Winter wheat fields scouted this week in Brookings, Beadle, Hand, Hughes, Lyman and Tripp Counties show varying growth stages from jointing to flag leaf. At flag leaf growth stage, a fungicide may be applied to protect the flag leaf from fungal diseases because the flag leaf is the largest contributor to grain yield. With a few exceptions, there is a very low level of fungal diseases developing in winter wheat at this time.
The main disease observed in winter wheat this week was tan spot. Tan spot is caused by a fungal pathogen Pyrenophora tritici repentis. This pathogen survives on wheat stubble and is observed more frequently early in the season when wheat is planted into the previous year’s wheat stubble. Although we have had conducive weather for tan spot to develop, the level of this disease varies a lot across fields (Figure 1 and Figure 2).
Figure 1. Winter wheat plants in a field in Brookings with lower leaves having moderate to severe tan spot symptoms. Notice the wheat stubble, which is the source of tan spot pathogen inoculum. Picture taken 5/21/2018.
Figure 2. Winter wheat plants in a field in Tripp County with no fungal diseases observed as of 5/21/2018. Such a field would not benefit from fungicide application at this time.
Growers are encouraged to scout their fields in order to determine the need for a fungicide at the flag leaf timing. If tan spot is not observed on the 2nd leaf below flag leaf, the risk of tan spot would be low. Growers can use the disease risk tracking tool to determine the likely risk for fungal leaf diseases. A list of fungicides and their efficacies against wheat diseases can be found in Management of Small Grain Diseases: Fungicide Efficacy for Control of Wheat Diseases. This information is put together by the North Central Region Wheat Pathology Working Group.
The other disease observed on winter wheat was wheat streak mosaic (Figure 3). This disease was observed in only two fields and also at a very low level. Wheat streak mosaic is caused by Wheat streak mosaic virus and is transmitted by wheat curl mites. Unlike the last two years, this past winter was very cold and longer, therefore the wheat curl mites were not as abundant and active after winter wheat emerged last fall. Wheat streak mosaic disease can only be managed preventatively. Once plants are infected, nothing can be applied to manage the disease. This disease is best managed by destroying volunteer wheat and grassy weeds before planting winter wheat in the fall. Crop rotation with a broad-leaf crop can also help to break the disease cycle.
Figure 3. Mild Wheat streak mosaic virus symptoms on a wheat leaf.