Powdery mildew is a constant challenge in wine production management. An in-depth understanding of this disease is crucial if you are to anticipate risks, protect your yields and optimise your investments.

This article looks at how to recognise, prevent and combat powdery mildew.

What is powdery mildew?

Powdery mildew is a fungal disease caused by the fungus Uncinula or Erysiphe necator. Along with downy mildew, it is one of the main vine diseases.

The damage can be considerable, leading to a significant drop in yields and major economic losses for winegrowers.

How can you recognise powdery mildew in your vineyards?

Powdery mildew affects leaves, young shoots, and sometimes even grape bunches.

• On young shoots: Growth is slowed, the internodes are shortened and the leaves curl. Susceptible grape varieties may develop a white fungal coating.
• On the leaves: oily spots appear on the upper side and a blackening of the veins on the underside. Later, a greyish, dusty down appears at the level of these spots.
• On bunches: Grapes become covered in grey dust, and the berries may burst, revealing the pips and allowing botrytis to develop. The bunches then smell of mould.

Under what conditions does powdery mildew develop on grapevines?

In vineyards, the development of powdery mildew is favoured by specific weather conditions, which are mainly caused by heat and humidity. The optimum conditions for the development of this disease are between 25 °C and 30 °C and 40 % to 100 % relative humidity.

Wind is also a factor that favours the development of the disease, as it allows the conidia (asexual spores of the fungus) to spread.

The presence of free water caused by heavy rainfall slows the development of powdery mildew.

How can you prevent the risk of powdery mildew?

Agronomic practices

It all starts with good agronomic practices to prevent the onset or development of the disease before you have to treat. Here are some examples:

• Prophylactic measures to control vine vigour: rational fertilisation, grassing the rows, green work (trimming, leaf removal, thinning, debudding), pruning.
• Aeration of the grapes for better penetration of the sprays: correct training form and defoliation.
• Choosing a grape variety with a low susceptibility to powdery mildew.
• Humidity management involves maintaining proper air circulation and avoiding excessively moist conditions to reduce the risk of powdery mildew infection.
• Regular crop inspection for early detection of the disease and early intervention, to limit the damage caused.
• Rapid removal of affected parts or plants to prevent or slow the spread of the disease.

The weather to anticipate powdery mildew

Weather conditions play a key role in the spread of powdery mildew, and can vary from one crop to another. That's why it's essential to use a weather reporting service that's as local as possible, with forecasts that are as reliable and accurate as possible. With Sencrop, you get :

• precise weather data that is continuously recorded and that you can access remotely to track in real time (precipitation, humidity, wind, leaf wetness).
• Sencrop forecasts: an automatic compilation of the most reliable forecast models at time T for your location and the observed data.

Decision-support tools specialising in powdery mildew

Disease modelling tools are now available to help you combat the risk of powdery mildew more effectively. They enable you to easily identify your at-risk crops and advise you on how to apply your treatments.

To assist in managing powdery mildew in viticulture, several Decision Support Tools (DSTs) are available, such as Vitimeto and Movida. These DSTs utilise your ultra-local weather data to provide tailored agronomic advice.

How do you treat powdery mildew?

As the initial symptoms of powdery mildew are not very visible, it is primarily managed through preventive measures when weather conditions conducive to disease development are identified. If preventive treatment proves insufficient and the disease spreads within your plot, swift action is necessary. The critical treatment window falls between the "separated flower buds" stage and the "bunch closure" stage.

There are several substances available for treating powdery mildew, each with its own specific mode of action. These include sulphur (authorised for organic production), azole/SDHI mixtures and other active fungicidal substinces.

You can also use the DSTs mentioned above to easily identify the risks of powdery mildew and receive recommendations on your treatment applications.

Conclusion

It is important to note that, whatever treatment method is used, integrated disease management remains essential to minimise the impact of powdery mildew in agriculture. This involves a judicious combination of cultural practices, varietal choices and the use of plant protection products, as well as constant monitoring of your vines to detect any outbreak of the disease at an early stage. By adopting a proactive and diversified approach, farmers can better protect their vineyards against powdery mildew and ensure sustainable, long-term yields.

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