Disease of the Month - Powdery Mildew vs Downy Mildew

 
  Hort Snacks - September 2017
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 Powdery Mildew | Downy Mildew

Powdery Mildew

Causal Organism: Erysiphe polygoni, E. cichoracearum

Crops Affected:
Cruciferous crops, lettuce, peas, rhubarb, cucurbits, strawberries, Saskatoon berries, raspberries, black currants

Disease Cycle:

  • Can’t survive without host plant tissue
  • Wide host range although each strain is very host specific
  • Develops in the spring & fall
  • Windborne spores are the main means of spread
  • Warm, dry weather (15-27C; inhibited by rainy, wet conditions) but conditions of high humidity
  • Poor air flow and shade due to dense shelterbelts or dense plant canopies
  • Cleistothecia (sexual spores) provide over wintering
Symptoms:
  • Spread may not be noticeable until most of the field is infected
  • Initially, may notice off-colour or talcum white spots developing
  • White, powdery or mealy, patchy, mycelial growth on the upper (and sometimes lower – depends on crop) leaf surfaces and all above ground plant parts (suckers, growing tips, stalks, flower calyxes, fruit); patches grow together over time
  • Leaves may gradually fade through shades of green and turn tan coloured; may be shrunken or die and abscise
  • Infected strawberry leaves curl upwards and may have purplish underside
  • Young raspberry canes may be distorted, shrunken, spindly and may die back
  • Plants may be stunted and reduced yields may be observed
  • Fruit may not mature and reach full size
  • Winter hardiness of raspberries can be affected
  • Mycelial growth on strawberry fruit does not have the watery soft rot associated with Botrytis
  • Cleistothecia may appear as disease progresses – dark, pin-head sized appear in host tissues
Management:
  • Avoid use of susceptible cultivars; use resistant cultivars if they are available
  • Ensure rotation to non-susceptible crops
  • Ensure adequate air flow and ventilation within orchard and crop canopy through adequate plant spacing, pruning to improve canopy ventilation and removal of any element that creates high humidity conditions
  • Prune out, remove & destroy infected plant parts, shoots and suckers, if incidence is low
  • Sanitation and removal of infested crop debris
  • Control non-crop hosts plants (weeds, volunteers, etc.)
  • Timely application of registered chemical controls (ensure rotation of chemical groups, if possible)
  • Regular wetting of the leaves can reduce disease development (although should not be considered a guaranteed control)
Powdery mildew on strawberry
Powdery mildew on cucurbit
Photos by Robert Spencer

Downy Mildew

Caual Organism:
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:
  • 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 an 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
Management:
  • 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.)
Downy mildew on peas
Photo by Doon Pauly
 
 
 
 
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on August 31, 2017.