Disease Update: Scab Reported in Late Maturing Wheat
Figure 1. Scab in a grower’s field in Lincoln County on June 30.
Figure 2. A wheat head partially bleached white by the scab fungus.
|Figure 3. (left) A wheat head partially bleached tan-brown by the scab fungus.|
July 2, 2009
Frequent rains prior to and during flowering increase the risk of wheat scab (Fusarium head blight), especially in late maturing wheat. In addition to frequent, heavy rainfall before and during flowering, other risk factors for scab are planting wheat into corn or wheat stubble and planting susceptible varieties.
Figure 4. Pink discoloration on a spikelet of a wheat head caused by sporulation of the scab fungus.
Most of this year's wheat crop escaped severe scab due to a prolonged dry period in May. However, frequent rainfall starting in late May and continuing through much of June raised the risk of scab in later maturing wheat, especially in western Nebraska. Some isolated fields are showing severe scab (Figure 1).
Scabby wheat heads are bleached white (Figure 2) or tan-brown (Figure 3), depending on the variety. Partial bleaching of wheat heads is diagnostic of scab. Bleaching usually starts in the middle of the head, but can start anywhere on the head and continue until the entire head is bleached. Scabby heads can appear in an entire field or in large areas of a field within a few days. A pink discoloration (Figure 4) on some scabby heads represents spore masses of Fusarium graminearum, the fungus that causes scab.
Figure 5. Scabby wheat kernels (left) and healthy kernels.
Scabby kernels can be shriveled or can appear chalky white or pink (Figure 5). These kernels are lighter than healthy kernels and may contain vomitoxin (deoxynivalenol or DON). Grain with a DON concentration greater than 2 ppm usually will be discounted at the elevator.
Recommendations for This Year
- Grain quality can be improved by increasing fan speed during harvest to blow out the lighter scabby kernels.
- Harvest and store grain from severely affected parts of the field separately.
- The moisture content of scabby grain should be about 12% before storage and should be maintained at that level to avoid deterioration of grain from fungus growth.
- Consider cleaning scabby grain. Cleaning can be achieved with air, screens, or a gravity table. To justify cleaning, the price of cleaned grain minus cleaning costs should exceed the price of scabby grain.
- Do not burn crop residue as this practice will not eliminate soil borne pathogens, depletes the soil of nitrogen, and increases the risk of wind erosion.
Recommendations for Next Growing Season
- In the fall, plant certified, fungicide-treated seed.
- If grain from fields with scab must be used as seed, treat it with a systemic fungicide before planting. Planting fungicide-treated seed will not prevent scab on wheat heads the next spring, but it will protect the seedlings from seedling blights caused by the scab fungus and other soilborne fungi.
- Avoid planting wheat into corn stubble. The scab fungus causes stalk and ear rots in corn and although the fungus can survive on stubble of small grains and some grasses, it survives best on corn stubble.
- Avoid planting wheat cultivars that are highly susceptible. No cultivars are highly resistant; however, some cultivars have good levels of tolerance.
- Consider applying a fungicide at the beginning of flowering if weather conditions favor development of scab. Fungicides registered for control of scab include prothioconazole + tebuconazole (Prosaro) and metconazole (Caramba).
To learn more about wheat scab, how to manage it, and the associated vomitoxin, see the Extension circular EC1896 Fusarium Head Blight of Wheat.
Extension Plant Pathologist