Integrating Disease Thresholds with TOM-CAST for Carrot Foliar Blight Management.

Cercospora carotae and Alternaria dauci cause foliar blight on carrot (Daucus carota subsp. sativus) and are managed with fungicides to prevent yield loss. Sprays are initiated prior to disease symptoms and reapplied frequently, but some of these applications may not be necessary when the threat of disease is low. Delaying the initial fungicide spray until disease symptoms are observed and applying subsequent sprays according to the TOM-CAST disease forecaster may reduce fungicide inputs. The objective of this 2-year field study was to compare preventive, calendar-based application schedules with an integrated management approach that incorporates disease scouting to initiate fungicide application and the TOM-CAST system for timing subsequent fungicide sprays to manage foliar blight on processing, fresh market, and cut-and-peel carrot cultivars in Michigan. Applications of the fungicides chlorothalonil alternated with azoxystrobin were made prior to disease symptoms (0% blight) or when the foliage became blighted at a trace, 5%, or 10% severity level. Fungicides were reapplied every 7 or 10 days or according to TOM-CAST using disease severity value (DSV) thresholds of 15, 20, or 25. Initiating fungicide treatment at a trace level of disease and timing subsequent sprays according to the TOM-CAST 15-DSV forecaster was comparable to the preventive, calendar-based fungicide regime. One to five fewer applications were needed, while fungicide costs were reduced by $21 to $141 per hectare, when spraying at the trace disease threshold and reapplying according to the TOM-CAST 15-DSV program compared with the 7- or 10-day intervals initiated at 0% blight. Fungicide programs initiated at 5 or 10% leaf blight often provided less control than programs initiated at 0% and trace disease. This study highlights the importance of initiating a fungicide program prior to advanced foliar blight and validates the TOM-CAST 15-DSV forecaster for managing Cercospora leaf spot and Alternaria leaf blight in three carrot production systems.