COMPARISON OF DECISION METHODS TO INITIATE FUNGICIDE APPLICATIONS AGAINST CERCOSPORA BLIGHT OF CARROT

In 1991 and 1992, two thresholds of a forecasting model were compared with two other decision methods for effectiveness in timing the first fungicide application against Cercospora blight of carrot induced by C ercospora carotae. The first fungicide application was made when : 1) the plants reached 15 cm in height (conventional method); 2) the inter mediate (middle) leaves of 50% of the plants were diseased (50% diseas e incidence threshold method); 3) the cumulative infection equivalence (CIE) was 14(forecasting model CIE 14); and 4) the CIE was 18 (foreca sting model CIE 18). In all four treatments, subsequent applications o f fungicide were made at 10-d intervals when there was no rain, and at 7-d intervals when there was rain. The CIE was calculated based on du ration of leaf wetness and temperature during the wet period, correcte d for high humidity and interrupted wet periods, and was cumulative st arting at crop emergence. For thresholds of CIE 14 and CIE 18, no yiel d losses were observed and the total number of fungicide applications needed was lower compared to conventional and 50% disease incidence th reshold methods. In a separate study, the CIE thresholds were related to the percentage of commercial fields that reached disease incidence thresholds of 50, 80 and 100% to establish low risk (CIE 11-15) and hi gh risk (CIE 16-20) thresholds. The forecasting of low and high risk C IE thresholds were too late for 3 and 19% of the commercial fields bec ause those fields had more than 50 and 80% of the plants diseased, res pectively.