Operational exposure of leaf wetness sensors

Leaf wetness duration (LWD) is a key factor in plant disease occurrence in many phytopathosystems and, consequently, an important variable in disease warning systems. Measurement of LWD is often problematic because of the lack of a standard sensor, and lack of a standard exposure protocol. Accordingly, operational aspects of LWD exposure were evaluated using data from experiments in three different climatic regions: Elora, ON, Canada; Ames, IA, U.S.A.; and Piracicaba, SP, Brazil. LWD sensors (flat, printed-circuit) were installed at different heights and angles, and above or inside different crops: turfgrass and corn in Elora; turfgrass and muskmelon in Ames; and turfgrass and cotton in Piracicaba. Visual observations of dew onset and dry-off were made for comparison with the different sensor positions. At Elora and Piracicaba, sensors deployed 30 cm above turfgrass and between 15o and 45o to horizontal showed the smallest errors in relation to visual observations of turfgrass wetness, for both dew onset and dry-off. Assuming the sensor at 30 cm and 30o as a reference for LWD measurements over turfgrass it was possible to identify significant differences among the different sensor heights and angles, showing that the position of the sensor had a strong effect on LWD measurements. Sensors at 190 cm measured shorter average LWD - 97 min for Elora and 54 min for Piracicaba - than sensors at 30 cm. No significant difference was observed between the sensors at 30 cm and 70 cm in both places. In Ames, the average difference in LWD between the sensors at 30 cm and 150 cm (both deployed at 45o) was 33 min. In relation to the angle of deployment, sensors at 0o and 15o measured longer average LWD - 38 min for Elora and 56 min for Piracicaba - than sensors at 30o and 45o. LWD measured by sensors near the standard screen height over turfgrass differed considerably from LWD measured by sensors in the canopy, especially during periods with less than 15h of wetness. In contrast, sensors at 30 cm over the turfgrass provided much more accurate estimates of crop LWD despite large differences in crop height and structure.