Wine Arcana

Concept

Disease resistance

Also known as PIWI

Disease resistance describes a grapevine's ability to limit infection or the development of a particular pathogen. It is a major goal of modern grape breeding because protecting susceptible vines requires repeated work through the growing season. Resistance is specific: a vine that withstands one disease may remain vulnerable to another.

Pierce's disease, caused by a bacterium, provides a different resistance target from the mildews commonly associated with PIWI. UC Davis's 2019 release of five resistant wine grapes illustrates breeding directed specifically at that disease.

Resistance and tolerance describe different advantages. Resistance limits the pathogen's infection or development; tolerance allows a vine to suffer less damage despite infection. Blanc du Bois, for example, can harbour the Pierce's disease bacterium while avoiding the decline seen in susceptible varieties. A tolerant vine can consequently remain part of the disease's host population.1. University of Georgia Extension, “Blanc du Bois,” Pierce's disease tolerance and symptomless hosts.

What PIWI means

PIWI abbreviates the German pilzwiderstandsfähig, meaning resistant to fungal diseases. In winegrowing it commonly refers to varieties bred for strong resistance to powdery mildew and downy mildew. The latter is caused by an oomycete, a fungus-like organism, but is conventionally included in the category.

Regent, Solaris, and Souvignier Gris are examples. Reduced need for protective treatments can also mean fewer tractor passes, less fuel use, and less soil compaction. The extent of those benefits depends on the vineyard and season.

Breeding and continuing protection

Breeders cross vines, then select offspring for agronomic performance and wine quality as well as resistance. Agroscope's Swiss programme combines resistance inherited through interspecific breeding with tests of the resulting grapes and wines. Its Divona account describes selection from thousands of candidates, including early biochemical tests of their defensive responses.

The same account makes the limits concrete: even for this strongly resistant variety, Agroscope recommends some protective treatment around flowering and fruit set according to disease pressure. Resistance reduces a management burden while monitoring remains necessary.

Other characteristics must also fit the site. Divona ripens early, and Agroscope cautions against the hottest situations and severely restricted water supply. This illustrates why a resistant grape is not automatically suited to every warming region. Winter hardiness, ripening time, water relations, and resistance to individual diseases are separate considerations in choosing planting material.

Making resistance last

Breeders can combine several resistance genes in one variety, an approach called pyramiding. INRAE's ResDur programme uses such combinations to make it harder for mildew populations to overcome protection. DNA markers help identify promising seedlings, followed by vineyard and wine evaluation.

Durability depends on the combination and the pathogen population. INRAE's 2026 summary of experimental work reports that an already defeated resistance locus can, in some combinations, compromise another locus's protection. The number of resistance genes alone is therefore an incomplete guide to lasting performance.2. INRAE Grand Est-Colmar, “Downy mildew-resistant vines,” 23 February 2026, institutional summary of research initially reported as a 2025 preprint; the result concerns particular resistance combinations. France's OSCAR vineyard network monitors resistant varieties after planting, including changes in pathogen populations and the appearance of other disease problems.

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