Evaluation of Disease Thresholds and Predictors for Managing Late Blight in Celery.

Late blight of celery, incited by Septoria apiicola, results in necrotic lesions on leaves and petioles and reduces yield when it occurs on marketable petioles. Michigan celery growers typically begin applying fungicides 1 to 3 weeks after transplanting and reapply fungicides at 7- to 10-day intervals to manage late blight. Using disease thresholds or predictors to prompt fungicide sprays may reduce the number of applications needed for effective disease control. Weekly treatments of azoxystrobin alternated with chlorothalonil were initiated early (1 week after transplanting), preventively (4 weeks after transplanting), or when disease symptoms were detected at a trace, 5%, or 10% level on 'Dutchess' celery plants in 2003 and 2004. The early and preventive programs were equally effective in preventing petiole blight and yield loss. The preventive program required three fewer applications, at a savings of up to $134/ha, when compared with an early fungicide program initiated 1 week after transplanting. Delaying the initial fungicide application until disease symptoms were detected resulted in high disease levels at harvest that were often similar to untreated control plants. Additional field trials were established in 2004 and 2005 at a research farm where S. apiicola inoculum was applied and at a commercial field where early blight, caused by Cercospora apii, developed from naturally occurring inoculum. A fungicide program that alternated chlorothalonil with azoxystrobin was initiated preventively and reapplied weekly or according to the Septoria predictor or TOM-CAST 10-disease severity value (DSV) and was compared with the standard, weekly application program initiated early. Combining the use of preventive initial applications with the Septoria predictor or TOM-CAST 10-DSV reduced the number of sprays by two to six while providing disease control that was comparable with the standard weekly fungicide program initiated early. These programs reduced fungicide expenditures by $71 to $213/ha compared with the weekly fungicide program initiated early.