Texas farmers remaining vigil about soybean rust

By KARI KRAMER | East Texas Edition

Extension plant pathology leader, Dr. Joe Krausz, delivers key points about the threat of soybean rust on Texas crops during a special meeting held in Lamar County. -- Photo by Kari Kramer

March 24, 2005 - In reaction to the recent spread of soybean rust across the southern region of the United States, Texas Cooperative Extension is attempting to educate their state's producers on the disease.

Dr. Joe Krausz, Extension program leader for plant pathology from Texas A&M University, spoke for over an hour to Texas producers on the origin, diagnosis, prevention, and management practices of soybean rust.

TCE-Lamar County recently hosted a program with the primary focus being soybean rust. Northeast Texas is a key point for soybean rust because of the area's close proximity to Louisiana, where soybean rust was discovered in 2004. The disease could easily spread throughout the entire state of Texas.

Krausz noted that in 2001 soybean rust reached South America, and in 2004, after hurricane Ivan, the disease surfaced in the United States.

"Spores can be moved thousands of miles. Ivan brought soybean rust to the United States," Krausz told the producers.

Soybean rust is caused by the fungus Phakopsora pachyrhizi (Asian soybean rust) and is spread through urediospores that are released from an infected plant. The spores are capable to traveling thousands of miles and contaminating hundreds of fields.

Soybean rust usually first develops on the lower leaf surface. Brown, tan, or reddish lesions will appear on the leaf's surface. Later, rust infected leaves turn yellow and fall off the plant. Soybean rust is not normally seedborne, according to Krausz, who added the seed does not normally carry the disease well.

After a plant has been exposed to soybean rust, if the conditions are ideal, the disease will thrive and pustules (bumps) will develop in five to 10 days. Pores will develop in 10 to 21 days and begin spreading spores.

"In a matter of a couple of weeks it can really really explode," said Krausz.

He added, "It's important to understand the disease cycle, to understand how one spore can go to thousands of spores."

For soybean rust to thrive, temperatures must be between 60 and 80 degrees Fahrenheit, humid, and the disease needs six hours of wetness to germinate. These are normal conditions for Northeast Texas and several other areas of the state. Soybean rust will not survive freezing temperatures, a fact that could offer East Texas some protection, according to Krausz.

"Diagnosing this thing is going to be a real challenge. There are a lot of leaf spots that look like this," said Krausz.

"The most early symptoms are little small freckles of yellow tissue," said Krausz, who then added a producer should next look for the pustules that form on the leaf's surface. According to Krausz, the lower leaves are in a thicker canopy and the dew sticks longer, providing the additional hours of wetness that soybean rust needs to germinate. Once the pustules mature (in a weeks time), pores will form and release the urediospores.

Krausz noted that several common soybean diseases and infections mimic soybean rust, but there are ways to tell the difference.

"What we're going to have to do is keep our eyes and ears open," suggested Krausz.

Bacterial pustule (not common in Texas), bacterial blight, frog-eye leaf spot, brown spot, and downy mildew can all have comparable characteristics to the beginnings of soybean rust.

According to Krausz, bacterial pustule will have cracked pustules, unlike the volcano shaped pustules/pores that develop with soybean rust. Bacterial blight does not have pustules. Downy mildew is a common problem for Texas soybean producers and reoccurs annually in the state. Unlike soybean rust, downy mildew has no pustules and is usually identified by the fuzz found on the infected plant.

Looking for volcano shaped pustules with spores is the best way to determine if soybean rust is the problem, the pathologist summarized.

Krausz advised producers to spend the greatest amount of time searching for soybean rust in areas that would serve as a better host. Shaded areas that provide more moisture, areas of the field that may hold additional water, and any area that promotes a longer period of leaf wetness are all areas, Krausz warned, that could encourage the disease to thrive.

If a producer believes they may have soybean rust, Krausz encourages producers to contact their county Extension agent, a plant pathologist, or a crop consultant. A sample should be collected, preferably placed between two paper towels or pieces of paper to keep the leaf flat and placed in a self-locking plastic bag. According to Extension documents, "Care should be taken to ensure the outside of the bags are not contaminated with spores."

Next, the date, exact location of the sample, the field location, county, and collector's name, address, and phone number should be recorded. The sample and a $30 fee should be sent to the Texas Plant Diagnostic Laboratory. The forms are available online at:

www.plantpathology.tamu.edu/extension/tpddl/forms.asp.

Fungicides have been found to minimize the effects of soybean rust. Fungicides, Krausz ad-dressed, should be applied up to twice yearly, during the reproductive (R3-R5) stage, or earlier if local reports indicate a rust problem in the area, and a second application should be administered 20 days after the first application.

Before choosing to use a fungicides, Krausz advised producers to take into account the rust pressure in the region, pressure of soybean rust in the field, the stage of plant growth, weather conditions and forecast (to determine if temperatures and conditions favor the spread of rust), yield potential and the price of soybeans. The fungicide must pay for itself.

There are several fungicide registered for soybean rust: Quadris, Bravo (not a systemic fungicide), Equus, and many Section 18 emergency fungicides such as Tilt, Propimax, Bumper, Laredo, Folicur, and Headline. These fungicides, according to Krausz, will also help manage other diseases.

Krausz noted that a systemic fungicide (a fungicide that is taken into the plant) is a better choice because it is not washed off by water. The systemic fungicides, according to Krausz, managed the rust better at different stages, but all produced relatively the same yield. Krausz indicated that more information on fungicides for soybean rust could be found at www.ipmcenters.org.

Krausz told producers that ground applications average about $4.50 per application but can reach $10. Air applications average $6.50 per application but can reach $12.

Krausz added that in the R5 stage, an air application will usually not penetrate the lower canopy and therefore a ground application may prove more beneficial.

With fungicides, Krausz advises alternating or mixing with other chemicals in certain cases in order to prevent the disease from becoming resistant.

"We're going to have to fool the fungus from developing resistance," said Krausz. "Ultimately we're going to have to develop rust resistant varieties."

Krausz left producers with the final thought that soybean rust can be beaten, but "it's going to take some scrutiny and real careful decision making."

For more information on soybean rust, contact the Extension agent in your county.