Resistance of a living organism to damage, especially through infections and poisoning, but also environmental conditions (in contrast to this, immunity means a complete defence mechanism or insensitivity). In viticulture, there are four points in particular where (depending on location and climate) resistance of grape varieties is very important, these are fungi, frost, phylloxera and drought (see drought and water stress). The term tolerance is often used instead in this context. Plants often develop natural resistance to their enemies over very long periods of time, up to several millions of years. A good example is the resistance of certain American grape varieties to phylloxera, which has several stages, or to both types of powdery mildew. The picture shows the difference between a non-resistant and a resistant vine when a phylloxera attacks the leaf. On the left, there is a prey-like gall with a phylloxera in it and eggs laid by it, and on the right, a weak reaction without gall formation. In extremely resistant vines, there is no reaction at all when a grapevine is bitten.