DEVELOPMENT OF A PATHOGEN GROWTH-RESPONSE MODEL FOR THE VIRGINIA PEANUT LEAF-SPOT ADVISORY PROGRAM

A new advisory program (ADV) to improve the efficiency of fungicide applications for control of early leaf spot of peanut was developed on the basis of growth responses of Cercospora arachidicola to specific environmental conditions. The new program, 89-ADV, assigned time-duration values to conditions conducive to infection (TDV(i)). Cumulative TDV(i) levels were used to determine the critical times for fungicide applications. Various spray thresholds (TDV(i) = 48, 72, and 96) of the 89-ADV program, along with a 14-day spray schedule and the original advisory program that was released in 1981 (81-ADV), were tested on Florigiant peanut during 1987-1989. Area under the disease progress curve (AUDPC) and leaf spot incidence at harvest were significantly lower in plots sprayed with chlorothalonil (1.25 kg/ha) according to the 89-ADV program than in plots sprayed according to the 81-ADV program. The plots of the 89-ADV program had a TDV(i) = 48 threshold. Higher advisory thresholds (TDV(i) = 72 and 96) in the 89-ADV program resulted in similar or better disease control than the 81-ADV program at fewer sprays per season. In 1988 and 1989, crop yield and value were significantly improved using the 89-ADV program with a TDV(i) of 48 in comparison with the 81-ADV program. No significant differences in yield or value were apparent in comparisons of the 89-ADV program and a 14-day spray schedule. Evaluations of spray programs under simulated disease environments constructed from historical weather data from 1983 to 1986 also demonstrated the superior disease control efficiency of the 89-ADV program with a TDV(i) of 48 to that of the 81-ADV program. In both field and simulated tests, the number of sprays per season with the 89-ADV program at TDV(i) = 48 and the 81-ADV program were similar. Based on these results, the 89-ADV program with a spray threshold of TDV(i) = 48 was adopted as the on-line advisory program in Virginia for growers at the start of the 1989 growing season.