Measurement and simulation of dew accumulation and drying in a potato canopy

Liquid water on the leaves of many crop plants plays an important role in the outbreaks of foliar diseases. While rain and irrigation contribute, the majority of wetting events in many areas results from cooling of leaves below the dewpoint temperature of surrounding air. Thus, most models of crop foliar diseases include factors related to both pathogen biology and environmental regulation of the presence of water on leaves. We measured and simulated leaf wetness formation to test the ability of a detailed, mechanistic soil-canopy-atmosphere model to predict wetness formation as a function of vertical position in the canopy, and to study the role of the near-surface wetness of soil beneath. Position of water formation is important because older, lower leaves tend to be more susceptible to some diseases, and the role of soil wetness has implications for the accuracy of mesoscale modeling of leaf wetness in lieu of in-field measurements. When random dispersion of leaves was assumed our simulation model generally underestimated water formation on leaves in the lower half of the canopy. Clumping of leaves within each horizontal layer of the simulation decreased water accumulation in the upper portion of the canopy and increased accumulation on lower leaves. Local near-surface soil wetness increased the duration of leaf wetness by about 2 h in our simulations. Thus mesoscale simulations that do not explicitly include local soil moisture, including irrigation, may frequently underestimate leaf wetness duration.