The Grapes Built to Fight Back

How PIWI grapes are challenging tradition, reducing sprays and reshaping the vineyard in a changing climate

In Japan, the arrival of spring, and with it a new season of beginnings and rebirth, is announced by the blossoming of cherry trees. With full bloom lasting for a mere three to five days, it is a special occasion that, over many centuries, has become deeply intertwined with Japanese cultural identity. In 812 AD, during the early Heian period, Emperor Saga is said to have held a cherry-blossom viewing event in Kyoto, marking the first recorded moment in an over 1,200-year-long tradition of observing seasonal change. The annual arrival of cherry blossoms was the subject of poems, ceremonies, courtly banquets and excursions, and over time an unusually consistent record emerged of when the cherry blossoms of Kyoto, or sakura, were in full bloom. These diaries and chronicles have allowed modern researchers to piece together a remarkable history of flowering from the medieval court to the present day, creating a uniquely long, if unintentional, record of how spring itself has shifted over time.

The main person credited with collating this information was Professor Yasuyuki Aono, a climatologist and phenology researcher associated with Osaka Prefecture University. The data shows a very clear trend, starting around the time of the Industrial Revolution in the mid-1800s, and as global warming has gathered pace, peak cherry blossom has arrived ever earlier, signalling, through phenological change, the gradual warming of the Japanese, and indeed the global, climate.

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The same changes that have advanced the arrival of spring in Japan are, of course, felt by the grapevine as well, with most established growing regions now experiencing a gradual rise in accumulated growing-season heat. This is often expressed in growing degree days, which is the sum of the average of the high and low temperature of each day, after subtracting a vine-growth base temperature of 10°C and excluding any negative results. It is a phenomenon that increasingly forces wine growers to adapt their techniques, and may soon make regions that are already marginal unsuitable for the cultivation of quality wine grapes. On the other hand, it has also opened the door to growing regions that, up until recently, would have been considered too cool to reliably ripen grapes, and vineyards can now be found across England and Canada, and even as far north as Norway and Sweden. Canada has also taken up the mantle of making eiswein, a highly concentrated dessert wine which requires the grapes to freeze while still on the vine, pulling moisture out of the grapes. This practice, believed to originate in Germany’s Rheinhessen region, is becoming increasingly difficult to make in increasingly mild winters of Europe as it requires a sustained, and relatively rapid drop to around -7°C for the grapes to freeze on the vine.

A gradually warmer climate can also have implications for disease pressure in traditionally cooler climates. Rising temperatures, altered rainfall patterns and higher humidity are increasingly coming together to create the perfect conditions for the spread of powdery mildew and, in particular, downy mildew, the latter requiring rainfall to thrive and spread. Overall, the disease pressure window for many growing regions in Europe has widened, at the same time as growing concern over the health and safety implications of copper-based fungicides is making their use more restricted, or, in the case of France, banned entirely.

Perhaps then there should be no surprise that the possibility of growing grape varieties whose traits include a strong resistance to both powdery and downy mildew, and indeed other fungal diseases such as botrytis and black rot, has attracted much interest. With both powdery and downy mildew being introduced to Europe from North America in the 19th century, alongside the broader biological shock of phylloxera and other introduced pests and diseases, they wreaked havoc across the grape-growing parts of the continent, until suitable methods of suppressing or killing the fungus responsible was discovered. These methods have remained relatively unchanged, with sulphur and copper still being the most reliable and cost effective tools available. Similarly, the workaround for dealing with phylloxera, came with the practice of grafting Vitis vinifera onto phylloxera resistant American-rootstock. The combined effect of this was a transformation of European viticulture from a largely self-sustaining agricultural tradition into one increasingly dependent on grafting, spraying and scientific intervention.

With new, non-vinifera rootstock becoming commonplace, it didn’t take long before hybrid varieties were developed in France. Breeders would cross European vinifera vines with American species that had co-evolved with these pests and diseases and therefore carried natural resistance. The goal of combining the wine quality of European grapes with the resilience of American vines was simple in principle but difficult in practice. Figures such as Albert Seibel, François Baco, Eugène Kuhlmann, and later the Seyve-Villard breeding lines became central to this first wave of hybrids. The hybrid varieties that resulted from these early efforts were widely planted across Europe, but the wine they produced proved disappointing when compared to the traditional varieties. Though many were undeniably robust, they were often rustic or lacking in nature, while some carried flavours associated with non-vinifera species such as the much-maligned “foxy” character, especially in labrusca-derived material. Despite tremendous efforts, the many hybrids that were planted in the late 19th and early 20th century fell to the sidelines in a wine market that favoured the flavours and qualities of pure vinifera derived wines. Furthermore they were often associated with overcropping, emergency viticulture and regions too cold, wet or difficult for “noble” varieties. Despite this however, work on hybrids did not stop, and the pressures that incentivised these efforts remain as real today as when work first began.

This gave rise to what might be called a second wave of hybridisation, with researchers using more sophisticated breeding methods in an attempt to produce varieties that offer disease resistance, while retaining the wine-making properties of the traditional varieties. German-speaking Europe in particular led the charge, with institutes such as Freiburg, Geisenheim, Geilweilerhoff and later Swiss and Austrian programmes becoming central to the development of modern resistant varieties. The German term used for these fungus resistant varieties is Pilzwiderstandsfähige Rebsorten, which while looking intimidating to non-German speakers merely means fungus-resistant grape varieties. The acronym for these, which unsurprisingly has caught on more among English speakers, is PIWI. Their ongoing development came with a marked shift in emphasis, away from mere hybrids, with whatever baggage came with that term, towards specifically bred varieties intended to reduce the need for spraying while retaining the ability to produce credible wine.

What follows is a closer look at some of the more important PIWIs, what they actually are and some of their strengths and weaknesses.

What PIWIs are not

It is worth noting that though PIWI grape varieties gain their properties through the combination of genetic traits, they are not genetically modified (GMO) but rather the product of conventional interbreeding. Breeders cross Vitis vinifera with resistant species or pre-bred resistant lines, then repeatedly backcross and select offspring that combine disease resistance with acceptable wine quality. As you can imagine, despite the prolific nature of vines, this work is painfully slow, with commercially available PIWI varieties usually being 25-30 years in the making. This is because breeders must make the initial cross, grow seedlings to fruiting age, screen for disease resistance, backcross or refine promising vines, test wine quality over multiple vintages and sites, then propagate and register the final variety.

In many ways the process is closer to the creation of varieties such as Müller-Thurgau, Marselan or Cabernet Sauvignon itself, except that the genetic base is wider because non-vinifera species are involved. A PIWI may include resistance genes from species such as Vitis rupestris, Vitis riparia, Vitis labrusca, Vitis amurensis or others, depending on the breeding line. While they are still hybrids, the repeated backcrossing with vinifera means that most will retain a very high proportion of vinifera ancestry, while retaining the carefully selected resistance traits the breeders are after. This avoids one of the key problems encountered in early hybridisation efforts, namely the prominent non-vinifera flavours of the resulting wine.

Vive La Resistance

Resistance should not be conflated with immunity. That PIWIs are resistant to powdery and downy mildew does not mean that these fungi cannot land, germinate and attempt to infect the plant. However, a resistant vine possesses inherited defence traits that make it much harder for the pathogen to establish itself, spread through tissue, reproduce, and damage the crop. It may seem like a fairly obvious semantic difference, but the key takeaway is that a given vine’s ability to fight off infection sits on a spectrum, and does not necessarily remain static over time. A variety may be highly resistant, moderately resistant, partially resistant, tolerant, or susceptible, and those categories can shift according to weather, pathogen pressure, tissue age, local pathogen population, and the particular resistance genes involved.

In broad terms, there are several overlapping mechanisms responsible for a vines ability to resist infection. These include more effective detection of the pathogen attack, enhanced local defences wherein the vine reinforces tissue or activates biochemical defences and hypersensitivity, in which infected cells die off rapidly, depriving the pathogen of living tissue to infect. The vine may also inhibit further sporulation of the attacking fungus and generally slow its progress through the canopy and bunch zone. The hypersensitive response is especially important in discussions of downy mildew as local necrosis, or controlled cell death, around the infection site dramatically limits the pathogen’s ability to sporulate.

Downy Mildew

The causal agent for downy mildew is Plasmopara viticola, a heterothallic oomycete that overwinters as oospores in leaf litter and soil. Oomycete’s are also referred to as water molds, and are not really to be considered a true fungus. They do however thrive in wet, humid conditions and can attack leaves, shoots, inflorescences and young berries. Once the oospores germinate, usually in moist and temperate spring conditions, they produce what is known as sporangia which in turn produces zoospores, or swimming spores. These sporangia and zoospores can be transmitted from the soil to the lower parts of the vine by rain splashing and wind, from where it will rapidly spread, particularly if conditions remain warm and humid.

Downy mildew in action

The various traits that allow for resistance to downy mildew are associated with specific genomic regions, or loci, which are called Rpv, simply meaning resistance to Plasmopara viticola. These loci are inherited from resistant Vitis relatives and do not all work in exactly the same way, nor do they all provide the same strength or durability of resistance. Some are major-effect loci that can sharply reduce disease development while others are partial or background contributors. Rpv3 is one of the most widely used in breeding, and Rpv3 has been described as a major locus on chromosome 18 associated with a race-specific hypersensitive response. I am no geneticist, and will happily admit that the genetic research in the article by Wilkerson et al. (2025) linked to above goes largely above my head. The key takeaway however is that we are able to identify key genomic sequences that are responsible for resistance to what is one of the most devastating, broadly dispersed and hard to eradicate diseases of the vine, which gives breeders of PIWIs a distinct edge when developing improved varieties.

Powdery Mildew

Powdery mildew, or Erysiphe necator, grows largely on the surface of green tissues and sends feeding structures into epidermal cells. It does not need free water in quite the same way as downy mildew and can become a chronic problem in warm, dry-to-humid conditions. It attacks leaves, shoots, inflorescences and berries, and berry infection can have severe oenological consequences with reduced quality, off-aromas, compromised skins, altered phenolics and increased vulnerability to bunch rot. I go into quite some detail on it in the below article which I suggest you check out if you’re interested in delving further into how powdery mildew operates.

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The Uninvited Guest: How powdery mildew shapes viticulture

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Similarly to downy mildew, researchers have identified particular loci for resistance which can be exploited when crossbreeding. For powdery mildew there are two primary families of loci which are called Run and Ren, respectively following the same naming convention as Rpv. Run standing for “Resistance to Uncinula necator”, an older name for powdery mildew, and Ren standing for “Resistance to Erysiphe necator”, its current taxonomic name.

In practical terms, powdery mildew resistance may mean reduced colony formation and smaller colonies, delayed disease development and less infection of berries and stems with generally lower disease pressure later in the season.

Pyramiding

With multiple different resistance loci identified for both downy and powdery mildew, breeders are able to systematically cross plants whose genetic material contain desired traits, and importantly, quickly analyse the genetic composition of the offspring to see whether the desired traits were combined or transferred. The key breeding technique used to achieve this is referred to as pyramiding, where multiple resistance loci are combined into the same plant. The benefit of this is that while a pathogen may evolve a way around one resistance gene, it is much less likely to overcome several unrelated resistance mechanisms at once.

In practice, a breeder will start with a range of plants that exhibit desirable traits. One may be exhibiting superior wine making properties, while another may contain the downy mildew resistant locus Rpv3. A third parent plant may contain another downy mildew resistant trait in Rpv12, while a fourth may display a powdery mildew resistant trait such as Run1. The breeder crosses these plants, then screens the seedlings to find individuals that have inherited the desired combination. Traditionally, this meant growing many seedlings and exposing them to disease pressure. Today, breeders often use marker-assisted selection, where DNA markers linked to known resistance genes are used to identify seedlings carrying the desired genes before they ever fruit. So instead of waiting several years to see whether a vine resists mildew, the breeder can test a young seedling’s DNA and ask, does it carry Rpv3? Does it carry Rpv12? Does it also carry Run1 or Ren3?

Over several generations of such crossbreeding, a pyramided vine may therefore contain a series of resistance loci, which significantly strengthens its ability to resist the specific pathogens it was bred to survive. A well bred PIWI may therefore be considered a layered, multi-disease resistant vine, and may as an example contain Rpv3 + Rpv12 for downy mildew, plus Run1 + Ren3 for powdery mildew resistance. As you can imagine, it gets significantly more complicated than this, as breeders need to juggle resistances with the many other properties that make a vine valuable, such as wine quality, yield, phenology, bunch structure, berry chemistry, winter hardiness, and adaptation to a given climate. This work is ongoing, but many PIWI varieties are already in commercial production, opening doors to new regions and creating some interesting wines.

Meet the PIWIs

What follows is a list of some of the more prominent PIWIs that have been developed and a brief description of their characteristics. It makes for repetitive and quite boring reading, but I think adds value as a point of reference. Feel free to skip to the Final Thoughts section should you not want to read through this list.

White PIWI varieties

Solaris

One of the most important northern European PIWIs. Early-ripening, high-sugar, disease-resistant, and useful in marginal climates. It can produce full-bodied whites, sometimes with tropical fruit, peach, citrus, and occasionally muscat-like or Sauvignon-like notes. Its weakness is that it can become blowsy, alcoholic or low in acidity if overripe.

It has enabled winemakers as far north as Norway to make wine, which truly marks a new frontier in viticulture.

A hint of fjord on the palate?

Souvignier Gris

A major modern variety. It often produces wines somewhere between Pinot Gris, Riesling, Sauvignon Blanc and Traminer, depending on site and winemaking. It can have good texture, decent acidity, stone fruit, citrus, herbs and sometimes a lightly spicy or phenolic edge.

This is one of the more serious white PIWIs because it can make wines with gastronomic weight rather than merely aromatic novelty.

Johanniter

Bred from Riesling-related lines, often compared loosely to Riesling or Pinot Blanc. It can show citrus, apple, peach and floral notes. Acidity can be good, though it depends heavily on site. It is generally regarded as reliable but not always thrilling.

Bronner

Often neutral-to-fresh, with citrus, apple and sometimes light herbaceous character. Useful for clean dry whites and sparkling bases. It can be slightly understated.

Muscaris

A highly aromatic variety with muscat-like perfume. It can produce attractive wines, but the obvious aromatic signature may dominate terroir expression. It is useful where a lifted, floral, grapey style is desired.

Cabernet Blanc

Despite the name, this is a white PIWI derived from Cabernet Sauvignon-related breeding. It often gives Sauvignon-like wines, with citrus, elderflower, green pepper, blackcurrant leaf, sometimes tropical fruit.

Floreal

One of the French resistant varieties now receiving more attention. It can make fresh, light-to-medium-bodied whites with citrus, melon, greengage, gooseberry or Sauvignon-adjacent notes. It reached broader UK consumer attention when Tesco introduced a Floreal wine, making it the first PIWI wine in a major UK retailer.

Voltis

Important partly because of Champagne. Voltis has been authorised on a limited experimental basis in Champagne’s appellation framework, which is symbolically significant even if its role remains small. It is valued for resistance and suitability for sparkling production, though it is not necessarily viewed as a grand still-wine variety.

Red PIWI varieties

Regent

One of the older and more established red PIWIs. It can produce deeply coloured, fruity reds with black cherry, plum, spice and sometimes smoky or earthy notes. In the right hands it can be good, but it can also be coarse, simple or over-extracted. Phenolic management is crucial for this variety.

Rondo

Important in very cool climates, including the UK, Denmark, Poland and Ireland. It ripens early and gives colour easily. Wines can be dark-fruited, herbal, sometimes slightly rustic. It is useful, but not always elegant.

Cabernet Cortis

A Cabernet Sauvignon-derived PIWI from Freiburg breeding. It can show dark fruit, spice, pepper, cassis and herbal Cabernet-like traits. It often has strong colour and phenolic content, which can be a virtue or a problem depending on extraction and ripeness.

Cabernet Cantor / Cabernet Carbon / Cabernet Jura / Cabertin

These belong to the broader Cabernet-type PIWI universe. They often aim for Bordeaux-like flavour architecture: cassis, dark fruit, herbs, tannin, colour. Some can be convincing while others can be angular or green if not fully ripe.

Divico

A Swiss red PIWI of real interest. It can produce deeply coloured, structured wines with dark fruit and spice. It has been taken seriously in Switzerland because it offers disease resistance without necessarily sacrificing ambition.

Artaban

A French ResDur red variety. It is intended for fruity wines with moderate alcohol and good colour intensity. The Julius Kühn Institute catalogue lists known resistance loci for Artaban including Run1, Ren3 and Ren9 for powdery mildew and Rpv1 and Rpv3.1 for downy mildew.

It is also worth diving into that catalogue as it gives more background to this subject and lists more varieties than I am covering here.

Vidoc

Another French ResDur red, with strong resistance to downy and powdery mildew. It tends to be later-ripening and capable of coloured, structured wines.

Satin Noir

A newer red PIWI that has attracted attention for colour, fruit and potentially more polished wine quality. Like many of the newer varieties, its reputation is still forming.

Final Thoughts

It is easy to be wary of new things, and for a subject as laden with history and tradition as wine, it is understandable that new varieties like PIWIs are met with some scepticism. Yet the benefits of disease resistant varieties, provided of course they also make delicious wines, are well worth the effort put into breeding them. Aside from economic viability in a rapidly changing climate, they represent an opportunity to dramatically reduce the use of fungicides, including copper and sulphur, and lean further into a truly sustainable form of regenerative viticulture than current disease management techniques allow.

If the wine is good, I see no reason to be dogmatic about where the vine gets its genetic material from. Though ideally we would be able to rely on own rooted Vitis vinifera varieties, the realities of global disease vectors and a warming climate requires a more pragmatic approach which PIWIs certainly represent. It will be interesting to see which of these relatively experimental varieties gain prominence, and what they can do in the hands of skilled winemakers.

As always, I hope the above has been interesting and that you learned as much as I did when researching it. Look forward to hearing your thoughts in the comments.

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