Asian soybean rust has not yet arrived in Missouri, but late-planted beans are at higher risk of damage if the fungal disease enters Missouri in late August or early September, said a University of Missouri Extension specialist.

Delayed maturity means the plants will still be filling pods at that time, and thus more susceptible to severe yield loss, said Allen Wrather, plant pathologist at the MU Delta Research Center in Portageville, Mo.

“There are estimates that yield loss will be as great as 60 to 80 percent if the fungus attacks when plants are just starting to reproduce,” he said.

Wrather will discuss soybean rust research and monitoring efforts at the MU Delta Research Center Field Day, Sept. 2.

Soybean rust has developed in Missouri three of the last four years. Its development and spread in a field is always weather-dependent, Wrather said.

“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,” he said. “Our worry is that the same will happen this year, and because of late planting, rust developing in mid-September could really damage yields.”

Soybean sentinel plots at the Delta Center, which is in the Missouri Bootheel, monitor for soybean rust as part of a statewide early-warning system. Twenty regional MU agronomists around the state take weekly leaf samples, which are sent to the Delta Center to be examined for rust. This year Wrather also established a toll-free soybean rust hotline funded with check-off dollars from the Missouri Soybean Merchandising Council.

Each Monday, Wednesday and Friday morning, Wrather records information on the status of soybean rust in Missouri and the United States. Farmers can call (866) 587-1206 at any time to receive updates on the spread of the disease.

“When rust gets close to Missouri and is threatening, we will provide daily updates so farmers know what could happen and can prepare to take action if rust does develop,” he said.

“Our effort with the early-warning system is to find rust when it first begins to develop in an area, because that is the time when farmers would need to apply fungicides to protect their crop.”

Once rust is easy to see on leaves, yield loss has already occurred, Wrather said. “You could still apply fungicides, but yield loss would still occur at that point.”

Producers should be careful identifying rust-like symptoms, as several other soybean diseases look similar to rust and can be mistaken for the disease.

The disease does not overwinter in Missouri because it needs living tissue to survive. In the United States, it can only overwinter on kudzu in parts of Texas and Florida, Wrather said.

When conditions are right, rust spores travel by wind and storm fronts. “The spores drift with wind currents and can travel for many, many miles before they fall out of the sky,” he said.

If spores do migrate to a field, weather must also be right for soybean rust to take hold. “If the spores land on a leaf and don’t have some moisture, they won’t germinate,” Wrather said. “Current science indicates that these spores need eight to 10 hours of leaf moisture to germinate so the fungus can enter the leaf, otherwise the spores will die.”

Drought in Florida, Texas, Alabama and Louisiana, where soybean rust now exists, is slowing its spread.

“If weather patterns change and rain becomes more frequent in those areas, then the disease could start to develop rapidly and spread to this area,” Wrather said.

To stay ahead of rust, farmers should call the hotline, listen to the radio and contact their local MU Extension office with questions, he said. “They should rely on us to provide information about the spread of rust. We will tell them using all sorts of media when rust has developed.”

For more information about Delta Center, see http://aes.missouri.edu/delta/.