It’s a fact: diseases able to harm the vines do exist since the end of the 19th century. In the region of Bordeaux, the oceanic climate make the vine particularly sensitive to fungal diseases, mildew and powdery mildew, that are microscopic fungus affecting the leaves and/or the grapes.
The vines must thus be treated; if not, there is no crop.
The conventional option (that is not-organic) favours chemical treatments known as “systemic”, what means that they get into the sap of the plant. These products have an action on the plant for 15 days; thereafter, they must be renewed.
The organic viticulture use only products that don’t stem from the chemical synthesis (for example the traditional “Bordeaux mixture”) and that are “contact products”, what means that they stay and protect the leaves and the grapes until they are “washed out” by an important rain.
In the organic way, the frequency of treatments depends thus on the weather (more often, if they are more “washing out” rains) and on the stage of the growth of the vines (you should treat as long as new leaves come). If the summer is dry, it’s therefore useless to renew the protection when the growth of the vines is stabilized, that is, in general, end of July.