Comparison of weather-based advisory programs for managing early leaf spot on runner and Spanish peanut cultivars

Weather-based advisory programs for scheduling applications of chlorothalonil to control early leaf spot of peanut were evaluated for two runner cultivars (Florunner and Okrun) and one Spanish cultivar (Spanco). Advisory programs included the following: (i) the Parvin, Smith, and Crosby program that utilizes daily infection indices based on the favorableness of relative humidity (RH) and temperature (T) for infection over a 2-day period (PSC); (ii) the modified PSC program for cultivars with partial resistance that reduces daily infection indices by 0.85 (0.85*PSC); (iii) the Virginia program that accumulates hours of T and RH favorable for infection with spray thresholds of 36 to 96 h (VA-36 to VA-96); and (iv) the AU-Pnuts program based on precipitation events and/or probability forecasts. Using the standard 14-day program, 6.7 sprays per year were applied to Spanco and 7 sprays per year were applied to the runner cultivars. Disease control, the reduction in area under the disease progress curve compared with the unsprayed treatment, was greater than 90% with the 14-day program for all cultivars. The PSC and 0.85*PSC programs failed to recommend needed sprays for all cultivars in 1992 and 1993 when disease incidence for these programs did not differ from the unsprayed control. For Spanco, the VA-36 (3.3 sprays/year) and AU-Pnuts (4.5 sprays/year) were the most effective advisory programs, providing 53 to 78% disease control. Except for the VA-48 and VA-60 programs in 1992, disease control on Spanco for VA programs was reduced with increasing spray thresholds of 48 to 96 h. The VA and AU-Pnuts programs provided better disease control on the runner cultivars than on Spanco. Disease control for the VA-36 (3.3 sprays/year), VA-48 (2.3 sprays/year), and AU-Pnuts (4.5 sprays/year) programs ranged from 62 to 95%, and was 80% or greater in 1992 and 1993, when levels of leaf spot were highest. The VA-60 and VA-72 programs also reduced disease incidence and area under the disease progress curve (AUDPC) for the runner cultivars each year compared with the control. In 1992 and 1993, when leaf spot reduced yields of Spanco, the VA-36, VA-48 (2.3 sprays/year), VA-60 (2.5 sprays/year), and AU-Pnuts programs were the only advisory programs that had yields equal to the 14-day program. Yields of the runner cultivars were not reduced by leaf spot in any year. The VA program was the most efficient advisory program tested. The optimum spray threshold for the VA program depended on the cultivar grown and the level of control desired. Spray thresholds of 36 h for Spanish and 48 h for runner cultivars have been implemented commercially in Oklahoma.