Leaf wetness duration (LWD) is one of the most critical variables involved in the development of plant diseases. Many pathogens require the presence of free water on plant organs to move and to start their infective processes. For this reason LWD is extremely significant in the management of crop protection activities and in particular, the successful use of weather-related disease forecasting models. Their operational application is a very important tool for reducing fungicide applications, for environmental safeguarding in high quality production systems, and for reducing the waste of resources and financial losses. Despite the relevance of LWD, no standard has yet been accepted for its measurement. For this reason the use of simulation models, based on agrometeorological variables, represents a valuable alternative to field monitoring. In this work a physical model based on the energy balance was applied for the simulation of LWD on a grapevine (Vitis vinifera). The model, developed in the United States on the cultivars Chardonnay and Concord, and in Australia on Cabernet Franc, was applied for the Sangiovese variety and was adapted for use with agrometeorological data easily available from standard weather stations. The model outputs were compared both with data measured by sensors in the 1995–2003 period and with visual inspections of LWD conducted on vines during 2003. Following, simulated and recorded LWD data were used as input by a model for the simulation of grapevine downy mildew (Plasmopara viticola) and the results were compared with observed data in order to establish the impact of different LWD data on the quality of model simulations.