Leaf wetness in dry beans under semi-arid conditions

Abstract

Leaf wetness measurements were made in and at the top of canopies of dry edible bean ( Phaseolus vulgaris L., cvs. A55 and Midnight) for two growing seasons. Two similar approaches were used for this measurement; a cotton cloth (acting as an artificial leaf) and a real leaf were placed on a grid network of independent, adjacent fine wires. This configuration acted as a variable resistor, infinite resistance when dry and decreasing resistance as the leaf became wet, as part of an electronic circuit with an output signal proportional to the degree of wetting. Atmospheric humidity was measured in and above the canopies using fine wire psychrometers. Difficulties encountered in making representative measurements of leaf wetness duration inside dry edible bean canopies were due to problems of representative location and the heliotropic nature of bean leaves. After sunrise, this latter phenomenon could randomly distribute water from the uppermost leaves to locations inside the canopy. Measurements made with the cotton cloth at the top of the canopy had the greatest duration of leaf wetness. This sensor and measurement location are ideally suited for making disease predictions where duration of leaf wetness is an important climatological input. In addition, predictions of leaf wetness duration from the comprehensive, mechanistic plant environment model, Cupid, are included.