Storms moving from west to east across Arkansas during the second week of September helped spread Asian soybean rust to over a dozen new counties in the state. However, most of the state's late soybean crop is no longer vulnerable to the disease, according to Arkansas Extension plant pathologist Scott Monfort.
By Sept. 21, soybean rust had been confirmed in 25 Arkansas counties. The most recently confirmed cases were reported in Lee, Phillips, Poinsett, Cross and St. Francis counties. On Sept. 19, rust was confirmed in Arkansas, Chicot, Drew, Randolph, Desha, Randolph and White counties. The disease was reported in Mississippi County on Sept. 17, in Jefferson and Lonoke counties Sept. 15, and in Clay and Pulaski counties Sept. 13.
The disease has also been confirmed in seven counties in Mississippi and 14 parishes in Louisiana.
“We think all the rust we're finding today (mid-September) probably came in last week and is just now showing symptoms,” Monfort said. “The disease cannot move that much in sunny weather. Most of the time, it needs cool and cloudy weather to sporulate. Once it's in the field, it needs plenty of moisture and humidity in the canopy to move through the field.”
Much of the state's double-crop soybeans have moved past the disease, according to Monfort. “When the ASR outbreak initially started in the southwest corner of the state, we thought at least a third of the Arkansas crop would be affected, or all of the late beans. But it stopped. We're probably down to 5 percent of the crop being vulnerable as of today (Sept. 19). I expect more than that has been sprayed, at least a quarter of the crop.”
The decision to spray a fungicide depends on the growth stage of the soybeans, noted Monfort. “If it's mid-R-5 and below, we're definitely recommending a spray. There are a lot of beans at late R-5 and they still have a week to go. They're good beans and we're trying to determine if we need to spray.
“In some situations, you may need to, in others you may not.”
Yield potential might also be a factor in deciding if a spray is necessary.
“The most important thing is to be aware of where the disease is relative to your location as well as the reproductive cycle of your crop. But this is a disease that we definitely can control.”
The outbreak also shows that the sentinel program for tracking the disease “is working. It's kept us aware of where it's going. Every place where we've found it this week, it's just at the beginning stage.”
To keep up-to-date on ASR in Arkansas, Louisiana and Mississippi call the toll-free, frequently updated ASR hotline at (866) 641-1847.