Comparing Disease Forecasters for Timing Fungicide Sprays to Control Foliar Blight on Carrot.

Alternaria dauci and Cercospora carotae cause foliar blight on carrot and can reduce yield in severely blighted fields. Historically, fungicides are applied every 7 to 14 days even though applications may be made when environmental conditions do not favor blight development. The purpose of this study was to compare a calendar-based application schedule with three disease forecasting systems for timing fungicide sprays to limit foliar blight, and included (i) an A. dauci disease forecaster, (ii) TOM-CAST, using a threshold of 15 disease severity values, and (iii) a disease forecaster developed to control C. apii on celery. Chlorothalonil was applied weekly or according to the forecasting systems to blight-susceptible 'Cellobunch' carrot plants in 2001 and 2002. Overall petiole health was poor ≥8.3; 10 = 100% petiole necrosis) when fungicides were not used. Although all disease forecasters maintained petiole health (≤5.3; 1 = healthy and vigorous), the TOM-CAST program had the best petiole health rating each year (≤2.8). TOM-CAST prompted 38 to 54% fewer applications than the weekly application schedule, resulting in a fungicide savings of $105 and $147/ha in 2001 and 2002, respectively, while providing similar blight control. The number of sprays also was reduced when fungicides were applied according to the A. dauci and C. apii forecasters, but acceptable blight control was not always achieved.