Warm Winter Woes

ROCKVILLE, Md. (DTN) -- It's been an unusually mild winter -- just ask Bob Kemerait's dog.

"I've only broken the ice in the dog dish twice this year," said the University of Georgia Extension plant pathologist. "That tells you how rarely it's been below freezing."

The implications of that are much larger than Kemerait's morning pet routine, of course. Some agricultural insects and diseases are likely to pose a much higher risk to row-crop growers this spring and summer, after much of the country experienced above-average temperatures this winter, scientists told DTN.

They include Asian soybean rust, which Kemerait has found alive and well in southern Georgia all winter long, as well as redbanded stink bugs, nematodes, southern corn rust, frogeye leaf spot and cotton leafroll dwarf virus.

RUST DISEASES STAYED CLOSE TO HOME

Normally, wintertime temperatures will chase Asian soybean rust and southern corn rust back toward their tropical hideouts in Central America, the Caribbean and the southernmost regions of Texas and Florida. Then the diseases' spores will spend the spring and summer hitchhiking on wind currents and storm systems up into the southern U.S. and -- in the case of southern corn rust recently --- into the Midwest.

But winter didn't have much bite to it this year, noted DTN Senior Ag Meteorologist Bryce Anderson. "For the past 90 days, from early December through early March, we saw above normal temperatures in all the lower 48 states," he noted.

That means soybean rust spores were able to hold a sleepover this year farther north, at least 60 miles north of the Florida border, Kemerait said. They bedded down on kudzu, a wild legume host of the disease that is widespread in many southern states.

"I could take you to six locations in Georgia right now where we've got kudzu that did not freeze back from last year, has already regrown this year and still has rust spores from last year, as well as rust spores on the new growth now," he said.

That doesn't necessarily mean the disease is going to race through soybean fields this summer -- a lot of factors play into rust's spread in the U.S. each year. But growers should be aware of the higher risk this year, Kemerait said.

"It doesn't guarantee we'll have an outbreak, but it means we've got both last year's inoculum and this year's inoculum out there," he said. "That's a concern."

Southern corn rust hasn't been spotted in Georgia yet, most likely because corn is its only host, he added. But volunteer corn plants in southern states and Central America could provide a bridge for the disease to arrive earlier than normal, given the mild winter, he said. "It's not that far away," he said. "It will probably be sooner rather than later."

REDBANDED STINK BUGS ALREADY ACTIVE

The redbanded stink bug is a fierce pest of soybeans, but it's rather wimpy about the cold. LSU AgCenter research has found that a week of freezing temperatures will kill 95% of a population. At 23 degrees, it only takes about seven hours to kill 90% of a stink bug population.

Unfortunately, those conditions didn't appear often enough in the southern U.S. this year, said Louisiana State University Extension entomologist Sebe Brown. As a result, the insects are alive, well and already actively feeding on a favorite host, crimson red clover, which will allow them to build their populations up earlier than ever before.

"Normally, in southern Louisiana, we start seeing redbanded stink bugs in clover at the tail end of February," Brown explained. "This year, we were catching them in the first week of January."

Growers already dealt with heavy populations of this pest in 2019, thanks to late soybean planting, he added. "We had a large population of adult stink bugs going into the winter and we're going to have a large population developing this spring, so we will see the consequences of that when we start growing soybeans this summer," he said.

See more on this risk here: https://agfax.com/….

FROGEYE LEAF SPOT READY TO LEAP INTO ACTION

History suggests frogeye leaf spot could be more troublesome than usual this year, Ohio State University plant pathologist Anne Dorrance warned growers in a university newsletter.

"It appears that when there are less than 10 days during the months of December, January and February of less than 17 [degrees] F, we have had reports of outbreaks of frogeye leaf spot," she wrote. The disease especially thrived in years where a high level of inoculum existed from the previous year and fields were planted to susceptible varieties. Yield losses above 35% occurred in those cases, and early fungicide applications were required, Dorrance reported.

She suggested no-till growers with a history of the disease in their soybean fields consider rotating to corn or another crop, ask for varieties with good tolerance to frogeye leaf spot and scout susceptible varieties carefully.

See the newsletter here: https://agcrops.osu.edu/….

ATTACK OF THE ZOMBIE COTTON PLANTS

Some cotton plants in parts of the Southeast were left standing into the winter due to overly wet fields, and temperatures never dropped low enough to fully kill them.

Although they appeared dead or dormant above the surface, those plants and others, such as winter weeds or cover crops, were likely to have active, living root systems this winter, Kemerait said. "That's a buffet for cotton nematodes," he warned. "That means they have a food source."

The risk of nematode injury and yield loss will be higher for many cotton growers in 2020 as a result, he said. See more on cotton nematodes, which include root knot, reniform and lance nematodes: https://www.cotton.org/….

A new cotton disease might also benefit from these zombie cotton plants.

Since 2017, cotton leafroll dwarf virus has been surfacing in cotton fields across the southern U.S., as far west as Texas. It causes leaf distortions and has a history of major yield losses in South America, although yield effects from it here are still uncertain. The virus is transmitted by aphids and survives on other hosts such as some common winter weeds.

The same leftover cotton plants that may be hosting nematodes this winter are also potential food for aphids, he noted.

"We'd hoped that standing cotton would freeze, but a lot of it didn't, and some of it is infected with this virus," he explained. "Those plants are currently feeding nematodes, and if they bud back out this spring, aphids might feed on them, too, and acquire the virus and spread it."

Kemerait doesn't recommend trying to spray insecticides to control these aphids, which don't need significant numbers to spread the virus. He just urges growers to avoid late cotton planting, and manage their standing cotton fields carefully.

"My main message is after this winter, stalk management might be important because of the nematodes and this virus," he said.

See more on these cotton pests here: https://extension.msstate.edu/… and here: https://www.cotton.org/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/CZ)

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