Four field experiments were conducted in 1998 and 1999 to evaluate the reliability of two forecast systems for carrot (Daucus carotae) leaf blight. Six fungicide spray programs based on accumulation of daily severity values (VDS) or infection equivalence values (EI) were compared to two calendar-based spray programs and one non-sprayed control. A data logger placed among carrot plants recorded 25 to 94 VDSs for Alternaria dauci and zero EI for Cercospora carotae. With the VDS-based system, sprays of the protectant fungicide chlorothalonil were reduced from 7.5 (7-day schedule) to 3.5 (VDS 3-5), 2.7 (VDS 6-8), and 2 (VDS 9-11) per crop cycle. Under the same conditions the systemic fungicide metconazole was sprayed 3.7 (14-day schedule), 2.5 (VDS 3-5), 1.7 (VDS 6-8), and 1.5 (VDS 9-11) times. The relationship of AUDPC to root weight was significant in two experiments (P = 0.005 to 0.016). Considering the results on root yield, the 3-5 VDS system equalled the calendar-based programs in 3/4 of the cases. Therefore, control of carrot leaf blight in local conditions may be based on such a system and systemic fungicides with efficacy similar to or higher than metconazole should be used.
