Add wheat streak mosaic virus to the list of possible causes of yellowing wheat in Oklahoma. Wheat streak mosaic virus is transmitted by the wheat curl mite, which oversummers on grasses such as volunteer wheat and corn. The wheat curl mite cannot survive more than two weeks without a green host, hence the recommendation to make sure that all grass plants are dead two weeks prior to planting. You can find more information on the wheat curl mite and wheat streak mosaic in OSU Fact Sheet EPP-7328 – Three virus disease of wheat in Oklahoma
The photo below is from our wheat variety trial at Kildare. As you can see there is significant yellowing in some of the plots. Our first thoughts were that either wheat soilborne mosaic virus and/or wheat spindle streak mosaic virus were causing the symptoms; however, the yellowing was present in many varieties that are resistant to both these diseases. The Disease Doctor, Bob Hunger, collected samples for analysis in the OSU Plant Diagnostic Lab. Tests showed that wheat streak mosaic was the culprit.
We are facing this problem because I did not follow my own recommendations. While the plot area was mostly clean at the time of planting, there was some volunteer wheat present. We planted anyway and sprayed glyphosate right after planting. In the past wheat streak mosaic virus was primarily a northwestern Oklahoma issue and we could get by with late burndown on wheat ground in central Oklahoma. Our Kildare plots are a prime example that this is no longer the case. We have to control volunteer grasses (wheat, corn, grain sorghum, etc.) in a manner that will break the green bridge for at least two weeks prior to planting.
So, what is next for our plots at Kildare? There are some variety differences in reaction to the wheat streak mosaic virus. We will rate plots and include this information in our wheat variety comparison chart. I anticipate the plots will continue to go downhill and it is yet to be determined whether or not we will be able to harvest useable data from the location. We will certainly try again next year and apply our turndown earlier.