Add another to the growing list of weeds, insects, and diseases that have developed resistance to the chemicals that farmers rely on to control pests and protect yields — strobilurin-resistant Cercospora sojina, the fungus that causes frogeye leafspot in soybeans. 

The strobilurin chemistry has been widely used as a first line of defense in preventing yield loss as a result of the disease.

“There was documentation in 2010 of strobilurin-resistant Cercospora in Illinois, Kentucky, and Tennessee,” Tom Allen said at the annual conference of the Mississippi Agricultural Consultants Association.

“In one particular field in Tennessee, frogeye leaf spot couldn’t be controlled with two 6-ounce applications of a strobilurin-based fungicide,” says Allen, assistant Extension/research professor, plant pathology, at the Delta Research and Extension Center at Stoneville, Miss. “In Mississippi, our best frogeye management tool in the past has been the strobilurin products.”

With resistant Cercospora, he says, laboratory studies have shown loss of efficacy for strobilurins as much as a thousand-fold, “and that’s pretty scary.”

There are other products labeled for the disease, Allen notes, and in order to maintain the effectiveness of the strobilurins, “I would suggest rotation with some of these alternate chemistries, based on the presence or absence of disease.”

Last year also saw a new problem for Mid-South soybean growers — soybean vein necrosis virus.

“It’s related to tomato spotted wilt virus, and is likely transmitted by thrips,” he says. “When the insect feeds on the leaves, the virus tends to follow the veins. In more advanced stages leaves developed a scorched appearance.

“It has been more of a problem in the Midwest, and in 2007 it was identified in Tennessee. It’s now pretty widespread in Arkansas, but not so much so in Mississippi. I found it in Mississippi in the latter part of the 2010 season. It typically is not found in the upper part of the canopy, and can be difficult to distinguish from some other diseases.

“We don’t know a lot about the disease yet, where it came from, or if it can be seed-transmitted, but research is under way in Arkansas to develop more information.”

Southern corn rust widespread in 2010

There was “a phenomenal outbreak of southern corn rust across the Mid-South and South,” last year, Allen says. “We hadn’t previously seen that much southern corn rust in early June.”

One of the reasons behind the spread of the disease, he notes, is that farmers in Mexico are growing more corn in areas where it hasn’t traditionally been grown. “This likely is a factor in a great deal of our southern corn rust, common rust, and rust situations in other crops.”

None of the rusts overwinter in the U.S., he says, except for a little soybean rust. “If we have rust in our fields, whether it be corn or even wheat, it has to blow in from somewhere else, like Mexico.

“We had a lot of questions from growers about southern corn rust last year, particularly if it would result in lodging, and how they could address that concern.

“For years, we’ve said that, based on previous research, applying foliar fungicides two weeks prior to black layer would likely not provide an economic return in most years.  But in a year like 2010, with high levels of southern corn rust and a prolonged conducive environment, fungicides made a difference.”

In a field trial at Stoneville, Allen says, corn treated with 6 ounces of Headline fungicide outperformed all other treatments, including other strobilurin and triazole chemistries.

“Are we ever likely to see that much southern corn rust again? Maybe. Will we see it in 2011? Probably not.

Was there any lodging as a result of the disease in my trials? Essentially, no.

“Is your corn going to fall over if you get southern corn rust at a high level? Possibly, but that would be more likely if high levels of infection occur earlier in the season. With late-planted corn, you may have to manage southern corn rust with more than one fungicide application — this is one reason corn following wheat is such a difficult sell.”

Aerial web blight found in several areas

Although some say 2010 was not necessarily a bad disease year for soybeans, there were some problem areas, Allen says.

“Aerial web blight was more a problem than in the past in the northeast and eastern areas of Mississippi. We didn’t see much in the Delta, but I think every northeast Mississippi field I was in had some aerial web blight. More typically, though, it has been a problem in soybean-rice rotations.

“It’s not easy to identify. You’ve got to get out and dig into the canopy to find it, usually in the lower portions, although it can be found higher up, depending on the time of the day. It’s one of those diseases that, if you don’t do something about it when you find it, it can take everything — leaves, pods, flowers, everything.”

Aerial web blight usually occurs in places where plants don’t get good air flow, Allen notes. As it progresses, plants will have reddish-brown leaves that are stuck together and matted. It can occur on petioles, pods, almost anywhere on the plant.

“Response to fungicides can vary widely from one field to another,” he says. “Just about every product gave an economic return when applied around R5 in trials conducted in east Mississippi.”

Managing soybean seed quality can be “a ridiculously complex challenge,” he says and can include early management decisions, environmental factors, pest management, and late management decisions.

“In 2010, we continued a massive seed quality study throughout Mississippi, with several different treatments, timings, and management practices. In almost all of the locations, the best returns were obtained when fungicide applications were made at R3.”

The Soybean Rust Hotline, which has been available to concerned members of the agriculture community since 2006, will be offered again in 2011, Allen notes.

“It’s a free service, sponsored by the Mississippi Soybean Promotion Board and BASF and it will have the latest information on the disease throughout the season.”

The toll-free telephone number for the Soybean Rust Hotline is 866-641-1847.