Foliar disease has shifted from early to late leaf spot, and fungicide shortages will mean a change in management plans.
• By Amanda Huber •
“It was the biggest leaf spot year we have had in a while,” says Albert Culbreath, University of Georgia plant pathologist. “We’ve shifted primarily from early leaf spot to late leaf spot.”
“Regardless of variety planted, most producers will need to apply fungicides and use some cultural practices,” Culbreath says.
Last year offered extremely heavy pressure in which to test varieties, and Culbreath says there are varieties with better resistance to leaf spot than the variety, Georgia-06G. On Peanut Rx, Georgia-12Y, Tifguard and TifNV-HiOL offer the least amount of risk points.
Earlier planting favors less leaf spot, as does reduced tillage, though it is only a five-point difference on the risk index.
Increase Use Of Cultural Practices
Bob Kemerait, University of Georgia Extension plant pathologist, says “Risk to leaf spot is of increased concern in 2018 because of limited supply of some important fungicides and concerns over current efficacy of some of our once ‘better’ products. Growers should develop a plan for best management of leaf spot diseases that includes variety selection, timing of application and fungicide selection.”
Most Fields At Risk
Dan Anco, Clemson University Extension specialist, says it was a problem even for some South Carolina growers with good fungicide programs.
“Late leaf spot did get out of hand in a number of fields,” he says, and field history greatly affects late leaf spot risk because leaf spot spores persist on peanut residue in the soil.
“Adjacent fields that had late leaf spot at the end of the previous season can be a source of significant infection this season.”
Then again, Anco says, late leaf spot spores can be carried for many miles in the wind and, therefore, any field is at some risk regardless of peanut history.
“All fields should be rotated out of peanut for a minimum of two years to reduce late leaf spot pressure,” he says.
Late leaf spot is diagnosed by the black spores on the underside of dark brown to black lesions on leaves. Yellow halos may or may not be present surrounding late leaf spot lesions.
Expect Fungicide Shortages
Plant pathologists across the peanut belt have been warning producers this spring about the likelihood of shortages of two commonly used fungicides: chlorothalonil and tebuconazole.
“The supply of some fungicides commonly used to protect the peanut crop from disease will likely be impacted in the upcoming season by production issues, especially in China,” Kemerait says. “Growers are encouraged to work with their Extension agents, consultants and chemical dealers to develop appropriate and effective fungicide programs based upon the level of risk in their fields and the anticipated availability of specific fungicides.”
“As always, an effective disease management program strongly benefits from the combined use of multiple integrated pest management practices such as variety resistance, length of rotation period and planting date,” Anco says.
In anticipation of a limited supply of chlorothalonil and tebuconazole in 2018, Anco offered example programs, shown in the chart, in the Clemson Cooperative Extension’s Peanut Money-Maker 2018 Production Guide.
“Where available supplies are further limited, the more effective 1.5 pint rate of Bravo listed may be reduced to the 1 pint rate or substitutions may be used,” Anco explains.