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An algorithm was developed for assessing disease risk and improving fungicide timing for control of Sclerotinia blight of peanut, caused by Sclerotinia minor. A 5-day index (FDI) of disease risk was calculated daily by multiplying indices of moisture, soil temperature, vine growth and canopy density and summing the values for the previous 5 days. Spray thresholds of FDI 16, 24, 32, 40, 48 were compared to a 60, 90, 120 DAP (days after planting) schedule and the standard demand program. Field trials in 1994 indicated that fluazinam (0.58 kg a.i./ha) applied at an FDI of 32 performed similarly to the demand program and was more efficient than the DAP schedule. However, the original FDI 32 algorithm triggered sprays 13 days subsequent to disease onset in 1995, indicating the need for improved vine growth and temperature parameters as well as DAP-dependent FDI thresholds. Results from 1996 and 1997 demonstrated that algorithms with new vine growth and temperature parameters coupled with DAP-dependent thresholds performed as well or better than the original FDI 32 algorithm, demand program, or DAP schedule. Protection intervals of 7 and 14 days improved the performance of iprodione (1.12 kg a.i./ha) while fluazinam provided protection for up to 21 days when applied according to the original FDI 32 algorithm. Planting date was evaluated for its effect on disease and fungicide use patterns. Late planting (20-28 May) delayed disease onset and reduced early season disease incidence three of the four years tested. When averaged across planting dates, the original FDI 32 algorithm performed as well or better than the demand program in 1994 and 1995, as did algorithms utilizing new vine growth and temperature parameters with DAP-dependent thresholds in 1996 and 1997. Chemicals for altering plant architecture were compared to defoliation by corn earworm and leaf spot for suppression of Sclerotinia blight. Chlorimuron (8.8 g a.i./ha) and withholding fungicide for leaf spot control demonstrated the most significant disease suppression and yield improvement. Results show the importance of fungicide timing and plant growth and canopy architecture modification for control of Sclerotinia blight of peanut. / Ph. D.

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