Quietly, Rust Becoming More Active In South

Soybean rust, which survives on leaves of kudzu as well as soybean plants, was not active early this year in Gulf Coast states, where it is annually detected first during the growing season. Hot, dry weather, including severe drought in Texas, has kept the rust spores from spreading through the air.

The most recent report was in a commercial research plot in Marion County, FL on Aug. 17. Two days prior, soybean rust was reported in nine new counties in Mississippi, all in commercial soybean fields, and soybean rust could no longer be detected in Texas. On Aug. 13, soybean rust was reported in Tensas County, LA on soybeans, and in Suwannee County, FL on kudzu. In 2009, soybean rust has been found in seven states and 54 counties in U.S., and in two states and five municipalities in Mexico.

“Soybean rust is continuing to develop in Florida,” says James Marois, University of Florida plant pathologist. “Since Aug. 5, five new counties have become positive, and there are now 14 positive counties in Florida. Tropical Storm Claudette is bringing large amounts of rain to the panhandle and expected to travel northwest.

Marois advises Florida growers and retailers to “be on the lookout for soybean rust. With the recent rains and the maturing crop we are likely to begin to encounter field infections.”

Soybean rust was detected Aug. 13 in a commercial soybean field just west of Stoneville/Leland. According to Tom Allen, Mississipi State University (MSU) plant pathologist and Troy Koger, soybean Extension specialist, “the soybeans were at the R7 growth stage. Levels of rust within the field were low. The soybean field had NOT received an R3/R4 fungicide application.”

The following day, soybean rust was reported in commercial soybean fields in Carroll, Grenada, Humphreys, Leflore, Montgomery, Sunflower, Yallabusha, and Yazoo counties. “Rust levels were extremely low in all of the fields save for one field in Montgomery County, immediately east of Winona, MS,” say Allen and Koger. “All soybean fiels were beyond R5.7, so they are out of the woods and would not require treatment.

According to Clayton Hollier, Louisiana State University plant pathologist, “three commercial fields of soybeans were found to be positive this week — all in Iberia Parish. Incidences were 18 percent, 7 percent, and 4 percent respectively and were in either R7 to R8 depending on location.”

Hollier adds that “Tensas Parish was positive for one location in a research plot with 10 percent incidence. The beans were in R6-7.”

As of Aug. 15, soybean plants are maturing throughout Texas, at least at pod fill or more mature. However, there are plants in the seedling stage in the Lower Rio Grande Valley, which will be the fall crop there, according to Tom Isakeit, Texas A&M University plant pathologist.

“There is still no rust at any the soybean sentinel sites in Texas,” Isakeit says. “The rust at one kudzu site, in Livingston, Polk County, in east Texas, is no longer detectable. Continued dry weather in central and southern Texas will not support establishment and development of rust.

“Recent rains in east Texas have not increased the risk there, as there is apparently no source of spores to start an epidemic,” he adds. “At this time, no preventative fungicides are recommended anywhere in the state.”

Isakeit also observes that “mineral deficiencies — particularly iron — are becoming apparent in some fields undergoing moisture stress. Some of these symptoms may resemble rust.”

Because of the proximity of the recent Arkansas and Mississippi detections, Missouri officials have put their soybean rust scouts on alert. The findings are about 250 miles from southeast Missouri.

“Soybean rust has not been detected in Missouri this year,” says Allen Wrather, University of Missouri (MU) Extension plant pathologist. “This is the first find of rust this year in those two states. The weather during the next few weeks will greatly impact the spread of rust.”

Soybean rust has been found in Missouri four of the last five years, but always late in September or October, near harvest. Wet weather in Missouri this spring delayed planting. “Soybean maturity and harvest will be later than normal,” Wrather says. “Rust that develops in September might damage late-maturing soybean plans and reduce yield.”

For most of the 2009 growing season, prevailing winds have been from the northwest, bringing cooler than normal weather to the region. That may change with the coming hurricane season, Wrather says.

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