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.”