Evaluation of disease forecasting programs for management of septoria late blight (Septoria appiicola) on celery
Septoria late blight caused by Septoria apiicola damages celery leaves (Apium graveolens), but its main impact is on the marketability of petioles. Three field trials (two in 2005 and one in 2006) were conducted at the Holland-Bradford Marsh, Ontario, to determine whether the number of fungicide applications required to manage septoria late blight could be reduced compared with the standard calendar spray program and whether disease management could be improved. Two disease forecasting programs (Tomcast at three disease severity value (DSV) thresholds and the Septoria Predictor) were assessed relative to a 7-day calendar spray program and a nonsprayed control. Each fungicide timing treatment, which entailed application of the fungicide Bravo 500 (chlorothalonil 50% a.i.) alternating with Champ 2FL (copper hydroxide 37.5% a.i.), was compared with Pristine WG (pyraclostrobin 12.8%, boscalid 25.2% a.i.) alternating with Champ 2FL on each of two celery cultivars. Pristine consistently provided superior control of septoria late blight compared with Bravo. Treatments were applied to two susceptible celery cultivars ‘Florida 683’ and ‘Sabroso’. Both cultivars responded in a similar manner to the treatments. Fungicide applications timed according to Tomcast at a DSV threshold of 10 or the Septoria Predictor resulted in septoria late blight severity levels on leaves and petioles comparable to the calendar spray program. Fungicide applications were reduced by 1 for the Septoria Predictor, by 1 to 3 for Tomcast at DSV threshold 10, and by 2 to 4 for Tomcast at DSV threshold 15. However, a yield reduction of 15% occurred in one trial when application of Bravo/Champ were timed according to Tomcast at DSV 15, indicating that the DSV 15 threshold was not always adequate for septoria late blight management using Bravo.