Variable rate fungicide application could help peanut farmers save money on soil-borne disease control, but Oklahoma State University agronomists and plant pathologists are still looking for the best ways to identify management zones within a field.

Chad Godsey, OSU Extension agronomist, said soil borne diseases such as sclerotinia may not require field-wide treatment. “We need to delineate the management zones that need sprayed,” he said during the recent American Peanut Research and Education Society annual conference in San Antonio.

Godsey and others tested several techniques to identify disease intensity throughout the field. Techniques included Green Seeker units in-season, soil sampling on half-acre grids, Veris machines to establish soil conductivity and satellite imagery from past growing seasons.

“Last year was the first time we tried variable rate fungicide application in a producer’s field,” Godsey said. “We applied Endura at various rates across the field based on the potential for sclerotinia to develop.”

He said satellite imagery “did define management zones as long as the resolution was fine enough to detect high disease areas and the imagery was from a year when disease pressure was high. Electrical conductivity did not correlate well to sclerotia counts.”

They also took soil samples and counted sclerotia to identify high populations and generate a prescription map for the grower to spray. Fungicide rates ranged from zero to 15 ounces of Endura per acre.

“We added Bravo to control leaf spot to assure that disease was not a contributing factor.”

They used a yield monitor to assess production across the management zones.

“We found no statistical difference in disease control and yield between our variable rate application strips and the adjacent strips where the producer applied a flat rate of 12 ounces across the entire strip,” Godsey said. “But we had light disease pressure in 2010. It was hot and dry in August and not a lot of disease pressure.”

If a grower were deciding between whole-field application and variable rate, however, he could save as much as $928 with variable rate, Godsey said. That figure is based on a 40-acre test field and a $2,200 treatment cost for the entire field.

“The VR program looks feasible to treat soil-borne diseases,” he said. “It could save money and could increase yield in fields with high disease pressure. In years with light disease pressure, targeted or variable rate application is a feasible option.”