Assess for Scab in Winter Wheat

A number of winter wheat fields in central South Dakota were scouted last week. Fusarium head blight (scab) severity was at low to moderate levels in several fields. Very few fields had severe scab. The level of scab in a wheat field depends on whether flowering coincides with rainfall, the susceptibility of the cultivar planted, and whether a fungicide is applied at flowering. Winter wheat is at ripening in most fields and this is when scab symptoms are most obvious. Once wheat starts to senesce and dry up, it may be difficult to differentiate between scab infected and non-infected wheat heads.

It is important to assess the level of scab in the field in order to determine the best approach to use when harvesting affected fields. It is also important to differentiate between bleached heads due to scab or due to other causes like insect damage or crown and root rots. For scab infected heads, look for bleached spikelets or entire heads with the peduncle still green. Occasionally, infected spikelets may have a pink-orange mass of spores at the base of the spikelet. For insect damaged plants or plants infected with root and crown rot pathogens, the entire plant may be killed and/or the peduncle is also bleached.

In addition to causing direct yield losses from shriveled scabby seed, the scab fungus also produces mycotoxins (mainly deoxynevalenol [DON]). Depending on the level of DON in grain, this can lead to dockage or even rejection of the grain at the elevator. The federal guidelines for acceptable DON levels in grain depend on the final use of the grain (human or animal consumption) and range from 1.0 ppm to 10.0 ppm.

Approaches for reducing scabby grain while harvesting infected fields include:

• Adjusting the combine fan speed to blow out scab infected kernels. Scab infected kernels are lighter than healthy kernels.
• Harvesting fields with high scab last to avoid contaminating grain from fields with less scab.
• Extra cleaning may be required if harvested grain is still scabby.
• Blending scabby grain with less infected grain to lower DON concentration.

Source: Emmanuel Byamukama, South Dakota State University