Supervised control of Cercospora leaf spot in sugar beet

Two approaches to supervised control of Cercospora leaf spot (CLS) in sugar beet are outlined, and their yield improving capabilities tested in field trials. One approach is based on action thresholds, which use disease severity assessments to indicate the need for a fungicide application. The second is based on measurements of within-crop temperature and relative humidity, to indicate the need for a fungicide application. Disease incidence (% diseased plants) in fungicide treated plots increased in a similar way to unsprayed plots. Incidence increased for all treatments before disease severity, assessed using the Agronomica scale, increased. There was little effect of fungicide treatments on model parameters for disease incidence and severity curves. However, fungicide treatments reduced disease severity by up to 50% and resulted in significantly higher relative sugar yields than the unsprayed treatment (set at 100%). Relative sugar yields from applying the action threshold for two early sprays (115%) and for both weather-based treatments (119% and 121%) did not differ significantly from the calendar sprayed treatment (119%), saving up to two fungicide applications. Fungicide treatments based on weather conditions delayed the increase in disease incidence, and were applied 1 week before treatments based on the first action threshold. Considering the significantly higher financial returns in 2001 and the positive experience with the weather-based system in 2003 and 2004, we conclude that sugar beet growers in the Netherlands should move towards using supervised control approaches for CLS in sugar beet although some further testing of the weather-based system will be necessary.