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Identification and Occurrence of Wheat Streak Mosaic Virus in Winter Wheat in Colorado and Its Effects on Several Wheat Cultivars. I. M. Shahwan, Graduate Student, Colorado State University, Fort Collins 80523. J. P. Hill, Assistant Professor of Botany and Plant Pathology, Colorado State University, Fort Collins 80523. Plant Dis. 68:579-581. Accepted for publication 12 January 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-579.

Winter wheat plants showing symptoms presumed to be those of streak mosaic were collected in northeastern Colorado in 1979 and 1980 to determine if wheat streak mosaic virus (WSMV) was the causal agent. Inoculum was prepared from each plant sample and hand-rubbed onto Carborundum-dusted Parker wheat seedling leaves. Six of 73 samples produced streak mosaic symptoms on inoculated seedlings. These six samples were confirmed as WSMV isolates by using appropriate indicator plants, stability in plant sap, occurrence of characteristic inclusion bodies, electron microscopy, and leaf-dip serology. Differences in symptom expression and percent infection were found among the six Colorado isolates when compared on Parker seedlings. The presence of WSMV was subsequently confirmed in 11 severely diseased commercial fields. In all cases, either nearby fields had many volunteers because of severe late-season hail damage or corn or millet had been grown in an adjacent area during the previous growing season. Volunteer wheat plants, corn, or millet apparently served as primary inoculum sources for early-fall infection of newly emerged winter wheat seedlings. Eight winter and four spring wheat cultivars were evaluated for symptom expression and yield loss after inoculation with a single WSMV isolate in the greenhouse. Significant reductions occurred within all cultivars. Regression analysis revealed no significant correlation between reductions in yield and plant height or fertile tiller number. Seedling disease severity ratings could not be used to predict relative rankings in percent reductions.