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 essenti al for development of white mold disease [caused by Sclerotinia sclero tiorum (Lib.) de Bary] in dry edible bean (Phaseolus vulgaris L.). Our goal was to monitor leaf wetness in field plots and predict leaf wetn ess from prevailing meteorological conditions. The dew point within th e canopy and the adjacent leaf temperature (infrared) were used to est imate leaf wetness in two dry bean cultivars of highly contrasting can opy architecture. These are simple, direct measurements that do not di sturb the canopy. Sensors were calibrated before and after field measu rement periods. Results were compared with the saturation air vapor pr essure deficit from a nearby weather station to derive an empirical eq uation for estimating leaf wetness. The measured leaf wetness period i n the dense canopy (cv. Tara) exceeded that in the open canopy (cv. St arlight) by 14% (plus an offset of 1.35 h). This corresponded to highe r white mold disease (WM) severity in the dense than in the open canop y. The respective WM severity for dense and open canopies was 93 and 3 2% in 1990, and 73 and 10% in 1991. The empirical relationship for est imating 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, a ccuracy approached 90%. Thus, nearby weather station data offer input to a method that in turn offers a means of assessing leaf wetness in T ara and Starlight, with potential for use with other cultivars. Theore tical considerations indicate that the empirical coefficients are a fu nction of canopy microclimate. Further studies should evaluate the rel ationship between these empirical coefficients and canopy architecture .