Disease of the Month - Downy Mildew

  Hort Snacks - March 2017
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 Range of species (Perenospora parasitica, P. farinose f.sp. spinaciae, P. rumicis, P. viciae, P. destructor, Bremia lactucae) – dependant on host crop

Crops Affected: beets, spinach, cole crops, radish, rutabaga/turnip, lettuce, rhubarb, onion, garlic, peas

Disease Cycle:

  • Affects a wide host range; species are specific to host; may be some specificity within host groups
  • Development is favoured by cool, moist conditions; temperatures between 10-15C and conditions with dew, drizzle or heavy fog
  • Sporulation occurs within 4-8 days, depending on species and environmental conditions
  • Spores spread by water splash or wind
  • Disease may stay in a dormant or latent state for a period of time
  • Overwinter / survival with oospores in soil, debris, plant parts, etc. – requires living host to grow and multiply
  • Symptoms can occur at any growth stage
  • Initial symptoms include small angular, pale yellow / yellow areas on the upper leaf surface and fluffy, white or grey, patchy mycelial mat on the under surface
  • Lesions enlarge and turn tan and papery
  • Systemic invasion can result in yellowish, and then greyish-black, necrotic tissues
  • Some crops (particularly root crops) can get a irregular, internal discolouration projecting downwards from the crown or soil line
  • Onions (and other bulb crops) have a greyish velvety growth on leaves that may appear purplish when full of spores; leaves may turn pale and yellow and then die
  • Spores may be produced on the lower leaf surface
Downy Mildew on pea leaves
Photo by Doon Pauly
  • Regular rotations out of specific host crops can reduce inoculum levels significantly
  • Plant into fields with good soil and air drainage
  • Bury debris and crop residues deeply to accelerate breakdown; dispose of cull piles quickly
  • Use disease-free planting material and seeds
  • Keep fields free from weeds
  • Apply protective fungicide sprays at appropriate times (typically early)
  • Resistant cultivars may be available
  • Ensure plants are free from stress (nutrient, moisture, etc.)
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on February 27, 2017.