E. A. Dorman, former Graduate Assistant,
B. J. Webster, Research Assistant, and
M. K. Hausbeck, Professor, Michigan State University, East Lansing 48824-1311
Alternaria dauci and Cercospora carotae cause foliar blight on carrot, causing yield reductions in severely blighted fields. Currently, fungicides are used on either a 7-day schedule or according to the TOM-CAST disease forecasting system. Organic production prohibits applications of most fungicides for blight control but does allow select copper-based products. The objective of this study was to use the TOM-CAST weather forecasting system to (i) assess the efficacy of copper hydroxide treatments for organic operations and (ii) evaluate the efficacy of reduced-risk products in an alternating spray program. Chlorothalonil, azoxystrobin, and copper hydroxide were applied alone or in alternating programs in 2001 and 2002. Reapplications were made on a 7-day schedule or according to TOM-CAST using disease severity value (DSV) thresholds of 10, 15, and 20. Area under the disease progress curve (AUDPC) data revealed that all application intervals significantly limited disease incidence on the foliage and petioles, resulting in healthier petioles at harvest compared with the untreated. The 7-day and TOM-CAST 10 DSV programs had comparable rating values for all parameters assessed and were more effective than the TOM-CAST 15 and 20 DSV programs in limiting petiole disease and maintaining overall petiole health. Copper hydroxide treatments resulted in higher AUDPC values and significantly different petiole health ratings compared with other treatments, yet P values indicated significant disease control compared with the untreated. The TOM-CAST 15 DSV program reduced sprays up to 40% compared with a 7-day interval and produced mean savings of $127/ha in 2001 and $137/ha in 2002.