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Fig. 2. Wheat seed with chalky white or pink scabby seed. (photo by Bill Willis).

Fig. 1. Wheat head 
 partially attacked by 
scab. 
(photo by Bill Willis)

Wheat scab (also known as Fusarium head blight) is caused by a fungus called Fusarium graminearum. It is a common problem in Europe, Asia, South Africa, and the Midwestern and Eastern U.S.A. Scab severity is very erratic and depends heavily on wet weather conditions. Scab can reduce both yield and quality of the grain. In Kansas, serious scab problems are usually limited to the eastern third of the state.

SYMPTOMS

Scab is usually first detected soon after flowering. Infected heads turn white, but leaves and stems may remain green. Sometimes, the portion of the stem directly below the head is turned a chocolate brown color. Often, only part of the head is attacked, giving heads a white and green appearance (Fig. 1). Close examination of spikelets often reveals a salmon-pink fungal growth on the glumes or at the base of the spikelets. Scabby wheat kernels are shrunken and chalky or pink in color (Fig. 2). Kernels above the infected spikelet will be shriveled if the fungus penetrates the rachis and cuts off the nutrient supply.

LIFE CYCLE

The scab fungus infects wheat, corn, oats, barley, and sorghum, and probably various other grasses. On sorghum, it causes stalk rot. On corn, the disease is called ear rot and it infects through the silks. Corn appears to be the major source of overwintering inoculum which starts the epidemic in the spring. However, other cereals may also provide some inoculum. Infection of wheat occurs through the anthers during warm, rainy weather. If conditions are dry during flowering, infection will not occur. Although the fungus can be carried in the seed, it appears that crop residue is the major source of inoculum.

YIELD LOSS

Yield loss is related to the percent of heads which are infected and the severity of infection. A rough estimate of loss can be made by collecting a random (no peeking when selecting heads) sample of 100 heads in a zigzag pattern across the field. Sort the heads into three piles: 1) healthy; 2) partially attacked by scab; and 3) completely attacked by scab. Estimated yield loss percent - (number of completely scabby heads) + 1/2(number of partially scabby heads). This method can be improved by hand- threshing some heads from each pile and adjusting the estimate up or down by comparing to the seed yield of healthy heads.

QUALITY LOSS

Scab often causes reduction in test weight, sometimes down to near 50 lb/bu. In addition, scabby kernels count as "damaged" in the grading process. These factors often combine to lower the grade of the wheat. Low-grade wheat must be blended with higher- grade wheat; this results in higher costs. Therefore, scabby wheat often suffers significant discounts. In that case, scabby wheat might be more profitably used as animal feed.

ANIMAL FEEDING PROBLEMS

The relative feed value of scabby wheat is often very good. However, scabby wheat may contain the mycotoxins DON (vomitoxin) and zearalenone (an estrogen analog). Neither of these toxins is considered carcinogenic or highly toxic, but they both can reduce the performance of livestock.

Since the mycotoxins are concentrated in the grain, baled wheat straw from affected fields should be safe for most uses. However, it should probably not be used for breeding livestock.

Swine are most sensitive to scabby wheat mycotoxins. As little as 1 ppm of DON can significantly reduce daily weight gains in pigs. Higher concentrations result in feed refusal and vomiting. Swine reproduction is also sensitive to disruption by the zearalenone mycotoxin. Non-breeding cattle and poultry seem to tolerate both toxins better than swine. If scabby wheat is used as feed, it should be tested for both DON and zearalenone content. Testing is available from the Comparative Toxicology Lab, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506. Phone (913) 532-5679. Submit at least 1/2 pound of a representative sample. Both grain and straw can be tested. It's best to take as many subsamples as possible and bulk them. The lab can make recommendations for appropriate dilution of feed which has high toxin levels. There is a $50.00 fee for this quantitative test, which includes DON, zearalenone, and four other common mycotoxins.

CONTROL

Some varieties like 2137 and Agripro Tomahawk are extremely susceptible to scab. These should be avoided in the scab prone areas of northeast and southeast Kansas. Agripro Hondo and the hard white wheat cultivar Heyne are moderately resistant to scab. They are able to resist spread of infection from the point of original attack. Karl 92 has intermediate resistance. No variety is considered highly resistant. You can spread your risk of infection by choosing several different varieties with different maturities because some will probably flower during conditions unfavorable for scab.

Foliar fungicides Tilt® and Folicur® are being used to combat scab in North Dakota and Minnesota as heading time applications.  Tilt (but not Folicur) has a section 24(c) special local needs label in Kansas that allows application up through the fully headed stage of growth. If rainy conditions are forecast, it is possible to apply Tilt during heading to reduce the risk of scab. Control results depend on getting excellent coverage of heads and that is difficult to achieve without a ground rig application.

Crop rotations that avoid planting wheat into corn stubble may be helpful in moderate epidemic years. However, in severe years, rotation makes little difference. Likewise, plowing-down corn crop residue may be helpful in some years but not in others.

Seed treatments are not effective in preventing head blight, but they may prevent seedling blight if scabby wheat must be used as seed. Several commercial treatments may modestly increase germination of scabby seed. Scabby seed should be cleaned very hard and germination should be tested prior to use.

At harvest, the percentage of scabby kernels can be reduced by setting combine fans higher during harvest. This will help blow the light, scabby seeds out the back of the combine.