How do you fight mildew and blight?

How do you fight mildew and blight?

Mildew and blight are a constant challenge in the management of agricultural crops such as tomatoes, vines and potatoes. Understanding this diseases in depth is crucial to anticipating risks, protecting yields and safeguarding investments.

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

What are mildew and blight?

They are a disease caused by micro-organisms called oomycetes, which reproduce via spores capable of surviving the winter and infecting plants when conditions are favourable.

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

How do you identify mildew and blight?

The visual symptoms vary from crop to crop, but generally include :

  • irregular, brown or yellowish spots on the leaves, accompanied by discoloured areas
  • a cottony texture on the underside of the leaves, in the form of white down
  • mildew can spread to the rest of the plant, leading to fruit deformation or abnormal growth areas.

Recognising these characteristics early is essential if you are to intervene early and prevent the disease spreading.

What are the conditions for the development of mildew and blight ?

The development of downy mildew is favoured by specific weather conditions, mainly heat and humidity. Stormy days with alternating rainy and hot spells are therefore particularly favourable for the development of this disease.

Downy mildew on vines

There are two stages in the downy mildew cycle: primary infection and secondary infection.

  • Primary infection occurs between March and May, when average temperatures are above 11ºC and rainfall is high (>10mm). These conditions allow the winter eggs called oospores to mature (the survival of these eggs over winter depends on the amount of rain that falls). The incubation period can last from 7 to 15 days, depending on temperature and humidity conditions, and the first symptoms (oil spots) appear on the plant.
  • Secondary contamination then begins. The spores from the primary contamination will, thanks to the wind, contaminate other parts of the plant. For this to happen, more than 2 hours of wetting is required at favourable temperatures. Secondary contamination can occur throughout the vine's growth cycle.

Blight in potatoes

Blight oomycetes are stored in the soil on potato debris or diseased tubers, which germinate and give rise to contaminated shoots. These spores begin to germinate and contaminate the first plants in the spring. The incubation period before the first symptoms appear lasts between 4 and 9 days, depending on temperature and humidity conditions. Subsequently, wind or rain disperses the spores from plant to plant and crop to crop.

The development of the disease is then conditioned by humidity and temperature conditions. Humidity of over 90% and an average temperature of between 15°C and 25°C are favourable.

Area of potato plot affected by blight

How can we prevent the development of mildew and blight?

Agronomic practices

  • Leave enough space between plants to limit the rapid spread of this parasite.
  • Disrupt the development cycle of downy mildew by rotating crops, thereby reducing the concentration of the parasite in the soil.
  • Regularly inspect crops and the surrounding environment to detect diseases early and intervene at the initial stages, thereby limiting the damage caused.
  • Limit stagnant humidity by avoiding excessive irrigation and ensuring adequate drainage.
  • Select a variety with low susceptibility to mildew/blight.
  • Limit the presence of regrowth
  • Manage piles of waste (lime treatment, covering, etc).

The weather to anticipate mildew

As we saw earlier, weather conditions play a key role in the downy mildew cycle, even down to a few degrees. That's why it's essential to use a weather data service that's as local as possible, with reliable, accurate forecasts.
With Sencrop you can access :

  • live weather reports for your crops, so you can monitor conditions in real time.
  • Sencrop forecasts, an automatic aggregation of the most reliable forecast model for your locality and for the data observed.
Sencrop forecast

How should mildew and blight be treated?

This disease is primarily controlled as a preventive measure, identifying weather conditions favorable to disease development. If preventive treatment proves insufficient and the disease spreads in your crop, prompt action is necessary.

There are several substances available for treating mildew, each with its own specific mode of action. In the vineyard, copper is the main substance used (e.g. Bordeaux mixture or oxychloride). That's why it's so important to anticipate and prevent risks. In the potato sector, we talk about ametocradine, cymoxanil or zoxamide, for example. Copper is used for organic management.

Other alternative solutions are also being tested, such as sodium bicarbonate.

To manage your treatment itinerary, you can refer to information shared by technical institutes.

Use the Sencrop spraying windows to identify the best time to apply your treatments. Weather conditions play a key role in the effectiveness of the product applied.


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

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