Wheat Scab Showing Up in State

By Jane Fyksen, Crops Editor
Thursday, July 10, 2008 11:06 AM CDT

Wheat head scab, also known as Fusarium wheat head blight, was noted at all four of the UW’s winter wheat variety trials at Janesville, Arlington, Lancaster and Chilton this season. This was not an unexpected discovery given the recent weather conditions during the flowering period. Growers are advised to be on the lookout for head scab in their wheat fields.

The discovery of wheat head scab also matched closely predictions for the risk of scab using the Fusarium Head Blight Risk Assessment Tool. Wheat head scab risk maps for Wisconsin can be obtained from www.wheatscab.psu.edu. Excessively wet weather favors this disease.

Fortunately, both the incidence and severity at the four UW testing locations are relatively low.

This is a good time to conduct a survey because the symptoms of head scab - part green head/part bleached and also salmon-colored heads - can be easily differentiated from other damage including hail and floret abortion, say Paul Esker, UW-Madison field crops pathologist, and Shawn Conley, soybean and small grain specialist.

Symptoms of scab include premature bleaching of one or more wheat spikelets or the entire immature head. Bleaching can start anywhere on the head and spread until the entire head is bleached. With continued wet weather, bleaching of infected wheat heads is expected to continue, which will increase disease severity.

During humid conditions, white or pink fungal growth with orange spore masses may be seen on the bleached spikelets. Blue-black fruiting structures can also form, giving the head a scabbed appearance, hence the name of this wheat disease. Bleached spikelets are either sterile or contain shriveled and/or discolored kernels, commonly referred to as Fusarium-damaged kernels. A high proportion of these kernels results in downgrading of contaminated wheat grain. Fusarium-damaged kernels are lighter than healthy kernels and can be removed or reduced by cleaning or setting the harvest combine to blow out “chaffy” grain, says Stephen Wegulo, University of Nebraska plant pathologist.

How do growers estimate the incidence and severity of wheat head scab? Esker and Conley suggest they go online and to the North Dakota State University website at http://www.ag.ndsu.edu/pubs/plantsci/smgrains/pp1095w.htm and Ohio State University at http://ohioline.osu.edu/ac-fact/0049.html. The North Dakota site has images for wheat varieties that have awns, while Ohio State’s provides images for awnless wheat varieties.

At the UW’s most uniform and prevalent head scab site (i.e. Lancaster), the Fusarium head blight index range is from 0.6 to 8.8 among varieties. This index is based on a range from 0 to 100 where zero is no sign of scab and 100 means all heads in the field are completely infected. A complete list of varieties and their disease ratings will be published along with yield data shortly after wheat harvest in early-to-mid-August (at http://www.coolbean.info).

At this point in the growing season, most winter wheat fields are past the labeled growth stage when fungicides can be legally applied.

“The disease can be caused by several species of the fungus Fusarium and is favored by wet weather during the growing season,” Wegulo reemphasizes. “The fungus also causes stalk and ear rots in corn and seedling blights in cereal grains. It can survive in soil or on corn, wheat and grass stubble. Scab also is more severe in reduced or no-till fields, especially if wheat follows corn.”

“Spores of the scab fungus are carried from fruiting structures on crop residues to wheat heads by air currents. Most infections occur during flowering because anthers and pollen serve as a food source for the fungus. Infected seed can transmit the fungus to emerging seedlings,” he continues. “This can cause severe seedling blight under favorable conditions for disease development. During warm temperatures, blight symptoms on heads appear within two to four days after infection. Therefore, a crop that appeared healthy a few days earlier can suddenly show widespread symptoms.”

Scab can be managed by planting tolerant cultivars, rotating fields out of cereal grains or grasses, and using a fungicide application at early flowering. As noted, fungicides can’t be legally applied to wheat after flowering.

Fungicides labeled for Fusarium head blight (Proline, Folicur, Caramb, and Tilt) can only suppress the disease. They cannot completely prevent disease development. It should be noted that spores of the Fusarium head blight fungus are produced abundantly during wet weather and become airborne. If excessively wet weather occurs before and during flowering, some level of Fusarium head blight will develop even if fungicides were applied at early flowering. Disease levels will be lower in fields that were sprayed with fungicide in a timely manner (at early flowering) compared to fields that were not sprayed, Wegulo explains.

Fungicides are effective in suppressing Fusarium head blight only if applied before infections occur, that is, at early flowering. Once plants are infected and symptoms are visible, it is too late to apply fungicides to suppress the disease, he says.

Mycotoxins concern

In addition to lowering yields and grain quality, the scab fungus also can produce mycotoxins. The most commonly produced mycotoxin is deoxynivalenol or DON. Grain with a DON concentration exceeding two parts per million (ppm) usually will be discounted at the elevator.

Handling scabby wheat takes extra care, say Wegulo and Michael Carlson, diagnostic toxicologist/analytical chemist at the University of Nebraska.

At harvest, increase the fan speed on the combine to remove some of the heavily infected grain, which usually is lighter than healthy grain.

Keep scabby grain separate. Consider keeping grain from heavily affected fields separate. Incidence and severity of scab varies field to field depending on the variety planted and local environmental conditions.

Test for DON. Scabby grain is likely to contain the toxins DON and, to a lesser extent, zearalenone. However, the presence of scabby grain doesn’t necessarily indicate high levels of these toxins - and vice versa, the University of Nebraska experts say. Consider testing grain from affected fields for DON and zearalenone concentration. The sample submitted for toxin testing should be representative of the entire truckload or bin of grain.

If the proportion of scabby grain is high, consider using seed cleaning equipment to remove or reduce scabby kernels. Cleaning doesn’t necessarily reduce DON levels as apparently healthy grain can have elevated concentrations of this toxin.

Be sure to stay safe. When handling grain, wear appropriate personal protective gear such as masks to prevent inhaling mold spores and grain dust which can cause allergy and breathing problems.

Scabby grain should be stored at or below 12 percent moisture to reduce the potential for deterioration during storage. Aerate to cool the grain soon after storage and continue cooling periodically as outdoor temperatures decline, until the grain is at approximately 25 degrees. Fungal growth and DON production stop after grain dries to a moisture content of 22 percent. Thus, DON production will not occur under the recommended storage conditions.

If possible, DON-affected grain should be kept separate from healthy grain. The marketing strategy for this grain will be influenced by many factors, including DON level, cleaning and/or blending costs, and contract obligations with elevators. In general, elevator discounts are highest at harvest and increase with the concentration of DON above two ppm. “Therefore, weigh the pros and cons, including the economics, of deferring DON-affected contracted wheat in the hope that discounts will reduce with time. Deferring delivery also gives time to clean and/or blend the wheat to improve quality,” advise Wegulo and Carlson.

Scabby or DON-affected grain can be used as livestock feed. For recommendations on using feed contaminated with DON or zearalenone, consult the University of Nebraska’s publication, “Fusarium Head Blight of Wheat” (EC1896) at http://www.ianrpubs.unl.edu/epublic/pages/index.jsp?what=publicationD&publicationId=1018.

To prevent or reduce seedling blights, scabby grain should be thoroughly cleaned and treated with fungicide before being used as seed for next season’s crop. Its better policy, though, not to save any seed from scab infected fields for replanting. Decreased seedling vigor and fall tillering coupled with increased potential for winterkill are associated with scabby wheat seed.

The UW team says severely damaged grain may be subject to price discounts at delivery and loads may be rejected. At most elevators, graders will look for scab-infected kernels and treat them as damaged kernels. USDA No. 1, 2 and 3 wheat can have up to 2, 4 and 7 percent damage respectively. If scab damage is high, the elevator may test the wheat for DON.

The UW experts stress that having scab doesn’t automatically mean grain will have DON, and, they note, healthy-looking kernels can test positive for DON.

The UW specialists contacted several elevators in Wisconsin last week and found they were in the process of developing plans for scabby wheat. The Port of Milwaukee exporter indicated they’d deal with scab during the season if they see a large incidence of diseased grain, but they weren’t expecting a problem.

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