Downy mildew, caused by Plasmapora viticola, is one of the endemic cryptogamic diseases of northern Spain that most commonly affects grapevines. Indeed, it is one of the diseases best known to viticulturalists worldwide. When climatic conditions are favourable, it attacks all organs of the plant, causing very serious crop losses and affecting the profitability and cost of wine. Given the climatic conditions of northern Spain, downy mildew attacks occur every year without exception, obliging growers to apply several treatments during the vine growth cycle. This not only increases cultivation costs enormously but causes environmental pollution and leads to problems during the fermentation of the must. Although the economic losses caused by downy mildew can be severe, greater knowledge of the pathogen’s biology, the availability of systemic treatments, and the better training of growers have helped reduce the risk of such damage. In recent years, several integrated programmes have been launched in the fight against this disease. Taking into account climatic conditions, the point in the vine growth cycle, and the development of the fungus, the aim is to reduce the number of treatments that need be applied to a minimum. Wine-producing countries are trying to develop disease prediction models for each of their growing regions. The German “Pro” [6] & “Freiburg” [8] models, and the Swiss “Winemild” model [1] are among the most well known. Infection with Plasmapora viticola leads to different symptoms depending on the organs attacked and the stage of its development: young leaves, adult leaves, or flowering stage, veraison, or ripening. Although the leaves can be affected even when very small, maximum sensitivity occurs when they are about 3 cm in diameter. The first symptoms are seen on the leaf lamina as ‘oil spots’. In the last phases of leaf disease, these spots become reddish-brown and acquire a typical mosaic pattern. Leaves become less sensitive as they age [3]. Vitis vinifera L. is very sensitive to downy mildew [4, 5, 10], although there are differences between different varieties and possibly even between different clones [10] . Many authors [4, 10, 7, 11] have reported that several factors appear to determine the degree of resistance possessed. Several laboratory studies [12] have investigated the resistance of different varieties of vine to downy mildew, but such results cannot always be extrapolated to the field. Different climatic, soil and cultivation conditions, etc., can all have great influence on whether the disease will appear and how it will develop. The qualitative methods currently used to measure resistance to disease have the problem of inherent subjectivity [5, 12]. A method is therefore required that can quantify the resistance of different varieties and clones in the field, which takes into account the factors that affect both plant and pathogen development, and which reduces as much as possible the subjectivity of current methods.