Leaf wetness duration measurement: comparison of cylindrical and flat plate sensors under different field conditons

Abstract

In general, leaf wetness duration (LWD) is a key parameter influencing plant disease epidemiology, since it provides the free water required by pathogens to infect foliar tissue. LWD is used as an input in many disease warning systems, which help growers to decide the best time to spray their crops against diseases. Since there is no observation standard either for sensor or exposure, LWD measurement is often problematic. To assess the performance of electronic sensors, LWD measurements obtained with painted cylindrical and flat plate sensors were compared under different field conditions in Elora, Ontario, Canada, and in Piracicaba, São Paulo, Brazil. The sensors were tested in four different crop environments--mowed turfgrass, maize, soybean, and tomatoes--during the summer of 2003 and 2004 in Elora and during the winter of 2005 in Piracicaba. Flat plate sensors were deployed facing north and at 45 degrees to horizontal, and cylindrical sensors were deployed horizontally. At the turfgrass site, both sensors were installed 30 cm above the ground, while at the crop fields, the sensors were installed at the top and inside the canopy (except for maize, with a sensor only at the top). Considering the flat plate sensor as a reference (Sentelhas et al. Operational exposure of leaf wetness sensors. Agric For Meteorol 126:59-72, 2004a), the results in the more humid climate at Elora showed that the cylindrical sensor overestimated LWD by 1.1-4.2 h, depending on the crop and canopy position. The main cause of the overestimation was the accumulation of big water drops along the bottom of the cylindrical sensors, which required much more energy and, consequently, time to evaporate. The overall difference between sensors when evaporating wetness formed during the night was around 1.6 h. Cylindrical sensors also detected wetness earlier than did flat plates--around 0.6 h. Agreement between plate and cylinder sensors was much better in the drier climate at Piracicaba. These results allow us to caution that cylindrical sensors may overestimate wetness for operational LWD measurements in humid climates and that the effect of other protocols for angling or positioning this sensor should be investigated for different crops.

Performance of cylindrical leaf wetness duration sensors in a tropical climate condition

“…The wetness onset and dry-off were detected later by the cylindrical sensors than by the flat plates. Sentelhas et al (2006) also observed that cylindrical sensors positioned horizontally indicated the dry-off later than flat plate sensors installed over turfgrass at 45º. This cylindrical sensor later dry-off may indicate that the cylindrical sensor cools down and warms up slower than the flat plate sensor, due to its higher heat capacity and different radiation geometry, resulting in a delay of the wetness onset and dry-off.…”

“…Its sensing surface faces a solid angle of 2π radians (360 degrees), which represents better some organs, such as stems. However, some studies have shown that cylindrical sensors should be used with caution, since there is no protocol for their deploying for standard measurements (Gillespie & Duan, 1987;Sentelhas et al, 2006). Gillespie & Duan (1987) verified that LWD measured by cylindrical sensors vertically deployed in an onion crop was two to three hours shorter, on average, than measurements obtained by flat plate sensors.…”

“…The regression analysis coefficients were significant by a t-test (p < 0.05), showing that the intercepts were not different from zero and the slopes were not different from Table 3 -Average leaf wetness duration over mowed turfgrass (5-cm tall) measured by flat plate sensors, deployed at 45º, and cylindrical sensors, deployed at different angles to horizontal (0º, 15º, 30º, 45º, and 60º), in Piracicaba, SP, Brazil. Comparing flat plate sensors in the standard position (30-cm height and deployment angle of 45º) with cylindrical sensors deployed horizontally, in Elora (43º38' N, 80º24' W, 369 m Altitude), ON, Canada, Sentelhas et al (2006) noticed that LWD obtained by cylindrical sensors was systematically longer than the flat plate measurements. According to these authors, LWD overestimation may be related to the occurrence of droplets of large volume hanging along the bottom of the sensors at the end of the wetting period.…”

“…Gillespie & Duan (1987) verified that LWD measured by cylindrical sensors vertically deployed in an onion crop was two to three hours shorter, on average, than measurements obtained by flat plate sensors. On the other hand, Sentelhas et al (2006) used cylindrical sensors deployed horizontally to measure LWD over turfgrass and other crops in a temperate climate, and observed that the mean LWD was about two hours longer than that measured by flat plate sensors, which was adopted as standard measurement.…”

Obtaining weather data for input to crop disease-warning systems: leaf wetness duration as a case study

“…Most electronic sensors were based on a design developed by Davis & Hughes (1970), which sensed the presence of wetness as a drop in electrical resistance across two adjacent circuits etched onto a printed-circuit board. Later workers found that coating these sensors with latex paint enhanced their sensitivity to wetness (Gillespie & Kidd, 1978;Sentelhas et al, 2004a) (Figure 1), that height and angle of deployment influenced sensitivity (Figure 2), and that electronic sensors could also be fabricated as cylinders rather than flat plates (Gillespie & Kidd, 1978;Gillespie & Duan, 1987;Lau et al, 2000;Sentelhas et al, 2004b;Sentelhas et al, 2007a). A recent innovation in flat-plate sensor design operates on the principle of electrical capacitance rather than resistance (Decagon Devices, Inc., Pullman, WA, U.S.A.).…”