In several of the winter wheat variety plots across South Dakota, the varieties rated as susceptible to scab (Fusarium Head Blight – FHB) showed noticeable incidence of infection. A recent survey conducted by SDSU Extension Agronomists also found levels of scab ranging from less than 10% in western South Dakota to over 80% in one field in the eastern part of the state, and in the 10-40% range in the south-central area.
The extent of infection varied from one field to another where scab was found. Variety resistance partially explains the difference between levels of infection from field to field in the same area. Difference in maturity between varieties also influences the level of scab infection as one variety may have been flowering during a period of wet weather while another may have flowered during drier conditions. Planting date also affects flowering date. The Fusarium Head Blight Prediction Center offers potential scab risk based on weather conditions. Fungicide applications during flowering are about 50-70% effective in controlling scab, and may also explain differences in the level of infection between fields. One of the main factors however, is the previous crop, and residue the wheat is planted into. Corn residue is the highest risk, with sorghum, millet, wheat, barley and oats following in importance.
The bottom line is that many wheat producers will be dealing with a scab infected crop during harvest. The best initial management strategy is to adjust the combine settings to throw as many of the scab infected kernels out the back of the machine as possible, while keeping as much good wheat as you can. The fungus sometimes moves to the center stem (rachis) of the head and causes “scab affected kernels” or “Fusarium damaged kernels.” These kernels may not be “scabby”, but shriveled and shrunken because the fungus killed the rachis and cut off the moisture and nutrient supply to the upper part of the head before those kernels were able to fill.
Scab reduces both yield and test weight, but the most devastating effect is the level of deoxynivalenol or DON (vomitoxin) that is often associated with scab infected grain. The presence of DON may result in substantial price discounts at the elevator and even rejection of purchase if the levels are high. While there is no standard for raw grain going into the milling process, the Food and Drug Administration (FDA) has established the level of 1 part per million (ppm) as a limit for finished grain products for human consumption. Higher limits exist for feeding to various species of livestock. Reducing the scabby kernels in a load of wheat will lower the DON level as well.
Crop residue such as straw is considered to be of value to be left in the field for organic matter, protective cover, and fertilizer nutrients. Baling straw behind the combine to be sold for livestock forage is seen as another source of revenue by some producers. This practice should be approached with caution when the crop is infected with scab. Research has shown that DON levels in straw can accumulate at much higher levels than found in the grain. Ten ppm is the highest level of DON suggested for grains and byproducts to be fed to any class of beef cattle, so testing straw from scab-infected wheat fields for DON is recommended. Some studies found DON levels in the straw in excess of 50 ppm, where the DON in the grain was closer to 10 ppm.