Source: South Dakota State University
By Emmanuel Byamukama, Extension Plant Pathologist
Fighting Stripe Rust
Stripe rust is continuing to develop on winter wheat and spring wheat. Several reports throughout the state indicate that this year is the worst we have seen for stripe rust on winter wheat in South Dakota.
If the flag leaf is covered with stripe rust pustules, it may be too late to apply a fungicide as fungicides mainly provide protection against new fungal infections. A few scouted spring wheat fields also had stripe rust beginning to develop.
Although high temperatures (> 75 F) can limit stripe rust infection, with the heavy inoculum pressure in the area, scouting is important in order to keep an eye on the level of development occurring in spring wheat.
If minimal stripe rust is observed on the lower leaves and wheat is flowering, a triazole fungicide at this stage will help with foliar fungal diseases and will also offer protection against Fusarium head blight (FHB, scab). This will minimize the number of fungicide treatments for foliar diseases and FHB. Some cultivars have good tolerance to stripe rust (Figure 1a).
Figure 1. Three winter wheat cultivars showing varying levels of susceptibility to different diseases: a) stripe rust, b) powdery mildew c) no visible disease. All three cultivars were planted the same day in the same field (side by side).
Powdery Mildew
Powdery mildew was found at low levels in a few fields in Brookings County. Powdery mildew favors warm and humid conditions.
The primary inoculum for powdery mildew is infected residues but some inoculum can also be blown by wind from nearby fields.
Some cultivars have good tolerance to powdery mildew (Figure 1b) and several fungicides on the market are effective at controlling the disease.
Fusarium Head Blight Prediction Tool
The national scab prediction tool indicates that the south-west part of South Dakota is at a high risk for FHB to develop in winter wheat (Figure 2). Some isolated spots in north-west, central and south-east South Dakota also have moderate to high risk for FHB.
FHB develops when wheat flowering coincides with rainfall. Inoculum for FHB can come from wheat and corn residue and can also be blown by wind from neighboring fields.
The leaf diseases prediction tool is showing weather conditions to be favorable for foliar disease development to occur in several locations.
When 6-8 “yes” are accumulated (Table 1), this would indicate infection by Tan spot, Stagonospora blotch and leaf rust is likely.
Scouting should then be done and depending on the level of disease and growth stage, a fungicide application should be planned. For leaf rust, it would also depend on whether inoculum has made it to our state from southern states.
Bacterial leaf streak is beginning to develop in winter wheat. This disease is encouraged by warm and humid conditions.
Bacterial leaf streak can be differentiated from foliar fungal diseases by the shiny ooze left on the leaf surface (Figure 3.).
The bacteria that causes leaf streak can also infect the wheat head causing black chaff (brown-black with necrotic streaks) and blotches mainly on the glumes. This disease cannot be managed by applying fungicides!
Use of clean seed, crop rotation and resistant/tolerant cultivars are the recommended management practices.
Figure 2. Fusarium head blight prediction for South Dakota from the national scab prediction tool. Red indicates high risk; yellow moderate risk and green low risk.
Leaf Diseases Prediction Tool
Figure 3. Prediction table produced by the small grains disease tool. Accumulating 6-8 “yes” indicates infection by the diseases in the table is likely.
Bacterial Leaf Streak
Figure 4. Shiny bacterial ooze on the leaf surface is an indicator of bacterial leaf spot. The disease usually starts to develop from the point where the leaf is curled.
