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Early rust increases soybean risk

Asian soybean rust has been found on actively growing soybeans 3-4 weeks earlier than in recent years in southwest Alabama and Mississippi, which may mean problems for thousands of acres of late-planted, full-season beans and all of the Upper Southeast double-crop beans.

John Mueller, long-time soybean rust monitor and professor of plant pathology at Clemson University’s Edisto Research and Education Centers says, “Our normal first find for rust has been between Aug. 15 and Sept. 15 in South Carolina. We have also been very wet but have seen no signs of rust yet.

“Despite rust being ahead of schedule in general for the South we should not get too worried yet. A week or two of drought could slow down the spread of rust and put us back closer to normal.”

However, there is one concern in South Carolina so far concerning rust. “Many of our early, full-season soybeans did not get planted on time. These early soybeans normally would escape significant rust pressure by maturing early. We will have more late soybeans and, therefore, more soybeans exposed to more rust this summer. We will need to keep a close watch on these late soybeans,” Mueller says.

As if on key, a stretch of 7-10 days with daytime temperatures above 100 degrees across northwest Florida and south Georgia and Alabama has slowed the development of rust on kudzu. Up until this time, rust watchers in all three states have reported rust to be very active.

North Carolina State University Soybean Specialist Jim Dunphy says the early arrival of rust in Alabama is not necessarily an indication North Carolina soybean growers will have to deal with the fungus.

Dunphy says, “Soybean rust has been detected in North Carolina every year since 2005. Rust was found in 17, 44, and 6 counties in 2005, 2006, and 2007, respectively.

“In spite of a favorable fall environment, rust was only found in five counties in 2008. Fungicides were not necessary to control rust and most foliar diseases were minor in 2008. Only in 2006 was it recommended that fungicides be sprayed in the southeastern counties on late-planted, late-maturity soybeans. Yield increases in these areas were on the order of 4-5 bushels per acre,” Dunphy says.

In Georgia, University of Georgia rust watcher Bob Kemerait says as of June 20, “Asian soybean rust is currently known only in a kudzu patch in Decatur County near Attapulgus and in a kudzu patch in Grady County near Cairo. Both sites are in extreme south Georgia. The rust is becoming more severe in those locations. However, we have not found additional infected locations yet,” Kemerait says..

Ed Sikora, a professor of plant pathology and long-time rust watcher says rust has been found growing and active on soybean plants in southwest Alabama. Sikora notes this is several weeks earlier than in previous years.

“Growers in Baldwin, Mobile and Washington counties with soybeans at the R3 growth stage or later should consider the use of a fungicide at this time if a fungicide has not been applied previously. Since it is likely these fields have been exposed to soybean rust, a tank-mix combination of a strobilurin and a triazole fungicide, or a prepackaged tank-mix of the two products, would be most beneficial,” he says.

Sikora reminds growers that fungicides used before bloom are not considered economical. Though symptoms of soybean rust can occur on soybeans before bloom, this has not yet been observed in Alabama. Soybeans that have reached the full pod-R6 growth stage should not be sprayed as there appears to be little benefit from fungicides applications after this growth period has been reached.

Earlier than usual rust along the Gulf Coast doesn’t necessarily mean soybean growers in the Carolinas and Virginia will see an increase in rust pressure in soybeans this year, but it is reason to be alert to rapid movements of the disease and to have fungicides ready.

Planting season rains and cool weather has delayed wheat harvest in some areas of the Carolinas and has delayed cotton and peanut planting from north Florida to central North Carolina. All these factors indicate a higher than usual percentage of soybeans being planted later than usual. In some areas soybean plantings may linger into July, which could put beans at a higher risk for soybean rust even under more normal time of occurrence.

Rust typically moves into the upper Southeast from Georgia, Alabama and Florida, but tropical weather patterns have pushed the disease-causing spores in a more west to east pattern, though later in the season. Though tropical storm and hurricane forecasts don’t show any immediate activity, Gulf of Mexico waters are already in the upper 80 degree range, creating an ideal environment for tropical storms.

While Asian soybean rust has been a constant threat to Southeast soybean growers since it’s arrival in the U.S. a few years back, it has never materialized as the yield-robbing monster it has been in Brazil and other soybean growing countries in the Southern Hemisphere.

Should tropical systems form and produce winds and rain that move generally west to east, Southeast growers could be in greater peril because active rust on soybeans has been found on soybeans in Louisiana 8-10 weeks earlier than in the past three years.

Though researchers are working diligently to develop a predictive model for Asian soybean rust, that tool isn’t available to Southeast growers just yet. Until then, being ready, willing and able to apply the right fungicide (preventative or protective) is the key.

Sentinel plots will give growers in the Carolinas and Virginia plenty of warning, but timing of fungicide applications will be critical to avoid widespread damage, if the troublesome little fungus reaches the area during the early growth stages of soybean production.


TAGS: Soybean
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