• Definition

  Downy mildew is a generic name for various fungal diseases, i.e. diseases caused to plants by parasitic fungi.
  It is triggered quite regularly as soon as there is too much moisture in the air and on the plants.

  There is not one but several forms of downy mildew, which are caused by different microscopic fungi, for example:
  • Plasmopara viticola which attacks the vines.
  • Phytophthora infestans that attacks potatoes and tomatoes

  Other fungi cause similar damage to lettuce, rose bushes or carrots.

  Development Lifecycle

  The fungi that cause downy mildew are microscopic in size and belong to the oomycetes, the oospore fungi. They produce a non-partitioned filamentous mycelium on which fruiting bodies bearing sporangia develop: small sacs containing asexual or sexual spores, depending on climate variations. Obligate parasites, they overwinter inside infected leaves, stems and fruit in the form of spores and mycelium. In the spring, the mycelium reactivates and the spores are dispersed by wind and rain, causing the first infection.

  During the growing season, when the climate is favourable, the mycelium develops and produces zoospores, asexual spores with flagella. A film of water on the tissues would promote their dispersion and germination. This is the most active phase where downy mildew multiplies rapidly and consolidates the secondary infection of the plant.

  Towards the end of summer, fungi produce oospores, resistant sexual spores, which ensure the survival of the species during unfavourable periods and preserve genetic diversity.

  Symptoms

  • On leaves: yellow spots that turn brown and then dry out in their centre, corresponding to a white down on the underside. Their extension can lead to the death of the foliage.
  • On petiole and stem: brown spots at different levels that can progress to the destruction of young plants or the breakage of adult stems.
  • On fruit: brown, hard and mottled spots with sometimes a white felting on the epidermis.

  Contributing factors

  • Relative humidity above 80%.
  • Temperatures between 15 and 25°C.
  • The succession of humid and fairly hot periods, of a stormy nature, favours the development and spread of the disease.
  • Dense vegetation.
  • Presence of weeds on the plot (which may be alternative hosts to the disease).

Recommended Prophylactic and Cultural Method

• Destroy plant debris and regrowth, sources of primary foci.
• Rotate crops with different species.
• Use healthy plants and, in case of infection, destroy plants with more than 25% of the foliage or fruit affected.
• Avoid proximity to potato and other nightshade plants.
• Dispose of plant debris by burial, incineration, etc.

Chemical method

• There are many treatments for this disease. It is recommended to recommend preventive treatment as soon as the conditions favourable to the development of the disease are observed.
• To prevent the development of habituation or resistance phenomena, favour applications based on products belonging to different chemical families and modes of action.

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