Soybean rust: Business as usual

At of the end of June soybean rust seemed to be behaving about as it has the past couple of years — active in Florida and the lower Southeast, but making no northward threats.

With a big crop of wheat in the upper Southeast, especially in South Carolina, it is expected that double-crop bean acreage will increase across the region. Rust is just one of the threats to later planted beans, but a potentially lethal one.

In Virginia, Soybean Specialist David Holshouser says, “We have 10 small sentinel plots planted, which include 3 maturity groups of soybeans. Sampling of our plots should begin within the next two weeks.

“In addition, with funding from the Virginia Department of Agriculture and Consumer Services and Virginia Tech’s College of Agriculture and Life Sciences, we will again be scouting commercial soybean fields throughout Virginia,” he adds.

In North Carolina, longtime Soybean Specialist Jim Dunphy says, “Soybeans are being inoculated with rust this week in Florida for experimental purposes. This poses little threat to soybean production since the inoculated soybeans are inside tents.”

Dunphy says this is a good sign for North Carolina soybean producers that they, have to inoculate, meaning researchers in Florida cannot count on natural infection.

North Carolina soybean growers will have some new resources from which to draw soybean rust management information in 2008. North Carolina will receive 2,000 copies of the booklet Soybean Rust Management in the Mid-Atlantic Region. These will be distributed to county offices on the basis of soybean acreage.

The older version of the Soybean Rust Management in the Mid-Atlantic Region, as well as the Soybean Disease Atlas, can be found at the SSDW web site A PDF version of the newer Soybean Rust Management in the Mid-Atlantic Region guide will be posted there soon.

Dunphy says, “we will receive 600 copies of the “Fungicide Manual” and these will be distributed to agents, crop consultants, and dealers as long as supplies last, though a PDF version can be obtained from”

The Fungicide Manual actually contains more information than most growers are likely to need, according to Dunphy.

Around the Southeast, soybean rust is viable on kudzu in eight or nine Florida counties (two or three that border on Georgia at this time. None of the Florida infection sites have shown much, if any spread. Development of rust on soybeans in the Florida Panhandle is progressing comparable to the past three years.

As was the case in 2007, south Georgia is hot and dry and no SBR has been found in Georgia to date. Since researchers have been tracking soybean rust, a consistent trend has been that hot, dry weather serves to significantly blunt the northward movement of the disease causing organism.

All sentinel plots in Mississippi are at reproductive stages and a late crop is currently being planted. Some of the state’s heaviest soybean acreage is still suffering from the effects of early June flooding, according the Mississippi State Extension Specialist Tom Allen.

Rust was found on May 27 in southern Louisiana, following heavy flooding, but no further spread of the disease has been documented, according to LSU Extension Specialist Clayton Hollier. No spraying for soybean rust has been necessary in the state so far in 2008, Hollier adds.

Alabama is expected to plant over 350,000 acres of soybeans in 2008, more than twice the 2007 acreage. While this could bode badly for soybean growers farther north in terms of soybean rust risk, so far the disease has shown no development nor movement in the state.

For the past three years, rust has been detected in Baldwin County in late June. This county borders the Gulf of Mexico and is adjacent to the Florida Panhandle and Southwest Georgia counties where rust typically starts its northward movement into the Carolinas and Virginia.

So far, Auburn University Plant Pathologist Ed Sikora says there has been no unusual soybean rust activity in Alabama, despite the large increase in acreage.

In Texas, the southern end of the state has been extremely dry, blunting both development and spread of soybean rust. Typically any problems encountered in central and north Texas production areas and into Oklahoma result from wetter than normal conditions in south Texas.

Rust may be active in Mexico on volunteer soybeans growing in corn on a limited acreage, but there is a good chance these volunteer soybeans have since been killed with herbicides. Soybean rust on jicama (yam bean, a crop grown in Mexico on as much as 100,000 acres) was sporulating, but has probably been harvested.

Despite the business as usual status of Asian soybean rust, the risk remains for soybean production in the upper Southeast because of the large increase in late-planted beans. High value wheat has mandated that growers wait until the crop reaches full maturity for harvest. Similar high values for other crops stands to create some delays in planting double-crop beans, and waiting can increase the risks.

“With the recent rains, it’s important that we forge ahead with soybean planting,” says Virginia Soybean Specialist Holshouser. “By late June soybean growers in the upper Southeast will lose about a half bushel of beans per acre with every day they delay planting.. With earlier planting, double-crop yields do not lag very far behind full-season yields.”

In addition to yield advantages, early planted double-crop soybeans also have less risk to soybean rust. “We know the disease can quickly move up the East Coast. Therefore, we’ll continue to keep a close eye on the states to our south,” Holshouser says.

For soybean rust, or other soybean diseases to occur three things have to happen:

• There must be a suitable host.

• The disease organism must be present.

• Environmental factors must be present to spread the disease.

In the upper Southeast in late June only one of the three factors existed — a suitable host. High humidity, days in the 60-80 degree F range, with a day or so of cloud cover is clearly possible this time of year. But, without all three factors, rust won’t be a problem.

If rust becomes a problem, growers have a good arsenal of fungicides for both prevention and cure of the disease.

In high-value soybeans, growers may be tempted to apply preventative fungicides. While any fungicide may increase yields by protecting the plant from other diseases, spraying when rust spores are not present or nearby is not recommended for Asian soybean rust.

The current system of sentinel plots has worked well in the past few years to give growers ample warning for either a preventative or treatment application of fungicides. The sentinel system is constantly being upgraded, and there is no reason to believe 2008 will be any different in terms of advance warning for rust.

Despite the business as usual progress of rust, growers are warned to closely monitor state and county Extension sites and be prepared to spray for rust, should the threat arise.

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