Rust Hits Soybean Fields

September 24, 2009 07:00 PM

Pam Smith, Farm Journal Seeds & Production Editor

Tom Allen has one word for the weather in Mississippi this year: weird. A wet May; scorching June; a cool, wet July and wet August and September set the stage for an unwelcome visitor in the form of soybean rust.
The state of Mississippi lit the USDA soybean rust map up like a Christmas tree over the past month. Allen, a plant pathologist with the Mississippi State University Extension Service, says the majority of the counties in the state will likely have rust confirmed before the season is over. "Fortunately, most of our soybeans are in the R5 stage or later.” Fungicide treatments are not considered economic beyond the R5.5 stage for rust management. Allen notes that preventative R4 treatments are thought to have contained most losses, but that was dependent upon geography within the state.
A field in Noxubee County was found to be severely infested with the fungus in early September. Allen says it is the most severe case of soybean rust to be found in Mississippi to date. "I'm guessing that field will experience 5% to 10% yield losses due to rust, but that's just a guess. It could be higher,” he says. "We'll have to see what it looks like at the end of harvest.” The 100-acre field near Brooksville was not treated with a fungicide.
July's cool and wet temperatures coupled with late soybean planting put the state at risk, he notes. "This is the earliest we've experienced rust in the Delta, where approximately 75% of our soybeans are grown. It's likely that rust had been present at this location for eight to 11 weeks,” says Allen. "Our July weather conditions were conducive for disease development.”
Dennis Reginelli, Extension agronomic crops agent in Noxubee County, said the field has at least half a dozen acre-or-less-sized circles of dead plants where the disease first occurred. The rust took hold on the plants, weakened them and made them susceptible to other diseases that defoliated the plants. "The entire field has rust spores in it,” Reginelli said. "In some places, you can actually see spores flying if you shake the plant foliage.”
Late planted soybeans remain vulnerable if they fall below the R-5.5 threshold, notes Allen. However, he notes that growers in northern states will also have to weigh the potential of frost should a treatment become necessary. "It's odd, but our weather patterns came from the north this year. Based on wind patterns, spores in Mississippi have been sent south this season.
An outbreak of soybean rust has already been confirmed in Kentucky—the earliest the disease has been found in that state. Illinois plant pathologists suspect rust is present in the southern regions of their state, but think most soybeans fall into the R5 or later maturity stages.
So far in 2009, soybean rust has been found in nine states and 164 counties, and in two states and five municipalities in Mexico. For more information on soybean rust or to monitor the disease spread, go to

You can email Pam Smith at

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