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, says University of Missouri (MU) Extension Specialist Allen Wrather.

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

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

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 says.

“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 mid- to late October,” he says. “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 U.S. 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 says.

“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 says. “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.