Soybean rust: Little damage in 2009

A larger than usual soybean crop in the Southeast, some beans were planted a little behind schedule thanks to late spring rains, and a predicted busy hurricane season — all pointed to problems with Asian soybean rust for the 2009 crop.

Like Hurricane Ida, Asian soybean rust was too late in the season to do much damage in the Southeast. Though rust made it from the Florida Panhandle to central Virginia by October, it arrived too late and in too little severity to create a need to spray with fungicides.

Rust was more widespread in 2009, than in 2008. In 2009, rust was documented in 16 states and 523 counties. In 2008, it was found in the same states, but only 392 counties. At the far end of the Southeast soybean range, Virginia growers finally saw rust for the first time in mid-to late-September. At the other end of the region, Georgia growers had to deal with rust as early as July, but not in sufficient numbers, nor in traditional soybean production areas, to cause major problems.

In Virginia, Virginia Tech Soybean Specialist David Holshouser says all fields had defoliated or were in the process of defoliating in the case of late planted, double-cropped soybeans by the time rust arrived.

In North Carolina, North Carolina State University Soybean Specialist Jim Dunphy says rust didn’t have any significant effect on this year’s soybean crop. Dunphy says North Carolina growers planted 1.8 million acres and yield is predicted at near 35 bushels per acre.

By the first week in November, rust was documented in 14 North Carolina counties. It is likely that rust spores were present in many if not all of our North Carolina counties, says North Carolina State Plant Pathologist Steve Koenning.

In South Carolina, John Mueller, a plant pathologist and rust watcher at Clemson University’s Edisto Research and Education Center in Blackville, S.C. says, “The cool, wet weather we had in late October and on into November helped spread whatever rust was present in South Carolina. However, this late-season spread of rust was not a threat to our soybean crop.”

“All of these soybean fields in areas where rust was found were late R-6 or later in maturity. We may have had some premature defoliation in a few fields that were not treated with a fungicide, but it was too late for this to hurt all but the very latest maturing soybeans,” Mueller adds.

Georgia has had sporadic reports of soybean rust throughout much of the growing season, but widespread damage did not occur, notes University of Georgia Plant Pathologist Bob Kemerait.

Since it was first reported in the U.S. in November of 2004, Asian soybean rust has been considered a major threat to soybean production. Despite its potential for damage, the fungal disease has caused only sporadic damage in the five cropping seasons since its initial occurrence.

The mild winter of 2007 resulted in rust over-wintering farther north than ever reported in the U.S. Despite its closer proximity to soybean producing areas, it failed to move south to north early enough to cause more than minor damage to soybean crops.

Drought in 2006 and 2007 are reported to have slowed down the northward movement of the disease. However, in 2008, and even more so in 2009 normal, to above normal rainfall across the Southeast, resulted in virtually the same movement of rust.

Hurricanes are considered to be a primary mover of rust spores. However, in years with numerous and large hurricanes, rust movement and/or severity of the disease was comparable to dryer, storm-free years.

Whether or not the U.S. soybean crop will ever be significantly affected by damage from Asian soybean rust remains to be seen. Based on the severity of the disease in Brazil and other South American countries, the potential threat is clearly abundant.

A high percentage of U.S.-grown soybeans are in the Midwest. Despite movement of the disease into these areas late in the season, there has been little or no damage in most years from rust.

While there was clearly more rust in Ohio, Illinois and other prime Midwest soybean production areas in 2009, compared to 2008, the progression of the disease slowed as traditional late summer dry weather patterns occurred.

Some areas of the Southeast were impacted by Asian soybean rust this past growing season. The disease developed early in southern Louisiana and spread northward to Arkansas in time to cause spray alerts.

In mid-September, Arkansas Extension specialists called for growers with beans in the susceptible R1 to R5 stage to spray for soybean rust. Prolonged cool, wet weather is the suspected cause for the higher level of rust damage in the state, compared to previous years.

“This year soybean rust has proven to be more aggressive than in any year since it first arrived in the U.S. and Arkansas,” says Scott Monfort, Extension plant pathologist at the University of Arkansas.

Other areas of the Delta had more sporadic problems with soybean rust, though most all fields in the area had some level of infestation during some parts of the growing season. In general, full-season beans planted in April escaped damage. Double-crop beans planted behind wheat, especially in the northern end of the region, also were susceptible to rust damage.

Regardless of the movement of Asian soybean rust, it is a relatively easy fungus to control with triazole and strobilurin fungicides. The system of sentinel plots designed to closely monitor the movement of rust, once again gave growers ample warning and ample time to spray beans that were in harm’s way.

Since its first appearance in U.S. soybean fields in 2004, Asian soybean rust has been a consistent threat to U.S. production, but so far it has been tracked well by university Extension workers and treated appropriately by soybean growers. The result has been little damage — so far.

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TAGS: Soybeans
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