Heat, drought dog Asian soybean rust

The good news is Asian soybean rust doesn't like hot, dry weather. The bad news is, neither do soybeans — at least not record heat and drought.

Across the lower tier of Southern states, from Louisiana to Florida, where rust over-winters and typically spreads northward, early season drought, plus long stretches of temperatures that ranged from the high 70s at night to the high 90s during the day, significantly slowed development of rust.

Though rust was found to over-winter on kudzu as far north as Montgomery County, in central Alabama, few infections have been reported on soybeans, either in sentinel plots or in commercial fields in the state in 2006.

If there has been a rust hotspot this season it would be along the Georgia border with the panhandle of Florida. Rust found on kudzu in the winter has been found to infect both sentinel plots and commercial fields in a relatively small area of south central Georgia and northwest Florida.

A brief respite from the hot, dry weather came in early- to mid-July, but an intense heat wave at the end of July and extending on into mid-August virtually stopped northward movement of rust.

Unfortunately, the intense heat, with 10-12 day stretches of 100 plus degree heat took a serious toll on all crops.

Combined Georgia, Alabama and Florida only produce 300,000-350,000 acres of soybeans annually. In the Carolinas and Virginia, growers are expected to harvest about 2.5 million acres, with a million and a half acres of that total in North Carolina. By comparison, the entire Southeast is expected to harvest about 30 percent of the total harvest in Iowa.

Tropical storm Ernesto provided some drought relief to parts of Georgia and provided additional rainfall to the Carolina's. More seasonal temperatures across the southern tier of states where rust typically forms is conducive to spread of rust in those areas and into adjoining states.

Ernesto made the worst possible loop in terms of spreading rust. The storm's path took it across the most heavily infested areas of north Florida and south Georgia, with a loop out into open water and back through the Carolinas.

Clemson University Plant Pathologist John Mueller says new infections in the central part of South Carolina appear to be active. “We now have rust in both Calhoun and Orangeburg counties, which are in the middle of our soybean growing area. Both locations appear to be active. In the first week of September, seven of 200 leaves collected at the Calhoun County site were infected. On at least half of these leaves more than 15 lesions were observed. This is compared to one leaf with one lesion the previous week,” Mueller says.

Post-Ernesto weather in South and North Carolina is moderately favorable for the spread and development of rust and other fungal diseases of soybean. Conditions are more favorable further south in the Carolinas. Labor Day rains received from the tropical storm, combined with predicted afternoon thunderstorms the first two weeks in September and relatively low temperatures may be favorable for the spread of rust within and between fields.

Mueller says, “it is appropriate to begin spraying for rust in any field in South Carolina where the yield potential is relatively high and the plants are at or past R3/R4, but not within 10 to 14 days of R6. A strobilurin should be included in the spray to increase control of diseases other than rust. A combination of a strobilurin and a triazole could also be used.

In South and North Carolina, where over 1.5 million acres of soybeans are grown annually, Zone 4 and 5 soybeans are past R6, the growth stage at which damage from rust in soybeans is greatly diminished.

Beans as far north as central Virginia are waist high and past the bloom stage. Unless some dramatic change takes place in September, it appears the bulk of Southeastern soybeans will not be affected by rust until late in the 2006 season‥

Further north in Virginia, no rust has been found in the 10 sentinel plots ringing the southern border of the state, nor in any of the 40 commercial fields being monitored.

Most full season beans, planted in April and May are at, or nearing, the point of being out of risk for rust. Many double-crop beans have not reached the R6 growth stage and remain at risk for late season infections.

In many cases, double-crop beans are not in the high yield categories and treating for rust may not be economically feasible.

Other than the finds in central South Carolina, rust found in Tift County, Ga., on Aug. 7, is the farthest north rust has been reported in the Southeast so far this growing season.

University of Georgia pathologists say although Asian soybean rust has been confirmed on kudzu in Miller and Brooks counties and on soybean research plots in Brooks and Decatur counties, it has really not spread much at all this year, likely due to hot and dry weather.

University of Georgia plant pathologist Bob Kemerait says, “Many growers may choose to wait to apply the fungicide application until we are able to document rust spreading in our sentinel plots and research plots.

For example, though rust was found on soybean plants in Attapulgus, Decatur County (1 leaf of 100) on July 3, we have not found the rust again. We have also not found rust in sentinel plots in Moultrie (SunBelt Expo) or elsewhere into September.”

The greatest risk to the largest acreage of U.S. soybeans probably will come from Louisiana, which could potentially spread northward to major production areas in the Midwest. However, extreme hot and dry conditions across the Delta and in northern Mississippi and Arkansas have slowed development of rust in those areas.

By the end of August, Asian soybean rust was confirmed in eight Louisiana parishes. “Our weather in late August was perfect for the development of rust,” says LSU Soybean Specialist, David Lancios, referring to conditions of low humidity, high moisture and moderate temperatures. Well over 50 percent of Louisiana's soybean crop had been harvested by mid-September, but Lancios warns, there is still a tremendous amount of acres out there that are still vulnerable to rust.

“We can control rust,” he said. “But we have got to spray.” Lancios reminded growers there also are other diseases that threaten soybeans. “We have other diseases that if left untreated are just as bad as Asian soybean rust,” he said. “I'm talking about aerial blight and Cercospora leaf blight.”

Rust was found in southwest Mississippi in kudzu and soybeans in late July. As of mid-August rust had spread within the kudzu field, but not within the soybean field. As of mid-July, no new rust finds were reported in Mississippi. Still Extension soybean specialists are recommending growers in southwest Mississippi evaluate their crop potential, and if the potential is high enough, to treat with a rust preventative fungicide.

Rust was found in south Mississippi on Aug. 1, said Mississippi State University Extension Service Soybean Specialist Alan Blaine. Statewide, Mississippi is expected to harvest about 1.6 million acres of soybeans.

”With many Mississippi growers, especially in the Delta, going to early planting of Zone 4 and Zone 5 soybeans, a large percentage of the state crop is already harvested, or will be by the time rust is a threat in central and northern Mississippi and on into Arkansas.

In Alabama, rust found over-wintering in Montgomery County appears to have not survived intense heat and drought in the area. Rust detected in kudzu in this area in the winter has not spread or moved, even within the over-wintering sites. As of the end of August only one case of rust has been reported on commercial or sentinel plots in Alabama. That site was subsequently destroyed and no new cases have been detected.

A soybean rust find was reported on soybeans in Liberty County in Texas on Aug. 27, seven miles to the east of the earlier (Aug. 20) find on kudzu. Throughout much of Texas, hot, dry conditions caused abandonment of many soybean fields and killed several of the sentinel plots. It appears most soybeans in the state are either harvested or near harvest, or have been lost to the drought.

The lack of early to mid-season tropical storms and hurricanes may have been a factor in delayed movement of soybean rust in 2006, compared to the record hurricane year of 2005. Though rust has not been a widespread problem in 2006, heat and drought have been deadly. Despite its potential for reducing soybean yields, rust is still much more manageable than heat and drought.

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