Genetic variation of Wheat streak mosaic virus in the Pacific Northwest

Megan Dawn Robinson

Wheat streak mosaic virus (WSMV) causes wheat streak mosaic, a potentially devastating disease of wheat. Symptoms of infection are a pale-green to yellow streaking mosaic on leaves, stunting, fewer and smaller seeds and with severe infection, shriveled seed and plant death. WSMV could have a devastating effect on the establishment of perennial wheat, which is being proposed as an alternative to annual wheat to prevent soil erosion. A survey for WSMV in the wheat growing area of Washington State was undertaken in order to understand genetic variation in the local population and relationship to isolates elsewhere in the world. Fifty local isolates were sampled during 2008, 2009 and 2010 in areas surrounding Lewiston, ID, Pullman, WA and the USDA Central Ferry Research Station, near Pomeroy, WA. Using four different methods of phylogenetic analyses, two well supported clades of WSMV were identified. Isolates that clustered within Clade A share sequence similarity with isolates from Central Europe, whereas isolates that clustered within Clade B are similar to isolates originating from Australia, Argentina and other regions in the United States. Nine isolates showed evidence of recombination; when the analysis was conducted without recombinant isolates, the same two well supported clades were observed. More polymorphic sites, parsimony informative sites and increased diversity were observed within Clade B when compared to Clade A. The observed diversity within both clades could make breeding for durable disease resistance in perennial wheat difficult if there is a differential resistance response to isolates from different clades. The replication of WSMV in wheat cultivars that differ in resistance was also investigated. At 25˚C, WSMV was able to replicate and produce symptoms in all three cultivars tested. Resistant cultivars Spitzer and CI 17881 exhibited an increase in the latent period based on positive ELISA ratios and symptom expression. However, ELISA ratios and disease ratings were equal to or higher than those observed with susceptible cultivar Stephens by the end of the experiment. This suggests that these cultivars are not exhibiting complete resistance in the growth chamber to WSMV since the virus was able to replicate and induce symptom expression.

Genetic variation of Wheat streak mosaic virus in the Pacific Northwest

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