CAUSES OF WETNESS PERIODS AND MEASUREMENT OF LEAF WETNESS DURATION FOR PLANT-PROTECTION PURPOSES

Mainly as a result of precipitation in the form of rainfall or dew, moisture on leaves plays a major role during the process of plant infection by a number of fungal pathogens. Leaf wetness duration and temperature must be taken into account in developing plant disease epidemic models or agricultural warning systems. Natural and artificial causes of canopy wetness are described. Leaf wetness duration (a variable that is difficult to define) is governed by both leaf and canopy structural parameters. Several physical principles are used to construct various types of sensors (mechanical instruments, resistance sensors, radiometric methods). A critical review of the instrumentation available for measuring leaf wetness duration is presented. Both leaf and canopy scales are considered in an effort to answer the demand from plant pathology and plant protection specialists.