Validation of a forecast system for onion downy mildew

ABSTRACT Aimed at validating a forecast system with different levels of daily severity values, compared to conventional spraying to control onion downy mildew, experiments were conducted in Rio do Sul, Santa Catarina State (SC), during the crop seasons of 2014, 2015 and 2016. Spraying regimes were established according to the daily severity values (DSV) of Wallin system (1962), assigning accumulated values of 6, 8, 10 and 12 DSV, and in the conventional system with spraying at every 5 and 7 days, compared to the untreated control. Except for the control, there was no significant difference among treatments for productivity in all cycles. The area under the disease progress curve (AUDPC), the final severity and the disease progression rate did not differ among treatments, but in the forecast system with DSV of 12, the spraying number was 42, 30 and 40% lower in the three years of evaluation, respectively, relative to the weekly application system.