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A Phomopsis Canker Associated with Branch Dieback of Colorado Blue Spruce in Michigan. MELISSA J. IGOE, Graduate Student; Michigan State University, East Lansing 48824. N. CURTIS PETERSON, Associate Professor, Department of Horticulture, and DAVID L. ROBERTS, Academic Specialist, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 79:202-205. Accepted for publication 10 October 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0202.

Branch dieback is a common problem of Colorado blue spruce (Picea pungens f. glauca) in Michigan nurseries. Isolations from 100 cankers obtained from 20 seedlings exhibiting dieback symptoms all yielded Phomopsis occulta. Both mycelia and conidia of P. occulta were shown to cause branch dieback. Root pruning of spruce resulted in an increase in lesion lengths from 8.0 mm to 32.2 mm and an increase in the number of seedlings with girdling cankers from 50 to 90%. Current year's growth had longer lesions (42.9-64.3 mm) after mycelial inoculations than older tissue (9.0-32.2 mm) had. Wounding was required for infection by P. occulta mycelia but was not required for infection by conidia. The results show that P. occulta causes branch dieback in Colorado blue spruce, especially on young tissue and root-pruned seedlings.