Article by Emmanuel Byamukama with contributions from Marie Langham and Bob Fanning.
The majority of winter wheat in South Dakota is at the flag leaf stage with some fields already flowering. Inspection of winter wheat fields in Douglas, Charles Mix, Tripp, Potter, Hughes, Brookings, and Lincoln counties indicate a few fields with significant disease development in the lower canopy. Generally, however, the majority of fields had minimal to no disease developing on wheat. Tan spot, powdery mildew, and bacterial leaf streak were the most frequently observed diseases. These diseases are favored by wet conditions, like we have had in the past few days. The causal pathogens for the above diseases survive on wheat stubble, although bacterial leaf streak can also survive in the soil and on the seed.
Wheat streak mosaic virus was the other disease that was found in winter wheat. Virus symptoms ranged from mild for several fields to severe stunting (one field, Figure 1). Previous surveys indicated that Wheat streak mosaic virus (WSMV) is the most common virus detected in wheat in South Dakota. However, other viruses have also been detected in South Dakota including Triticum mosaic virus (TriMV), High Plains virus (HPV), and Barley yellow dwarf virus (BYDV). Three of these viruses, namely WSMV, TriMV and HPV, have the same vector, the wheat curl mite (WCM), (Figure 2). BYDV is transmitted by aphids. The WCM are very small and can only be seen with magnification. WCM feed and reproduce in the rolled wheat leaf blades and are favored by warm and dry conditions. During the summer, as wheat matures, they move to the exposed leaf surface and are blown by wind to other hosts including corn, sorghum, grass weeds and volunteer wheat. Viruses transmitted by WCM produce similar symptoms so that it is hard to tell them apart based on visual symptoms. These symptoms
can also be confused with herbicide injury. Normally, herbicide injury initially causes yellowing and eventually the plants die. Herbicide injury should also affect plants uniformly in the field or at least according to a drift or spray pattern. Virus infection does not kill the plants, and infected plants may either have a gradient (symptom severity decreases from the edge of the field) or may be scattered throughout the field. Virus infected plants are usually stunted, produce fewer tillers, and the tillers grow close (prostrate) to the ground.
Unfortunately, once virus infection has taken place, there is nothing that can be done to manage the disease. For viruses transmitted by the WCM, which are the most predominant and damaging, infections in fall are the most important, in terms of causing significant yield loss. WCM-transmitted viruses can be effectively managed through elimination of volunteer wheat and grassy weeds. The WCM survives from one season to another on the so called 'green bridge' (volunteer wheat and grassy weeds) (Figure 3). Therefore, destroying this green bridge between seasons will control WCM and ultimately WSMV. Planting of wheat in fall should be delayed until late September to early October or at least two weeks after volunteer wheat and grass weeds have been sprayed with a herbicide.
Another effective management strategy for viral diseases is planting host resistant or tolerant cultivars. Evaluation for WSMV resistance/tolerance among the common winter wheat cultivars has been ongoing at SDSU. Evaluation of the cultivars includes symptom severity, stunting, delay in heading, yield loss, and loss in test weight. Wheat cultivars with the lowest yield losses in last year's WSMV nursery include: Art, Camelot, Jerry, Mace, McGill, and Robidoux. Mace has shown field resistance during testing in Nebraska and Kansas. Some varieties that have been evaluated multiple years (since 2007) and performed in the group with the lowest yield losses include: Expedition, Jerry, Hawken, Overland, Tandem, and Wendy.
At this growth stage (heading through flowering), a disease to watch for is Fusarium head blight (FHB, scab). The pathogen that causes FHB, Fusarium spp, survives on corn and wheat residues. The most significant impact by FHB is when infection takes place between head emergence and shortly after flowering. Warm and moist weather conditions favor the disease development. In addition to causing direct yield loss through bleached heads and scabby seeds, Fusarium spp also produce a mycotoxin, deoxynivalenol (DON). Elevated levels of DON in grain can lead to rejection at grain elevators.
A well timed fungicide application can reduce FHB and DON accumulation in grain. For fields with a history of FHB, wheat following wheat or corn, risk of FHB may be high, depending on weather conditions during heading. The national Fusarium Prediction Center provides predicted risk for FHB based on weather conditions. Growers are encouraged to visit the site and zoom in to the weather station nearest to them for predicted FHB risk. The triazole class of fungicides (such as Prosaro or Caramba) are recommended for managing FHB (consult the fungicide label for rates and pre-harvest interval).