Limber pine dwarf mistletoe (Arceuthobium cyanocarpum (A. Nelson ex Rydberg) Coulter & Nelson; Viscaceae) severely parasitizes limber pine (Pinus flexilis James) and several other white pines, including western white pine (P. monticola Dougl. ex D. Don) and whitebark pine (P. albicaulis Engelm.), over an extensive geographic range in the western United States (1). However, limber pine dwarf mistletoe has not been previously reported on sugar pine (P. lambertiana Dougl.), another white pine found within the range of limber pine dwarf mistletoe (1). In August 2009, we found a sugar pine infected with limber pine dwarf mistletoe approximately 0.8 km northeast of Tahquitz Peak in the San Jacinto Mountains, California (33°45′24′′N, 116°40′24′′W; elevation 2,640 m). The infected sugar pine was 13 inches (33 cm) in diameter and had 13 infections on five of its lower branches. Ten of the infections were producing mature male and female mistletoe plants with open flowers or developing fruits, respectively. Two of the infected branches were forming witches' brooms in response to infection by limber pine dwarf mistletoe. The infected sugar pine was growing within 3 m of four limber pines severely infected with limber pine dwarf mistletoe. The male and female plants produced on the infected sugar pine were morphologically identical to those growing on the infected limber pines. Limber pine dwarf mistletoe can be distinguished from sugar pine dwarf mistletoe (A. californicum Hawksw. & Wiens), the principal dwarf mistletoe parasitizing sugar pine in California, by its smaller plants (mean height 3 cm versus 8 cm) and flowering period (August to September versus June to July). In an attempt to determine the relative susceptibility of sugar pine to limber pine dwarf mistletoe, we conducted a survey of the infested limber pine stand. Because there were no additional sugar pines growing in the area, it was impossible to assess the general susceptibility of sugar pine to limber pine dwarf mistletoe, but the production of many mature plants from 10 of the infections on the sugar pine suggests this tree species may be highly susceptible. However, this is currently the only known location where sugar pine co-occurs with limber pine dwarf mistletoe (1), so assessing the susceptibility of sugar pine to this dwarf mistletoe will depend on locating additional sites where they co-occur. It should be noted also that previous surveys in the San Jacinto Mountains failed to detect infection by limber pine dwarf mistletoe on sugar pine (1). Specimens of limber pine dwarf mistletoe on sugar pine were collected and deposited at the Deaver Herbarium (ASC), Northern Arizona University, Flagstaff (Accession No. 92697). To our knowledge, this is the first report of limber pine dwarf mistletoe parasitizing sugar pine.
References: (1) F. G. Hawksworth and D. Wiens. USDA For. Serv. Agric. Handb. 709, 1996.
