Onion (Allium cepa L.) is grown worldwide for its fleshy bulbs which are used as food and medicinal purposes. In Egypt, onion is the 2nd major export crop after cotton. Downy mildew of onion, caused by Peronospora destructor (Berk.)Casp. is considered one of the most destructive disease of onion and has a wide geographical distribution includes Egypt. P. destructor is a polycyclic pathogen: many infection cycles can follow one another during an onion-growing season. When weather conditions are favourable, the fungus can complete its cycle in a short time and the disease can cause severe yield losses and affect negatively the Egyptian national income. Therefore disease management relies on routine applications of both protectant and eradicant fungicides (4 – 6 sprays) throughout the season but maintaining control in the life of the crop and timing applications effectively is difficult. In addition, reducing fungicide applications on onions is extremely desirable for the environment and consumer. A computerized forecasting model for onion downy mildew named by the author ( ODM-Cast ) was developed and field validated during 2006/2007 and 2007/2008 onion growing seasons in a disease hot spot cultivation site with a susceptible cultivar(Giza 20) and downy mildew disease severity was confirmed by the visual presence of leaves typical symptoms on onion plants in untreated plots. An advanced wireless telemetry Agro-weather station (Adcon A733 AddWave) which established within the crop canopy was used for monitoring the weather microelements such as: air temperature, relative humidity, leaf wetness, precipitation, global radiation and wind speed 24 hour a day. The results showed that ODM-Cast forecast model successfully indicated the disease daily infection potential and reduced the number of sprays in both years compared with the time table fundamental sprays in both 2006/2007 and 2007/2008, respectively. The basic roles of system analysis for model development and validation are discussed in details .