wild both in the Red List of China's Biodiversity—Higher Plants Volume in 2013 and by Qin et al. (2017, Biodiversity Science, 25, 745–757). Other than the specimens collected by Joseph Charles Francis Rock in 1929 (Davidian, 1978, The Quarterly Bulletin of the American Rhododendron Society, 32, 81–85), the species had not been collected in the type location in the Kulu Mountains in Muli county, south-west Sichuan, although four specimens of Rhododendron aff. adenosum were collected in 1984 and 2008 in the Luoji Mountains (c. 130 km from the type location) in Puge county, south-west Sichuan. These four specimens did not, however, have flowers, precluding definitive identification.
With the support of the Second Qinghai-Tibetan Plateau Integrated Scientific Expedition Project (2019QZKK0502), the Biodiversity Survey and Assessment Project of the Ministry of Ecology and Environment of China (2019HJ2096001006) and the Science and Technology Basic Resources Investigation Program of China (2017FY100100), field investigations were conducted in May 2020 in both the Luoji Mountains and the type location in the Kulu Mountains. In the Luoji Mountains we confirmed that the specimens of R. aff. adenosum were wrongly identified, as they had a campaniform rather than infundibuliform floral shape and a different number of flowers per inflorescence (> 8 vs 4–8 in R. adenosum). During 2 days in the Kulu mountains, however, we rediscovered one individual of R. adenosum. As there could be additional individuals in the region, we intend to survey the surrounding mountains for the species.
As only one individual R. adenosum is currently known it should be categorized as Critically Endangered on the IUCN Red List and as a Plant Species with an Extremely Small Population (Ma et al., 2013, Biodiversity and Conservation, 22, 803–809). In addition to field surveys, propagation experiments will be performed once seeds have been collected in the autumn, and we are discussing with local nature reserves ways to protect the single known individual.
