Timing the application of fungicides to control potato early blight (Alternaria solani) in multi-location field trials in Denmark.
Field experiments were carried-out at three locations in Denmark (i.e. Flakkebjerg, Sunds and Billund) in order to evaluate different spray strategies to control potato early blight. The treatments evaluated in the experiments were: (1) untreated, (2) a standard application in which fungicide application started at row closure, (3) starting fungicide application at the onset of first symptoms (First Symptoms), (4) starting fungicide application at 14 days after onset of first symptoms (Late Application), (5) fungicide application according to the maturity of the potato (Maturity-Based Model), (6) Modified TOMCAST, in which first spray was done at 330 physiologic days (Pdays) and subsequent spray was done according TOMCAST Disease Severity Value (DSV) threshold, and (7) TOMCAST+Maturity-based model, in which we combined the TOMCAST DSV and Maturity-Based Model. In general, our results showed that starting spraying from onset of symptoms could provide sufficient early blight control comparable to standard practice without any yield penalties. Starting application of fungicide later in the season resulted in lower disease control. The Modified-TOMCAST, Maturity-Based Model and TOMCAST + Maturity-based models controlled early blight effectively in all the experimental locations. In general, our results showed that low area under disease progress curve (AUDPC) values were associated with the fungicide treatments compared to the untreated. However, fungicide applications which started at 14 days after first symptoms resulted in lower disease control compared to the standard and other fungicide treatments. Starting application at the onset of symptoms resulted in low AUDPC blight control comparable to the standard treatment. The decision support models showed a potential for optimizing the control of early blight by better targeting fungicide applications.