UG99: A future threat to U.S. wheat growers

Wheat growers in the upper Southeast have had a tough year — disease decimated harvests last spring, fall rains prevented planting on a big scale and early spring drought has added to the woes.

If that plate of trouble isn’t enough, wheat farmers around the world are being urged to keep a watchful eye out for UG99, a nasty variant of the stem rust fungus that once ravaged wheat crops across the country.

UG99 was first detected in Uganda in 1999 — hence the name — in eastern Africa. It has since spread to other African countries and was recently found in Iraq, Iran and Afghanistan. UG99 isn’t alone, there are a number of other variants of stem rot popping up around the world.

Speaking at a recent Virginia Field Day, veteran Virginia Tech plant pathologist Erik Stromberg notes getting UG99 in the U.S. is likely more a matter of when, not if.

“With all the military personnel we have circulating through the Middle East, it’s going to be difficult, if not impossible, to prevent the disease-causing spores from coming back to the U.S on equipment or clothing,” Stromberg says.

Stromberg notes that scientists in the U.S. are treating the threat from UG99 much like they did the threat from Asian Soybean Rust. Many of the same sentinel plots used to monitor the progress of Asian Soybean Rust will be used to monitor the movement of UG99 and other variants of stem rust.

Historically, stem rust has been a significant disease of wheat and barley in the Southeast and throughout grain producing regions of the country. Over the past 20 years or more, highly successful breeding programs have produced wheat and barley varieties with resistance to the disease-causing fungi.

“Over the past few years there have been sporadic outbreaks of stem rust throughout the U.S. In all these cases, the culprit has been new races of the fungi that can overcome plant resistance to the subsequent disease,” Stromberg says.

One of the Virginia Tech researcher’s biggest fears isn’t that UG99 will make it to Virginia and other Southeastern states, rather a big concern is continuation of funding for sentinel plots and similar projects that are quite capable of keeping farmers aware of threats from the disease and to make recommendations for controlling it in an economically feasible way.

“We have fungicides in place that will protect plants from the fungus. The problem is knowing when or whether to use these materials and simply the cost of using fungicides on grain. The bigger long-term problem is developing new varieties that are resistant to UG99 and similar variant strains of the disease-causing fungi,” Stromberg says.

A first start for growers, Stromberg contends, is being able to differentiate among the three most common wheat and barley rust diseases in the upper Southeast.

The three rust diseases — stem rust, stripe rust and leaf rust — aren’t simple to identify and distinguish from one another, but with a little effort and training it can be done fairly easily, Stromberg says.

As part of the rust education effort, the Virginia Tech researcher and his colleagues have developed an illustrated Wheat and Barley Rust Identification Guide that can be obtained by contacting the Plant Disease Clinic in the Department of Plant Pathology, Physiology and Weed Science at Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061-0331.

Stem rust, as the name implies, attacks primarily the wheat or barley stems. It is commonly found on leaf sheaths and blades and in severe cases can infect grain heads.

Stem rot is characterized by orangish, red blisters caused by oval-shaped or elongated lesions on both side of plant leaves.

Leaf rust most commonly occurs on the leaf blade. Infections sometimes occur on leaf sheaths and very rarely occur on the stems of plants or on the grain head.

Lesions caused by leaf rust are brown — and rarely show the orange coloration of stem or stripe rust lesions. These lesions are more round and uniform in shape than the other two diseases.

Stripe rust most commonly affects the leaf blades. In severe cases it can affect leaf stems and grain heads.

Lesions caused by this fungus are small and round and merge to form stripes on both sides of the leaf blade. These lesions are more yellow, but have a slight orange.

“All three diseases have unique interactions with common varieties of wheat and barley. These interactions can modify the disease symptoms, resulting in reduced lesion size and varying amounts of tan and yellow tissue surrounding the lesions,” Stromberg explains.

The amount of yield damage these diseases can cause in wheat and barley is highly variable. Anytime the leaf surface is damaged, some level of photosynthesis is affected. How much depends on growth stage, other stresses, and varietal susceptibility to name just a few.

Needless to say, wheat and barley farmers in the upper Southeast don’t need UG99 or other disease variants. Wheat production for the 2009-2010 season may end up dropping more than 50 percent due to the combined effects of disease, flooding and drought during critical head development stages.

Keeping a watchful out for UG99 and similar disease variants is critical rebuilding the losses in wheat production.

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