August 20, 2008
Threat of soy rust fades in Midwest, late soy still at risk
Experts across the Midwest are beginning to sound the "all-clear" on Asian soy rust for yet another season, after the fungus again failed to gain a significant foothold within the US soy crop.
"The soy rust risk for Iowa is very low, at this time. I think it is safe to say that our full-season soy crop is now out of the woods and is no longer in danger," said Iowa State University extension plant pathologist Daren Mueller. "Soy rust finds are way behind where they were for this date in 2007. Soy rust currently is restricted to selected states in the Deep South, and there has been very little change in the status of the disease over the past month."
Although two new infections were found on the Georgia-Florida border earlier this week, the US Department of Agriculture points out that the airborne plant pathogen has still only been found in about 30 counties of Alabama, Florida, Georgia, Louisiana and Texas during 2008, compared to 75 counties in eight US states, as of mid-August 2007.
"We have also had a lot of rain this year, especially during June and early July. However, in many areas the faucets' turned off until recently, or it is still dry, making it very unfavourable for rust development," said Ohio State University plant pathologist Anne Dorrance.
Soy rust - which flourishes in mild, damp weather - has been largely bottled up by drought in the Delta, southern Plains and Southeast this season. The spread of rust is greatly reduced during times of dry weather, as its airborne spores are quickly killed by desiccation and direct sunlight.
"Most of the soy planted in early to mid-season is past or almost past the growth stage where fungicides can be sprayed or where rust could have much of an effect on yield," said Clemson University soy pathologist John Mueller. "Until we get very wet weather several days in a row, or rust is recorded moving northward in Georgia, the need for individual growers (in South Carolina) to scout fields is minimal."
Experts agree that most full-season soy in Iowa, Minnesota, Kentucky, Ohio and Indiana have developed to the point where yield damage from rust is unlikely, although they note weather patterns have recently changed, putting later-planted fields at risk.
"Florida and areas along the Gulf coast are now receiving rainfall typical for this time of year, with afternoon showers being common," said North Carolina State University plant pathologist Steve Koenning. "With a large late season soy crop in Georgia, Alabama and Mississippi, the potential need to make fungicide applications for rust in North Carolina - on our late crop - is at least moderate."
The greatest risk of rust damage to any single state's soy crop undoubtedly lies in Missouri, which seeded 5.3 million acres to the crop, despite extremely wet spring weather resulting in fields being planted nearly a month later than normal.
With seeding so far behind schedule, many Missouri soy fields will still be filling pods in late August and early September, a state of plant development particularly susceptible to rust damage.
"Last year, the weather was such that rust invaded the area early, in mid-September, which was unusual. In the past it has only developed in middle to late October. Our worry is that the same will happen this year," said University of Missouri plant pathologist Allen Wrather. "There are estimates that yield loss will be as great as 60 percent to 80 percent, if the fungus attacks when plants are just starting to reproduce."
Although rain and wind whipped up by Hurricane Edouard failed to cause any discernible increase in soy rust infections across Texas earlier this month, some experts feel that Tropical Storm Fay - which made landfall in southern Florida Tuesday morning - may be more successful in jump-starting spread of the disease.
"Asian soy rust has been very slow to develop in Georgia in 2008, however, conditions are now excellent for the disease to progress quickly," University of Georgia extension specialist Bob Kemerait reported to the USDA's public rust Web site Tuesday. "We have plenty of moisture in the state, much of the commercial (soy) crop is at a vulnerable reproductive stage, the disease is finally beginning to spread, and there is the imminent threat of tropical storm Fay moving spores from Florida into production areas of Georgia."
Consequently, Kemerait advised farmers who have fields in the projected path of Fay, which is forecast to traverse Georgia and reach central Alabama by Sunday, who have not applied a fungicide within the past 12 days, to "strongly consider," spraying their soy crop now.
Soy rust, which affects yield by causing premature defoliation of infected plants, is a major plant pest in South America and Asia. It first appeared on the continental US about five years ago, after a Caribbean hurricane carried spores from infected fields in northern South America into Louisiana.