The resistance properties of PIWI varieties are an excellent tool for helping to manage mildew in grapes, but genetics may still need support in higher-risk seasons, as Hutchinsons agronomist Rob Saunders explains…
PIWI grape varieties have been attracting more interest in recent years as growers look to reduce their reliance on an increasingly limited armoury of fungicides.
The disease resistance bred into these hybrid varieties, some of which have been around for several decades, gives them greater resistance to common fungal diseases, namely powdery and downy mildew, than many classical varieties. Indeed, the term PIWI is a German acronym, “Pilzwiderstandsfähige Reben”, simply meaning “fungus-resistant grape varieties”.
Wine GB figures show PIWI varieties account for the fifth, sixth, and ninth most planted grape varieties in the UK, proving particularly popular in the Midlands, North, and West, suggesting growers in these often wetter regions may be seeking an extra defence to higher disease pressure. Their early ripening ability is likely to be another key draw for many, especially in more marginal grape-growing regions.
Can we rely on genetics alone?
While varietal resistance is a valuable tool for helping growers manage mildew risk, it is by no means a “silver bullet” and should be seen as part of any integrated disease management strategy.
As viticulturist Sam Doncaster, a vine technician with Rebschule Freytag in Germany’s Pfalz region explains, disease resistance in many PIWI varieties currently grown in the UK is controlled by just a single gene per fungal infection type (e.g. powdery mildew), therefore it only offers a single line of defence. This could potentially be overcome if, or when, a fungus mutates, leaving the vine at risk of infection.
The white variety, Bianca, mainly grown in Hungary, illustrates such changes. Despite possessing high resistance to both mildew types when it was first developed several decades ago, in the late 2000’s scientists discovered a downy mildew isolate capable of overcoming the variety’s resistance mechanism, thereby making it more susceptible to downy mildew.
Fungal mutations occur naturally all the time, but selection pressure will be greatest in higher disease risk seasons, such as 2024 (downy mildew) and 2025 (powdery mildew).
“When I hear growers [of resistant varieties] say ‘we intend to not spray our vines’, I feel uncomfortable as it is only a matter of time before mildew will turn up. Once found on the vines, there is no going back,” Sam said.
Protecting genetics
Breeders across Europe are working hard to develop resistant varieties that use multiple genes to combat the risks associated with the early ‘monogenic’ resistance, while still meeting the quality and taste expectations winemakers require.
However, that work is still ongoing and currently in the UK, most PIWI varieties feature single gene resistance. Supporting this with targeted use of crop protection products will add an extra line of defence against fungal infection, reducing selection pressure and the associated risk of mutations that can overcome genetics.
Fungicide requirements may be less than for classical varieties, but relying on single gene resistance alone, is not advised.
In some seasons, when disease pressure is low, timely use of copper and sulphur, combined with a good nutrition strategy could be enough to help keep vines healthy, however when pressure is higher, extra, well-timed fungicide defences will be necessary.
Recognise too that there is no specific varietal resistance to the other major fungal threat, Botrytis, although some varieties can have more favourable bunch architecture that means berries are less prone to rupturing, potentially reducing disease risk.
In-season monitoring is key
Careful disease risk monitoring throughout the season is essential for any variety, resistant or not, so use local weather data and established risk models (such as RIMpro) to help predict any likely increases in disease pressure, and plan suitable control strategies.
Hutchinsons is currently working with partners to develop a novel system for detecting downy mildew spores in the vineyard, to assess the correlation with RIMpro predictions.
A prototype system was trialled in a Kent vineyard last season, featuring a cyclone air sampler to collect spores, the DNA from which is then analysed in the lab to identify those present. Testing and research will continue throughout this year, with the eventual aim of developing a fully automated system that can provide growers with early warnings of disease infection. Watch this space…
