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Beginner

Scientific Name
Sphaerotheca macularis f. sp. fragariae

Identification

• White patches of fungus develop on the lower leaf surface, causing the leaf edges to roll up.
  
• The lower leaf surface develops purplish-to-reddish grey blotches along the margins, with little or no powdery sporulation.
  
• The upper leaf surface may develop red speckling and purplish discolouration.
  
• A white powdery substance may form on the fruit.
  
• Fruit is smaller, with raised seeds and a lightly scarred surface.

Often Confused With
Leaf scorch
Thrips damage on fruit

Period of Activity
High humidity and warm weather (15– 27°C, 60- 80°F) are needed for powdery mildew to develop on strawberries. In early warm springs with extended periods of high relative humidity, the disease will establish early.

Scouting Notes
Scout for leaf infections in new plantings and after harvest in renovated plantings. Scout when days become short (end of July) and when night temperatures become cool. Look for infected fruit on susceptible varieties (Cavendish, Seascape). Be sure to scout Seascape in spring, summer and fall. Control on this variety requires an aggressive fungicide program at the first sign of leaf disease.

Thresholds
First sign of leaf disease on Seascape; otherwise no thresholds have been established.

Advanced

Scientific Name
Sphaerotheca macularis f. sp. fragariae

Identification
White patches of fungus develop on the lower leaf surface, causing the leaf edges to roll up. The lower leaf surface develops purplish-to-reddish grey blotches along the margins, with little or no powdery sporulation. The upper leaf surface may develop red speckling and purplish discolouration. In Ontario, infection of the flowers and fruit is not common but does occur on some varieties, reducing yield and quality. A white powdery substance may form on the fruit. Fruit is smaller, with raised seeds and a lightly scarred surface. Although fruit and blossom infections can cause economically important losses, infection of leaves after harvest probably has little, if any, effect the following year.

Often Confused With
Leaf scorch (Both diseases cause purple blotches and flecking on the upper leaf surface. Only powdery mildew causes leaf edges to curl upwards.)
Thrips damage on fruit (Both can cause raised seeds, and fruit skin to be bronzed or cracked. Use a hand lens to looks for white mycelium around the seeds, diagnostic of mildew, or bronzing on the underside of calyx tissue caused by thrips.)

Biology
Powdery mildew is caused by the fungus Sphaerotheca macularis, which infects only wild and cultivated strawberries. This pathogen cannot survive without living host tissue. It overwinters in infected plants. Spores produced from this fungus are carried short distances to new uninfected plants by wind. Unlike the spores of most fungal diseases, powdery mildew spores do not need free moisture to infect. The spores are short-lived but can germinate 4- 6 hours after landing on dry leaf surfaces in high humidity. Spore germination is suppressed by rain. Leaf symptoms are visible about 6 days after infection.

Period of Activity
High humidity and warm weather (15– 27°C, 60- 80°F) are needed for powdery mildew to develop on strawberries. In early warm springs with extended periods of high relative humidity, the disease will establish early. Typically however, symptoms are most apparent in mid-late summer, when cooler nights and the resulting high humidity provide good conditions for infection.

Scouting Notes
Scout for leaf infections in new plantings and after harvest in renovated plantings. Scout when days become short (end of July) and when night temperatures become cool. Look for infected fruit on susceptible varieties (Cavendish, Seascape). Be sure to scout Seascape in spring, summer and fall. Control on this variety requires an aggressive fungicide program at the first sign of leaf disease.

Thresholds
First sign of leaf disease on Seascape; otherwise no thresholds have been established.

Management Notes

• This pathogen is able to develop fungicide resistance, therefore alternate between fungicides from different families and follow resistance management practices.
  
• Contaminated transplants are thought to be a major source of infection. Purchase plants from an accredited program.
  
• Research in some areas of Canada indicates that post-harvest infections generally do not reduce yields the following year.
  
• Susceptible cultivars include: Annapolis, Governor, Jewel, Startyme, Seascape and Veestar.