Where wheat streak mosaic was a severe problem on some wheat fields in 2011, will it be safe to use that wheat as seed this fall? Can wheat streak mosaic virus be carried over into the following season from the seed?

Viruses in general are mobile within the wheat plant. They have a hard time getting through the barrier formed between the developing kernel and the rachis and making it into the seed. However, Australian scientists have reported seed transmission by wheat streak mosaic virus (2005 Plant Dis. 89:1048-1050). They report seed transmission rates of 0.5-1.5% from infected genotypes. I don't recall that seed transmission has been reported in the U.S. Therefore, the potential exists to introduce the virus into new areas via seed.  

Obviously, however, without the mite vector, the virus will not spread from the 0.5-1.5% infected plants that develop from the seed. If the mite is present, it most likely will already be a vector of the wheat streak mosaic virus; therefore, the impact of seed transmission within an individual field seems very small.

With regards to using the grain from the field with wheat streak mosaic virus for seed, the major problem will probably be that it will be shriveled. All else being equal, small seed often produces less vigorous plants than large seed. A producer who uses small seed and plants according to bushels per acre will plant more seeds per acre than someone who uses large seed. If the germination rate of the small seed is still high, that tends to compensate for the less vigorous plants that come from small seed compared to large seed, according to my data. But, if the producer is planting by seed number per acre, then it is best to use large, dense seed. The key is whether the seed has high germination and good seedling vigor.