By Emmanuel Byamukama
Winter wheat fields scouted the week of May 27, 2019 in South Central South Dakota were found with wheat streak mosaic virus (WSMV) and tan spot developing at very low levels. Both diseases were found in non-rotated wheat fields. Wheat streak mosaic virus is transmitted by the wheat curl mites (WCM) and the virus overwinters in volunteer wheat and grassy weeds infected in the fall. Tan spot is caused by a fungal pathogen Pyrenophora tritici repentis, which overwinters on wheat stubble.
Wheat Streak Mosaic Virus
Wheat streak mosaic disease symptoms currently being observed are discontinuous light-yellow streaks interspersed with green streaks (Figure 1). However, WSMV symptoms vary depending on the weather conditions and the cultivar planted. The current weather conditions favor mild WSMV symptoms, whereas warmer and dry conditions can lead to severe systemic plant yellowing and stunting.
Figure 1. Wheat streak mosaic virus symptoms on wheat leaves.
Wheat streak mosaic virus cannot be managed once infection takes place. However, preventative practices if implemented before planting are effective in controlling WSMV. These include:
- Rotate wheat crop away from small grain crops to avoid volunteer wheat which is the source of WSMV inoculum
- Destroy any volunteer wheat and grass weeds at least two weeks before planting.
- Select resistant/tolerant winter wheat varieties. See the rating of winter wheat cultivars in the 2018 South Dakota Winter Wheat Variety Trial Results Regional Summaries.
WSMV symptoms should not be confused with nutrient deficiency symptoms currently being found in wheat. Due to excessive moisture in the fields, fertilizer has not been applied to several winter wheat fields and some of these are showing nutrient deficiency symptoms. Plants with nutrient deficiency symptoms have leaves that are light yellow in color and some leaves may have green veins (Figure 2).
Figure 2. Nutrient deficiency symptoms on winter wheat in Hanson County. These symptoms can be confused with virus symptoms.
Tan spot starts to develop in the lower leaves and lesions are characterized by a small dark brown center surrounded by a yellow halo (Figure 3). Continued lesion expansion leads to coalition of the lesions leading to premature death of the infected leaves. Tan spot tends to be more severe in non-rotated wheat fields.
Figure 3. Tan spot symptoms developing in winter wheat.
Tan spot can be managed by:
- Crop rotation to avoid wheat stubble which is the source of inoculum.
- Selection of resistant/tolerant varieties.
- Applying a fungicide when disease pressure warrants it.