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First Report of Colletotrichum gloeosporioides on Arceuthobium tsugense subsp. tsugense in Canada

Although reports of Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. in Penz. occurring as a hyperparasite of western hemlock dwarf mistletoe (Arceuthobium tsugense (Rosend.) G.N. Jones subsp. tsugense) have been previously published (1), there are no collection or herbarium records to support this finding (B. Geils, personal communication). Given this ambiguity, this report is the first record of C. gloeosporioides on A. tsugense subsp. tsugense based on voucher collections deposited at the Pacific Forestry Centre (DAVFP #25277). Western hemlock dwarf mistletoe is a parasitic plant that occurs on western hemlock (Tsuga heterophylla (Raf.) Sarg.), causing growth reduction, degradation of merchantable wood, and reduction in reproductive fitness of its host. Dwarf mistletoe infections result in significant reduction in volume of mature stands of T. heterophylla in coastal British Columbia (B.C.). Aside from traditional silvicultural control methods, a potentially important control includes the use of fungi deleterious to the growth of dwarf mistletoe. C. gloeosporioides has been identified as a hyperparasite of numerous Arceuthobium spp., including A.tsugense subsp. mertensiana Hawksworth & Nickrent (1). In October 1996 and May 1997, lesions were observed on shoots of A. tsugense subsp. tsugense collected from two field sites, Cowichan Lake (48°56′N, 124°23′W) and Duncan (48°45′N, 123°50′W), B.C. Acervuli containing masses of spores and dark setae were observed within lesions, and conidia from the acervuli produced pure cultures of C. gloeosporioides. The fungus was identified based on conidial and cultural characteristics (2). Conidia were cylindrical to elliptical in shape and measured 12.5 to 15.0 × 2.5 to 4.0 μm. On potato dextrose agar, colonies developed a white to patchy, dark gray mycelial mat with conidia produced in mycelia and in distinct salmon-colored masses. Colony growth occurred at 10 to 30°C, with optimum growth at 20°C, while maximum germination was observed at 30°C after 24 h, with germination occurring between 10 and 35°C. Branches with healthy mistletoe shoots were cut from western hemlock trees and placed in Hoagland's nutrient solution. A spore suspension (10⁶ conidia per ml) was brushed onto mistletoe shoots at a rate of approximately 1 ml per replicate, with 8 to 10 replicate branches, each containing a cluster of >5 shoots. Controls were treated with sterile, distilled water and branches were incubated in the greenhouse with 55% relative humidity. The fungus was re-isolated from acervuli that developed on inoculated shoots after 14 days, and these results were repeated in a second experiment. The fungus was not isolated from the controls. Koch's postulates were fulfilled by inoculating shoots of A. tsugense subsp. tsugense and re-isolating the pathogen from symptomatic stems. C. gloeosporioides could be an important control measure for western hemlock dwarf mistletoe.

References: (1) F. G. Hawksworth and D. Weins. 1996. Agric. Handb. 709, USDA, Forest Service, Washington, D.C. (2) B. C. Sutton. 1980. The Coelomycetes. Commonw. Mycol. Inst., Kew, England.