Erysiphe polygoni, E. cichoracearum;
other genus and species specific to each host crop

Crops Affected:
Cruciferous crops (broccoli, Brussels sprouts, cabbage, cauliflower, radish, and rutabaga), lettuce, peas, rhubarb, Cucurbits (cucumbers, pumpkins, squash), strawberries, saskatoon berries, raspberries, black currants

Disease Cycle:

• Fungal pathogen
• Pathogen cannot survive without host plant tissue
• Powdery mildews have a wide host range, although each strain is very host specific
• Typically develops in the spring & fall, however correct conditions can shift the timing
• Windborne spores are the main means of spread
• Cleistothecia (sexual spores) provide over wintering

Conditions for Development:

• Warm, dry weather (15-27°C)
  • Conditions of high humidity
  • Inhibited by rainy, wet conditions
• Poor air flow
• Shade due to dense shelterbelts or dense plant canopies

Symptoms:

• Spread may not be noticeable until most of the field is infected
• Initially, may notice off-colour or talcum white spots developing on leaf surfaces
• 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
  • Leaves may be shrunken or die and fall off
• Cleistothecia may appear as disease progresses
  • Dark, pin-head sized bodies appear in host tissues
  • Variations in the appendages of cleistothecia can assist in species identification

Crop specifics

• Infected strawberry leaves curl upwards and may have purplish undersides
  • Mycelial growth on strawberry fruit does not have the watery soft rot associated with Botrytis
• 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

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
  • Adequate plant spacing
  • Pruning to improve canopy ventilation
  • 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
  • Should not be considered a guaranteed control

• May contribute to the development of other diseases