Downy mildew

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Mike Pearson (Courtesy of EcoPort,
Is this a Minor Pest?
Minor Pest Title

Downy mildew (Peronospora destructor)

Minor Pest Description

It attacks young plants, appearing as white specks, usually confined to the oldest leaves of young plants. A greyish white mould develops rapidly in cool damp weather and progresses down the sheath, and plants eventually fall over and dry up. Optimum temperatures for fungal growth are between 13 and 20 °C. Because of the temperature requirements of the fungus, the disease is more serious in higher cooler areas.

The fungus survives in seeds, bulbs, sets, and on plant debris. Spores are carried long distances by air currents. The fungus can infect onion, Welsh onion, Egyptian onion, garlic, shallot, leek and possibly some other species of Allium.


Minor Pest What to do.
  • Use healthy seeds or sets.
  • Use resistant varieties, if available.
  • Rotate at least 3-years free of onions or other Allium species (e.g. garlic).
  • Wider spacing of plants help reducing humidity and downy mildew.
  • Preventative treatments with rock powder can reduce the attack of this disease.
Minor Pest Position
Minor Pest Firstcontent
Pest Type
Other Crops
Grapes, hops
Host Plants