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Soybean rust in 2006 Beware ill winds bearing gifts

Mississippi's lone encounter with Asian soybean rust in 2005 had a happy ending.

“We had one producer in Mississippi who had soybean rust in three fields in George County,” says Billy Moore, retired Extension plant pathologist with Mississippi State University.

“When we found it outside Lucedale, we suggested he spray with a fungicide. He sprayed with Headline. He had heavy rust just prior to leaf drop but made an excellent yield with that one application of fungicide.”

Moore, who has been working “part-time” with MSU's Soybean Management and Application of Research and Technology or SMART program almost since its inception, says Mississippi soybean growers may wonder what all the fuss was about since Asian soybean rust seemed to be a no-show in 2005.

But rust could have turned up in more places than George County if environmental conditions had been right, he told farmers attending the Delta Ag Expo in Cleveland, Miss.

“We had spores throughout Mississippi last year, including in the Delta,” he said. “There were even spores found in the northern United States at a population high enough that you could identify them as the fungus that causes soybean rust.

“But the one thing that was missing was the proper environment.”

Moore said three ingredients are needed for the development of Asian soybean rust: (1) A susceptible host, (2) a disease pathogen and (3) the proper environmental conditions.

“100 percent of our soybeans are susceptible because they have no resistance to soybean rust,” he said, noting that researchers are investigating 24 lines for possible resistance to the disease.

“You have to have a pathogen that has to be blown in. If it's not blown in, you won't have the disease because the fungus is not there. And it was brought in last year, late in the season. If you have don't have the proper environment, you won't have rust.”

Leading up to the 2005 season, growers heard predictions of a 40 percent reduction in soybean yields in the Mid-South and 10 percent nationally if Asian soybean rust became established at the critical stage of development or R1/R2 or flowering.

“Didn't happen, did it?” said Moore.

“Those predictions were made prior to the Environmental Protection Agency giving approval for a number of rust control combinations that will prevent the disease if applied at the proper time.

“I don't think we will have that kind of loss because you folks know almost as much about soybean rust as we do. You've heard enough about it.”

That's not to say that the disease shouldn't be treated with respect — and with fungicides — when the proper conditions exist for its development. New research by LSU AgCenter scientists shows just how prolific the fungus can be when those conditions occur.

Ray Schneider, the LSU plant pathologist who was the first to find soybean rust in the United States in 2004, had test plots near Quincy, Fla., where rust occurred in some farmers' fields and on the University of Florida Experiment Station last fall. His research was conducted over a five-day period.

He and fellow researchers calculated that the plots were producing 80 billion urediniospores per acre per day for each of those five days. “That's a lot of fungi,” said Moore.

“If you consider that 50 percent of those are dead when they come out of the canopy, which is very likely because solar radiation kills them, and, if you consider that another 25 percent were dead by the time they reached the field, you have 25 percent left. You would still have enough urediniospores being produced by one acre of soybeans to contaminate several million acres of soybeans in this country. It just takes the right environment.”

Moore used maps produced by Bart Freeland of USDA's World Agricultural Outlook Board in Stoneville, Miss., to illustrate the location of Asian soybean rust discoveries in the United States in the fall of 2004 and in 2005.

In September 2004, Hurricane Ivan brought spores in from South America and deposited them in nine states. Rust was subsequently discovered on late soybeans in 14 locations from south Louisiana to southeast Missouri to South Carolina.

As expected, the disease did not overwinter north of a line running along the Gulf Coast areas of Louisiana, Mississippi, Alabama, Florida and cutting across the central portion of Florida in 2004-05.

“It depends on how cold the temperatures get,” said Moore. “But what's the situation that exists out there right now?” (Moore was speaking on a rainy, January day with temperatures in the 40s. January 2005 was the second warmest in Mississippi in 50 years.)

Even though the disease did not survive the winter of 2004-05 in the Mid-South, scientists believe Asian soybean rust spores were blown into the region from overwintering sites in central Florida in April and again in June. The latter occurred beginning on June 8 when mean air currents shifted and began blowing to the northwest.

“On June 11, Tropical Storm Arlene went through the same area where urediniospores were being blown from known infected areas into the Gulf,” he said. “It picked up those urediniospores and moved them straight up the Mississippi-Alabama state into Georgia and into southern Mississippi.

“Alabama and Georgia were having frequent rains. It was so dry in parts of south Mississippi they were having to feed cattle. This was the difference in environmental conditions — you have to have moisture.”

A short time later, scientists discovered soybean rust in sentinel plots near Foley in south Alabama and in the Florida panhandle. More discoveries followed in Alabama and Georgia, which received more rainfall than south Mississippi. Rust did show up in a small area of a sentinel plot in southeast Mississippi on July 13.

Those weather patterns continued into the fall with numerous discoveries of the disease being made in Alabama, Florida, Georgia, North Carolina and South Carolina. Scientists also found outbreaks of the disease in southeast Texas.

What will happen in 2006? Moore said he recently talked with Scott Izard, a Penn State University researcher who is working on the computer modeling program that could help predict where rust will occur, triggering fungicide applications for the disease.

Izard was in a kudzu plot in Florida and had been in several areas of the state looking at other kudzu plots. “The kudzu was loaded with rust,” said Moore. “They had five kudzu plots with fungus down there last fall. Why they didn't kill them I don't know. We tried to talk them into (doing so), but, nevertheless, they left them down there.”

In Texas, scientists have also been watching a field plot of kudzu, one acre in size, between Beaumont and Houston. “The Extension plant pathologist told me that 90 percent of that plot is dead and that he was planning to check on the rest of it the next week,” said Moore.

“Hopefully, those spores in Texas will be dead by the time we plant our soybeans because we can get wind blown in from Florida or we can get wind blown in from Texas, and, in the Midwest, it can do the same thing.”

Sentinel plots will again be the early warning system for Mid-South soybean producers, said Alan Blaine, Extension soybean specialist and interim head of the Northeast Research and Extension Center at Verona, Miss.

“We're not going to have as many as we did last year, but we're going to plant those across the coastal area and then come up the Mississippi River a short ways,” he said. “We will plant them extremely early because what we found in the field is that once the plant starts blooming it becomes much more susceptible to the disease.

“If we put those plots in in late January or March before you begin planting, they will bloom earlier and we should be able to pick up the disease then.”


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