ESTIMATING LEAF WETNESS IN DRY BEAN CANOPIES AS A PREREQUISITE TO EVALUATING WHITE MOLD DISEASE

In semiarid regions, wetness of leaves or other plant parts is essential for development of white mold disease [caused by Sclerotinia sclerotiorum (Lib.) de Bary] in dry edible bean (Phaseolus vulgaris L.). Our goal was to monitor leaf wetness in field plots and predict leaf wetness from prevailing meteorological conditions. The dew point within the canopy and the adjacent leaf temperature (infrared) were used to estimate leaf wetness in two dry bean cultivars of highly contrasting canopy architecture. These are simple, direct measurements that do not disturb the canopy. Sensors were calibrated before and after field measurement periods. Results were compared with the saturation air vapor pressure deficit from a nearby weather station to derive an empirical equation for estimating leaf wetness. The measured leaf wetness period in the dense canopy (cv. Tara) exceeded that in the open canopy (cv. Starlight) by 14% (plus an offset of 1.35 h). This corresponded to higher white mold disease (WM) severity in the dense than in the open canopy. The respective WM severity for dense and open canopies was 93 and 32% in 1990, and 73 and 10% in 1991. The empirical relationship for estimating hourly leaf wetness explained 81% of the variance in the open canopy and 70% of the variance in the dense canopy in 1991; in 1992, accuracy approached 90%. Thus, nearby weather station data offer input to a method that in turn offers a means of assessing leaf wetness in Tara and Starlight, with potential for use with other cultivars. Theoretical considerations indicate that the empirical coefficients are a function of canopy microclimate. Further studies should evaluate the relationship between these empirical coefficients and canopy architecture.