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Disease Note.

An Epidemic of Septoria Leaf Spot on Populus trichocarpa in the Pacific Northwest in 1993. G. Newcombe, Washington State University, Puyallup 98371-4998. G. A Chastagner, Washington State University, Puyallup 98371-4998; B. E. Callan, Pacific Forestry Centre, Victoria, B.C., Canada, V8Z 1M5; and M. E. Ostry, U.S. Forest Service, North Central Forest Experiment Station, St. Paul, MN 55108. Plant Dis. 79:212. Accepted for publication 9 January 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0212E.

During cool, wet weather in late spring and early summer of 1993, a leaf-spot epidemic developed on black cottonwood, Populus trichocarpa Torr. & Gray, in Washington, Oregon, and British Columbia. By late August, the severity of leaf spotting ranged from mild (isolated spots) to severe (coalesced spots on chlorotic or necrotic leaves with some spots on petioles). Up to 50% of the leaves of some trees were severely infected. Hybrid poplars were also affected to a lesser degree; lesion spotting was more severe when hybrids were grown in stands than in stool-beds. Hybrids with P. maximowiczii Henry parentage were more severely infected than those with P. deltoides J. Bartram ex Marsh. Leaf collections for fungal identification were made from P. trichocarpa trees at 61 sites. Fifty-one were from Eugene, OR, to Bellingham, WA, and from the Pacific Ocean to sites east of the Cascade Mountains. Ten sites were in British Columbia from Vancouver Island, north to Terrace. For each collection, ten pycnidiospores from each of ten pycnidia were measured for length; means ranged from 54 to 80 ?m, which is within the range reported for Septoria populicola Peck (1). Septoria musiva Peck, the cause of Septoria canker and leaf spot of many Populus spp. and hybrids elsewhere in North America, was not found in the collections.

Reference: (1) G. E. Thompson. Phytopathology 31:241, 1941.