Comparison of wetness sensors and the development of a new sensor for apple scab prognosis

Apple scab prediction is based on the model of Mills which describes the risk of Venturia inaequalis infection using leaf wetness duration and air temperature data. The quality of prognosis relies on sensor quality. Six brands of commercially available leaf wetness sensors were compared at the Julius Kuehn-Institute (Germany) apple orchard from 2011 to 2013. Also a new wetness sensor was developed and optimized from 2009 to 2015 in cooperation with Thies Clima company. The Thies-wetness sensor was equipped with a glass–ceramic surface, an adjustable heating system to avoid dew and an adjustable cooling system to prolong wetness duration of sensor surface. Prototypes were tested in climate chamber before exposition in the orchard to achieve sensor settings with drying properties broadly similar to apple leaves. The market-ready prototype showed suitability for prognosis in field assays in primary seasons of 2015–2017 and maybe integrated into prognosis of apple scab without further modeling. The Thies-sensor was shown to be more reliable, uniform, sensitive and more suitable for forecasting than the purchased models.