Arceuthobium gillii Hawksw. & Wiens (Viscaceae) is primarily distributed in the Sierra Madre Occidental of Mexico from central Durango and northern Sinaloa into Chihuahua and Sonora (3). In Mexico, it commonly parasitizes Pinus leiophylla Schiede & Deppe var. leiophylla and var. chihuahuana (Engelm.) G. R. Shaw, P. lumholtzii Robinson & Fern., and P. herrerai Martinez and rarely infects P. arizonica Engelm. and P. cooperi Blanco (3). This dwarf mistletoe also occurs in the Chiricahua, Huachuca, Santa Rita, Rincon, and Santa Catalina Mountains of southern Arizona and the Animas Mountains of southwestern New Mexico (1,3). In the United States, A. gillii has only been reported to parasitize P. leiophylla var. chihuahuana (1,2,3). The host range of A. gillii has consistently not included P. engelmannii Carr. (2,3). However, we have located a small population of P. engelmannii naturally infected by A. gillii in the South Fork of Cave Creek, Chiricahua Mountains, Arizona. The infected P. engelmannii occurred approximately 2.6 km west of the South Fork Cave Creek Picnic Area, Coronado National Forest, along Forest Trail 243 (31°50′53″N, 109°12′14″W, elevation 1,670 m). Only four P. engelmannii were infected: one tree had four infections as indicated by small, dense witches' brooms and branches with large, spindle-shaped swellings, and three trees had one infection each based on the presence of brooms. One branch was collected from the tree with four infections because observations of the branch with binoculars indicated it had dwarf mistletoe shoots. Only four male shoots of A. gillii were on the branch and they were consistent morphologically with other male plants of this dwarf mistletoe collected from nearby P. leiophylla var. chihuahuana. The infected P. engelmannii were all growing in close association with P. leiophylla var. chihuahuana severely infected with A. gillii. The only other dwarf mistletoe reported to infect P. engelmannii in the Chiricahua Mountains is A. vaginatum (Willd.) Presl subsp. cryptopodum (Engelm.) Hawksw. & Wiens. This dwarf mistletoe was not present or anywhere near the Cave Creek population of P. engelmannii. In addition, plants of A. gillii can be easily distinguished morphologically from those of A. vaginatum subsp. cryptopodum by their color (2,3). Plants of A. gillii are consistently green to greenish brown, while plants of A. vaginatum subsp. cryptopodum are consistently orange to reddish brown. A specimen of A. gillii on P. engelmannii has been deposited at the Deaver Herbarium, Northern Arizona University, Flagstaff (Accession No. 75392). To our knowledge, this is the first report of A. gillii on P. engelmannii. This host-dwarf mistletoe combination is evidently very rare because other investigators have reported that P. engelmannii was probably immune to A. gillii (2,3).
References: (1) F. G. Hawksworth and M. Weiss. Southwest. Nat. 20:418, 1975. (2) F. G. Hawksworth and D. Wiens. Brittonia 16:54, 1964. (3) F. G. Hawksworth and D. Wiens. Dwarf mistletoes: Biology, pathology, and systematics. USDA For. Serv. Agric. Handb. 709, 1996.
