Leaf Spot In A Dry Year

Increased presence of disease shows susceptibility of varieties and weakening of fungicides.

Georgia peanut growers experienced problematic leaf spot diseases this year because of susceptible varieties and weakening fungicide treatments, according to Albert Culbreath and Tim Brenneman, plant pathologists at the University of Georgia Tifton Campus.screen-shot-2017-01-02-at-12-13-37-am

Brenneman says this year’s dry conditions should have set up an environment that was less favorable for leaf spot.

“It displays the magnitude of the problem,” Brenneman says. “If we’re having trouble in a dry year, we could really have a serious problem in a wetter year.”

Cultivar Selection Still Critical

Brenneman says that leaf spot could have hurt overall peanut yields in some places this year, but that the dry conditions will be the biggest factor.

“The irrigated crop is not bad,” Brenneman says. “Some dryland areas are horrendous and have been zeroed out for insurance already. The earlier United States Department of Agriculture crop estimate predicted very high yields for Georgia, but as the crop has been coming in, it looks like that projection was optimistic.”
Almost all varieties grown in Georgia, Alabama and north Florida are susceptible to leaf spot, including Georgia-06G, Georgia’s most widely grown variety.

“Growers must be aware of their cultivar selection,” says Brenneman. “Some of the high-oleic peanuts are especially susceptible to leaf spot.” Certain buyers are willing to pay a premium for those cultivars with that oil chemistry since the products made from them have a better shelf life.

“Growers must realize that if they choose to grow those varieties, they must stay on top of their spray programs and not cut any corners.”

Try A Fungicide Combo

Culbreath and Brenneman are working to combat leaf spot in the short and long term. Culbreath says that improving fungicide efficacy is his short-term solution for stopping the disease. He says mixing the less effective fungicides has been more effective than spraying them separately.

“We’re working with experimental fungicides with multiple companies,” Culbreath says. “Some of the experimental fungicides are much more effective than what we have now and represent a lower risk in terms of applicator and environmental exposure. They’re still going to be expensive, but fungicides are a very important part of our leaf spot management program.”

The long-term goal for leaf spot is to help develop varieties that are resistant to the disease.
Culbreath says they are helping determine the relative resistance levels in hundreds of peanut lines and from that will determine the genes or gene groups that are responsible for the resistance.

“With that information, geneticists can develop molecular markers for those genes that a breeder can use to screen a lot of lines quickly for resistance. Hopefully, we will find multiple types of resistance and will be able to use the markers to combine multiple types of resistance into one variety,” he says.

Cultural Practices Are Key

Culbreath has also been working on improving leaf spot control in organic peanut production.

“Brian Jordan, a graduate student, has found that several advanced breeding lines from UGA peanut breeder Bill Branch, such as Georgia-12Y, have the potential to do well without fungicides when used with good rotation and early planting dates to dodge the epidemic,” Culbreath says. “Resistance or tolerance and crop rotation are essential for that type of production, but I think we can put together an integrated system that will work for organic production.”