Agronomy and Horticulture Department


Date of this Version



MAGAREY, SEEM, WEISS, GILLESPIE & HUBER, Estimating Surface Wetness on Plants, Micrometeorology in Agricultural Systems, Agronomy Monograph no. 47.


Copyright 2005. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America.


Plant surface (leaf, stem, petiole, etc) wetness or leaf wetness is an important agricultural weather variable for the prediction of plant diseases (Yarwood, 1978). The estimation of surface wetness has received considerable attention from a diverse group of scientists but unfortunately, no standard for measuring surface wetness has been widely accepted. Since surface wetness is not a true meteorological variable, as is temperature or precipitation, its measurement has not been routinely made as part of a general observation program by most national meteorological agencies. The main problem of not observing surface wetness on a routine basis has been the lack of agreement about a standard sensor design and protocol for the use of sensors. In addition, most sensors measure surface wetness indirectly and have different physical properties. In order for a sensor to represent a particular crop, plant organ, or environment, the sensor should be calibrated with visual observations of surface wetness. Unfortunately, the collection of visual observations is labor intensive and difficult, since there is no universally accepted definition for plant surface wetness.

In this chapter, we will attempt to define surface wetness in physical terms and review methods for determining surface wetness. We also will consider solutions to the lack of surface wetness standardization from both a measurement and simulation perspective. And finally, we will also recommend the most suitable protocols for estimating surface wetness.