rainfall, a component of climate which affects grapevines in many and conflicting ways.

For vines depending directly on rainfall (see dryland viticulture), there needs to be enough rain, at the right times, to promote adequate growth and to avoid severe water stress during ripening. On the other hand, more than enough rainfall can lead to excessive vegetation growth and a poor canopy microclimate, especially on soils high in nitrogen. It can also cause waterlogging on soils prone to it (see drainage). Similarly, rainfall can promote fungal diseases such as downy mildew and botrytis bunch rot by wetting foliage and fruit.