July 19, 2007

Figure 1. Scab on wheat heads.

Figure 2. Pink discoloration on a wheat head representing sporulation of the scab fungus.

Figure 3. An irrigated field with scab.

This year Nebraska wheat growers have had to deal with a disease known as scab, or Fusarium head blight. The disease is caused mainly by the fungus Fusarium graminearum, but can also be caused by other species of Fusarium.. Scab occurs on all small grain cereal crops. Infection of wheat heads occurs mainly during flowering. The same fungus that causes scab also causes stalk rots on corn. It overwinters on residue of host crops and can survive on wheat grain. Scab usually is more severe when wheat follows corn because the causal fungus survives better on corn residue than on residue of other host crops.

Symptoms

Scab starts as small water-soaked spots on glumes. These brownish spots spread in all directions on the head and can start in the middle, at the base, or at the top of the head (Figure 1). Eventually the entire head can appear bleached. During humid conditions, a white or pink fungal growth with orange spore masses may develop on glumes (Figure 2). Dark fruiting structures known as perithecia may develop, giving the head a scabbed appearance, hence the name scab.

Favorable Conditions

Scab is favored by wet, warm conditions. During periods of continuous moisture and temperatures ranging from 77°F to 86°F, head blight symptoms can appear within three days after infection. Hence, a crop that appeared normal several days earlier can suddenly show widespread symptoms. Infections also can occur at lower temperatures during prolonged periods of continuous moisture.

This year excessive rainfall and favorable temperatures in south central and eastern Nebraska before, during, after flowering contributed to scab epidemics in wheat fields.The risk of scab development is higher in irrigated (Figure 3) than in rain-fed fields.

Implications

There are several implications of scab. Floret sterility and poor grain fill can result in significant yield losses. Scabby grain reduces test weight and lowers grain quality, which can lead to discounts at elevators. The species of Fusarium that cause scab produce harmful mycotoxins including deoxynivalenol (DON), also known as vomitoxin, and zearalenone. The more common of these toxins is vomitoxin, and its presence in grain can be more important than the yield loss resulting from scab.

It is important to note that grain from fields that had scab can be healthy looking and yet contain unacceptable levels of vomitoxin. The only way to be sure of the level of vomitoxin in grain is to test the grain. Testing is best done by a specialized lab or service. Commercial test kits also are available. To obtain accurate results using these kits, carefully follow instructions.

Vomitoxin Levels

Vomitoxin levels in wheat grain delivered to Nebraska elevators has ranged from 0 to 16 ppm, with levels from severely affected areas averaging 4 to 6 ppm.

Elevator Discounts

Discounts taken at the elevator usually are dockage, foreign material and contrasting classes. Dockage consists of any non-wheat material that can be easily removed from grain. Foreign material consists of all non-wheat material that remains after removing dockage and shrunken and broken kernels. Grain with contrasting classes is grain contaminated by volunteer wheat grain of a different class.

In addition to the above discounts, grain can be discounted if it contains vomitoxin levels above 2 ppm. Discounts due to vomitoxin will vary among elevators. Some elevators may reject grain altogether if vomitoxin levels exceed 2 ppm.

Recommendations for Grain with Vomitoxin

• It is not practical to remove vomitoxin once it is present in grain.

   

• Grain contaminated with vomitoxin should be stored separately from uncontaminated grain.

   

• Determine how much vomitoxin in ppm is present in your grain, by submitting samples to a specialized lab or service. Some testing kits are set at a given vomitoxin level and give a negative result if the level is below the set level and a positive result if it is above. These kits cannot determine the actual level in ppm, which is what you want in order to determine how the grain can be used should it be rejected at the elevator.

   

• Grain with vomitoxin levels that are not excessively high can be blended on-farm with uncontaminated grain to lower the vomitoxin level.

   

• Contaminated grain can be fed to animals that are less sensitive to vomitoxin. 

For more information on scab and how to manage it, see the June 22 issue of CropWatch.

Stephen Wegulo
Extension Plant Pathologist