Resistance is not a promise of invulnerability. It is an advantage a vine has over a pathogen – one that the pathogen can erode through adaptation. For PIWI breeding, the crucial question is therefore not simply whether a new variety resists mildew. It is also how long, and under what conditions, that protection holds.
The subject is timely: Geisenheim University is planning a workstream on grapevine breeding and microbiology for its international alumni week from 22 to 28 November 2026. This is an opportunity to take a closer look at collaboration between laboratories, breeders and vineyards – without implying any findings or statements from participants at this stage.
When the pathogen learns
PIWI refers to fungus-resistant grape varieties. Many carry genetic traits originally derived from other Vitis species, which give the European cultivated grapevine protection against powdery or downy mildew. This protection rests partly on the plant recognising particular features of the pathogen and triggering a defence response.
But pathogens are not unchanging adversaries. Variants can emerge within their populations that are less readily recognised by the plant’s defences. Under the pressure exerted by widespread resistance, such variants have an advantage: they can multiply more successfully and weaken the variety’s protection. INRAE research from 2026 describes different pathways of adaptation in the pathogen responsible for downy mildew. This underlines why effective resistance is not automatically lasting resistance.
This does not allow a fixed lifespan in years to be determined. It depends, among other things, on which pathogen strains are present, which resistance genes a variety carries and how extensively it is grown. Site and weather conditions also influence when and how visibly a disease appears. A single year without visible infection therefore proves neither lasting resistance nor its breakdown.
More than one line of defence
One way to strengthen resistance is to combine several resistance factors. Specialists refer to pyramiding when several genes targeting the same pathogen are brought together in a single variety. If the pathogen has to overcome several defence mechanisms at once, this can be more difficult than overcoming a single resistance. This is no guarantee either: resistance genes differ in their effects, and pathogens can continue to evolve.
Breeding therefore involves more than testing whether a seedling responds to a pathogen in the laboratory. Trials over several years, different pathogen strains and observations at different sites are crucial. The OIV recommends field trials over several years to assess mildew resistance; trials for downy mildew should be conducted without chemical crop protection or resistance enhancers. This allows the vine’s natural response to be assessed more accurately.
The vineyard as an early warning system
Testing does not end when a variety is officially approved. The French OSCAR network monitors resistant varieties in commercial vineyard plots, records disease development and collects pathogen samples. These are examined in the laboratory for characteristics such as aggressiveness and virulence. Networks of this kind connect vineyard observations with genetic and biological analyses – and can reveal changes that are difficult to put into context from a single plot.
For winegrowers, it is not only a major breakdown in resistance that matters. Recurring infection, differences between varieties or regions, and the crop protection required should also be recorded. PIWI does not automatically mean “never spray”: the varieties can also be affected by other diseases, and growers must adapt their measures if resistance becomes less effective.
Durability is therefore not achieved in the breeding laboratory alone. It also depends on winegrowers, researchers and breeders systematically sharing their observations. Documenting unusual infections and examining pathogens allows subsequent breeding efforts to be more precisely targeted. The question of resistance is therefore not a countdown but an ongoing task: selecting resistance genes, testing their effectiveness under real-world conditions and responding in good time when the pathogen finds new ways through.
Original reviewed by the editors · 5 November 2026
