An advisory program to determine the timing of fungicide application for control of early leafspot of peanut, caused by Cercospora arachidicola, was developed based on growth responses of the pathogen to specific environmental conditions. The advisory program (89-ADV) assigned time-duration values to conditions conducive for infection (TDVi). Cumulative TDVi levels were used to determine when fungicide applications were needed. Various spray thresholds (TDVi=48, 72, 96, 120) of the 89-ADV program were compared to a 14-day spray schedule and an advisory program that was released to growers in 1981 (81-ADV). Leafspot incidence, area under the disease progress curve (AUDPC), spray number, yield and value were used to evaluate the performance of spray programs. Preliminary trials used Florigiant peanut and the fungicide chlorothalonil (1.26 kg/ha). The 89-ADV program with TDVi=48 performed exceptionally well for three consecutive years in field tests and in simulated disease environments reconstructed from historical weather data. Benefits of this program compared to the 81-ADV program included significant improvement of leafspot control, and improved crop yield and value. Based on performance, the 89-ADV program was delivered to growers as the on-line peanut leafspot advisory in 1989. Subsequent evaluations of the 89-ADV program included cultivars and fungicides in large multi-factorial experiments. Three classes of cultivars were identified: class I or highly susceptible, Florigiant and NC 9; class II or moderately susceptible, NC 7 and NC-V11; and class III or moderately resistant, NC 6. The efficiency of fungicide sprays was improved through effective leafspot control with about three fewer sprays per season than the 14-day spray schedule when chlorothalonil at 1.26 kg/ha, diniconazole at 140 g/ha or terbutrazole at 126 g/ha was applied on class I cultivars according to the TDVi=48 threshold of the 89-ADV program. The same efficiency was achieved when chlorothalonil or terbutrazole was applied on class II cultivars according to the TDVi=96. Cupric hydroxide at 1.79 kg/ha plus sulfur at 1.04 kg/ha or terbutrazole at 126 kg/ha with TDVi=96 as well as chlorothalonil at 1.26 kg/ha with TDVi=120 resulted in efficient control of disease on the class III cultivar. The spray adjuvant SoyOil 937® at 0.5% of spray volume consistently improved the performance of chlorothalonil, and allowed a reduction of application rate from 1.26 to 0.95 kg/ha without sacrificing disease control. The integrated use of cultivar resistance, fungicide, spray adjuvant, and TDVi thresholds of the 89-ADV program contributed to a reduction of fungicide input and improved disease control. / Ph. D.