Resistance of a living organism to damage, especially through infections and poisoning, but also environmental conditions (in contrast, immunity means a complete defence mechanism or insensitivity). In viticulture, there are four main points where (depending on location and climate) resistance of grape varieties is very important: fungi, frost, phylloxera and drought (see drought and water stress). The term tolerance is often used instead in this context. Plants often develop a natural resistance to their enemies over very long periods of time up to several million years. A good example is the resistance of certain American grape varieties to phylloxera, which is a multi-stage disease, or to both types of mildew. The picture shows the difference between a non-resistant and resistant vine when a phylloxera bites the leaf. On the left, a prey-like bile with phylloxera inside and eggs laid by the phylloxera. On the right, a weak reaction without bile formation. For extremely resistant vines, there is no reaction at all when a phylloxera bites the leaf.
Resistance depends, on the one hand, on whether pathogens such as bacteria or viruses, toxins or even living beings have the necessary living conditions, and, on the other hand, on defence mechanisms or on defence mechanisms of the organism concerned that are automatically activated immediately after infestation. It can be completely to weakly pronounced. Fungal tolerance refers to the fact that the fungal pathogen can multiply on the host plant, but does not damage the host plant, or only to a small extent. This is caused by glucanases and chitinases, which are parts of the complex defence mechanism of plants. They cleave the cell wall building blocks glucan and chitin, which are found in many fungal cell walls, and thus inhibit or prevent fungal growth. Phytoalexins have a similar effect. When infested by microorganisms such as fungi or bacteria, they are produced by the vine itself as a defence reaction.
When breeding new grape varieties or rootstocks, great importance is attached to, among other things, high resistance to fungi and virus strains, to pests (in the case of rootstock vines, especially phylloxera) and frost resistance. The picture shows the difference between European and American vines when a phylloxera bites at the root. With the completely resistant American vines, no swellings(nodosities or tuberosities) are produced, thus depriving the phylloxera of food. Rather, a protective cork tissue is formed and thus the penetration of moisture and microorganisms is prevented.
Fungus-resistant grape varieties are particularly in demand in organic (ecological) viticulture and are known as PIWI varieties. Genetic engineering opens up new possibilities. For example, successful experiments have been carried out to implant two barley genes in the Riesling variety, thereby conferring fungal resistance. In addition to the great savings in vine protection costs due to the fact that no pesticides are needed and the time saved in the work process, the positive environmental protection achievements that can be achieved should also be emphasised. See also a complete list of all pests and diseases under vine enemies.
Sheet: By Joachim Schmid, Geisenheim - Photographed by himself, CC BY 3.0 de, Link
Root: From Abesadze, Makarevskaja and Tskhakaja, Georgia 1930 - from Joachim Schmid, Frank Manty, Bettina Lindner: Geisenheim Grape Varieties and Clones, Geisenheim Reports 67, 2009, ISBN 978-3-934742-56-7, GFDL 1.2, link and link