E. A. Santos

Spatial variability of leaf wetness duration in cotton, coffee and banana crop canopies

Despite the importance of leaf wetness duration for plant disease epidemiology, there has been little attention paid to research on how its variability relates to different cropping situations. The objective of this study was to evaluate the spatial variability of leaf wetness duration (LWD) in three crops, comparing these measurements with turfgrass LWD, obtained in a standard weather station. LWD was measured by electronic sensors in three crops with different canopy structures and leaf area: cotton, coffee and banana. For the cotton crop, cylindrical sensors were deployed at the lower third and on the top of the canopy, facing southwest. For the coffee crop, flat plate sensors were installed in the lower third of the canopy facing northeast and southwest; in the middle third facing northeast and southwest; and inside and on the top of the canopy. For the banana canopy, cylindrical sensors were used to measure LWD in the lower third of the canopy and in the upper third of the plant. Turfgrass LWD was simultaneously measured in a nearby standard weather station. The LWD showed different patterns of variation in the three crop canopies. For coffee plants, the longest LWD was found in the lower portions of the canopy; for the banana crop, the upper third of the canopy showed the longest LWD; whereas for the cotton crop no difference was observed between the top and lower third of the canopy. Turfgrass LWD presented a good relationship with LWD measured on the top or in the upper third of the crops. Thus, the estimate of crop LWD can be perfomed based on turfgrass LWD, this being a useful tool for plant disease management purposes for crops in which the longer LWD occurs at the upper canopy portion.

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

ABSTRACT: Leaf wetness duration (LWD) measurements are required for disease warning in severalagricultural systems, since it is an important variable for the diagnose of plant disease epidemiology.The cylindrical sensor is an inexpensive and simple electronic LWD sensor initially designed tomeasure this variable for onions, however some studies show that it may be helpful for standardmeasurements in weather stations and also for different crops. Therefore, the objective of this studywas to assess their performance under tropical climate conditions, in Brazil, having as standardmeasurements those obtained by flat plate sensors, which have presented very good performancewhen compared with visual observations. Before field assessments, all LWD sensors used in ourstudy (flat plates and cylinders) were white latex painted and submitted to a heat treatment. Laboratorytests were performed in order to determine the resistance threshold for the sensor to be considered wetand the time response of the sensors to wetness. In the field, all cylindrical sensors were initiallydeployed horizontally 30-cm above a turfgrass surface in order to assess the variability among them withrespect to LWD measurements. The variability among the horizontal cylindrical sensors was reduced byusing a specific resistance threshold for each sensor. The mean coefficient of variation (CV) of LWD datameasured by the cylindrical sensors was 9.7%. After that, the cylindrical sensors were deployed at fivedifferent angles: 0o, 15o, 30o, 45o, and 60o. Data of measurements made at these angles were compared withthe standard measurement, obtained by flat plate sensors at the same height and installed at 45°. Thedeployment angle had no systematic effect on LWD measurements for the local tropical conditions,since the correlations between flat plate and elevated cylinder measurements were very high (R