Growth stage important Asian soybean rust in east Arkansas

The arrival of rain and cooler temperatures has aided the recent spread of Asian soybean rust in Arkansas. Since the end of August, ASR infections have shown up in two northwest Arkansas sentinel plots and, on Sept. 8, in east Arkansas' Jackson County.

“Up until yesterday (Sept. 7), despite heavy scouting, we hadn't found any recent ASR outside sentinel plots around Fayetteville in Washington County and Alma in Crawford County,” said Scott Monfort, Arkansas Extension plant pathologist. “Extension agents have been diligent in sending samples in. We've been receiving leaf samples all week. The numbers of those have especially increased from all the counties in the Arkansas River Valley.”

Lab tests showed nothing on samples until the Jackson County leaves were checked. Two leaves out of 50 (at 2 to 5 percent of the leaf area) were positive for the disease.

“The pulled leaves didn't show any disease from an initial, cursory look. But we always incubate the samples overnight to see if ASR begins sporolating.

“Early this morning, I got a call from the lab saying, ‘We've got it.’ It's at low incidence and low severity. We caught it right at the beginning. That points to the sentinel plots operating as they were devised to.”

Fungicide recommendations have been updated. “Everyone in the Arkansas Delta should spray a fungicide on their late-planted soybeans.”

At the same time, farmers need to pay closer attention to their soybeans and check growth stages. “I was at recent meeting and some growers weren't sure about growth stages. Some of the beans we looked at were already past the vulnerable stage. So, before spraying, make sure it's worth the expense. Some soybeans — those with a 20-bushel yield potential, for example — may not be worth spraying.”

Are there still wheat beans at R-3/R-4? “I believe a majority of the double-crop soybeans are at R-4/R-5. Some are even borderline R-6. Statewide, we estimate there are about 200,000 acres still vulnerable enough to spray.

“If this ASR situation is just beginning and a producer's (east Arkansas) beans are mid-R-5, by the time rust is able to build and cause trouble, his crop will have matured past being vulnerable.”

How long before the bulk of the 200,000 acres is safe from ASR? “We're probably three to four weeks out, so there's still time to worry about this. The weather forecast says the rains should stop (around Sept.12) and fields can begin to dry out. But I don't know how long we'll have before the next storm system rolls through.”

Right now, if soybeans are R-4 or beyond, “we're suggesting using a triazole only. Triazoles have some curative activity. If someone still has beans R-3, or younger, then the farmer should consider a strobilurin along with a triazole.

“If you're in, or near, Jackson County, you should make a spraying decision quicker than others. But there's still no need to panic — we've caught it early.”

To keep up-to-date on ASR in Arkansas, Louisiana and Mississippi call the toll-free, frequently updated ASR hotline at (866) 641-1847.

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