A software implementation of PLASMO (Plasmopara Simulation Model) model for downy mildew (Plasmopara viticola Berl. et De Toni) of grapevine (Vitis vinifera L.) development forecasting is presented in this paper. The computer program has been developed to facilitate validation and further improvements and to allow direct model use in vineyard management. The model simulates the development of downy mildew on the basis of climatic conditions. The inputs to the model are: temperature and leaf wetness for the inoculation phase, temperature and relative humidity for the incubation phase and the type of treatment applied. Numeric or graphic outputs are available. The program provides useful information for identifying fungicide application times on the basis of the actual downy mildew development. Field validation tests performed in untreated rows show a good correlation between field observed and model simulated infections. Moreover the model identifies only three treatment distribution periods, while with the traditional method six treatments were applied.