First Report of Arceuthobium sichuanense, a Dwarf Mistletoe, on Pinus tabuliformis in Qinghai Province, China

Abstract

Arceuthobium sichuanense (H. S. Kiu) Hawksw. & Wiens belongs to family Viscaceae, one of the mistletoe families of flowering plants that are parasites on several species of the genus Picea (Pinaceae) in the southwest of China, including Qinghai, Sichuan, and Tibet Provinces (Wang et al. 2016). However, it has not been reported on Pinus trees (Pinaceae) (Wu 2003). Hawksworth and Wiens (1970) considered that A. sichuanense parasitizes only Picea, and A. pini Hawksw. & Wiens parasitizes only Pinus (Kiu 1984). However, in June 2012, A. sichuanense was observed parasitizing Pinus tabuliformis Carriere in a mixed conifer forest of P. tabuliformis and Picea wilsonii Mast. in Beishan Forest Farm (36°56′14.5″N, 102°30′16.3″E, elevation 2,314 m), Haidong District, Qinghai Province, China. One infected P. tabuliformis was observed, approximately 3 m from a P. wilsonii 14 m tall that was severely infected with A. sichuanense. This diseased tree was infected as indicated by a small, nonsystemic witches’ brooms and a branch with large, spindle-shaped swellings. Male dwarf mistletoe plants were observed producing mature aerial shoots and staminate flowers. These shoots were compared morphologically with A. sichuanense collected from nearby P. wilsonii, and this was sufficient for a positive identification of the dwarf mistletoe species found on P. tabuliformis. A. sichuanense can be easily distinguished morphologically from A. pini by the much shorter shoots (2 to 6 cm in A. sichuanense versus 5 to 20 cm in A. pini) and smaller staminate flowers (1.5 to 2 mm in A. sichuanense versus 2 to 2.5 mm in A. pini) (Hawksworth and Wiens 1970, 1993; Wu 2003). An area of 1.5 ha around the infected P. tabuliformis tree was carefully examined for A. sichuanense, and only five infections were discovered on three P. tabuliformis trees. Most trees had no infection, even where the P. tabuliformis crown intermingled with that of a severely infected P. wilsonii. Therefore, we would tentatively classify P. tabuliformis as a rare host of A. sichuanense. The only other dwarf mistletoe reported to infect P. tabuliformis is A. pini (Wu 2003). This dwarf mistletoe was not present in the Beishan Forest Farm population of P. tabuliformis and P. wilsonii or anywhere near this area. There are reports and publications about A. pini on Pinus densata Mast., Pinus griffithii McClelland, and Pinus yunnanensis Franch. (Wu 2003), but it has not been seen on P. tabuliformis so far. To our knowledge, this is the first report of A. sichuanense parasitizing P. tabuliformis and the only known instance of A. sichuanense parasitizing a Pinus species found in China (Wu 2003). Specimens of A. sichuanense on P. tabuliformis were collected and deposited at the museum of Beijing Forestry University (accession no. BJFC-120619). A. sichuanense appears to be an extremely rare occurrence on P. tabuliformis at the present time; hence, this dwarf mistletoe is not considered a management concern in this pine species.