Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Strawberry -- Common Leaf Spot
 
Cause: Mycosphaerella fragariae (asexual: Ramularia brunnea), a fungus that overwinters on old infected leaves and as sclerotia. Conidia are produced abundantly in lesions on infected leaves. Sclerotia in dead leaf material produce conidia in the spring. Conidia are rain-splashed to healthy tissues. Only young leaf tissue is suceptible to infection and then, only after a minimum of 12 hours of leaf wetness. Long wet periods (several days) and warm temperatures (over 50°F) favor disease development in the spring and in summer after renovation. Several physiologic races of the fungus have been identified. ‘Olympus’, ‘Puget Reliance’ and ‘Shuksan’ are very susceptible.
Symptoms: Foliage is at first dark red or purplish, gradually becoming grayish or almost white with age. Fully developed spots are about 0.12 inch in diameter, with a whitish center and reddish margin. They are scattered widely over the leaf surface, reducing leaf function. Infections occur in moist weather and are most severe in spring and fall.

The fungus also can infect fruit in what is called black seed disease. Berries usually have one or two spots but may have as many as 10. Spots are brownish black, hard, and leathery and appear on one to several achenes. Fruit does not rot but discolors under the spot.

Note the small leaf spots with gray centers and purple borders.

Black seed and necrotic leaf spots.

Note that most of the leaf spots on this plant are near the ground and on older leaves.

 
Cultural control: Removing foliage after harvest reduces inoculum in the field. Mow leaves of June-bearing plants 0.5 inch above the crown after harvest. Do not mow everbearing or day-neutral types. Leaf spot can still be a problem after renovation unless supplemented with chemical protection.
Chemical control:
  1. Cabrio EG at 12 to 14 oz/A. Do not apply more than twice sequentially or more than five times per year. May be used at harvest. 12-hr reentry.
  2. Captan 80 WDG at 1.9 to 3.75 lb/A. May be applied up to the day of harvest. Hi-Yield Captan Fungicide 50 WP may be used in Oregon home gardens at 2 Tbsp/gal water. 24-hr reentry.
  3. Fixed copper. Begin applying when plants establish and continue weekly through the season. Discontinue if signs of phytotoxicity appear.
    1. Champ Formula 2 at 1.3 to 2 pints/A. 24-hr reentry.
    2. Cuprofix Ultra 40 Disperss at 1.25 to 2.5 lb/A. 12-hr reentry.
    3. Kocide 3000 at 0.75 to 1.25 lb/A. 24-hr reentry.
    4. Nu-Cop 50 DF at 2 to 3 lb/A. 24-hr reentry.
  4. Pristine at 18.5 to 23 oz/A. Do not use more than 2 consecutive applications or more than 5 times/year. Can be used day of harvest. 12-hr reentry.
  5. Rally 40 WSP at 1.25 to 5 oz/A. Applications may be made up to the day of harvest. Do not apply more than 30 oz/A/season. 24-hr reentry.
  6. Syllit FL at 2.4 to 3 pt/A. Do not apply within 14 days of harvest. 48-hr reentry.
  7. Thiram Granuflo at 3.4 to 4.4 lb/A. Do not use within 3 days of harvest. Pest not on label but can be used for control. 24-hr reentry.
  8. Topsin 4.5 FL at 15 to 20 fl oz/A plus another fungicide. Do not apply within 1 day of harvest. 12-hr reentry.

Chemical control for non-bearing nursery production only.

  1. Bravo Weather Stik at 1.5 to 3 pt/A. Do not use if Omite 30 W will ever be used. Washington only (SLN WA-880013b). 12-hr reentry.
Notes: When used for Botrytis control, iprodione has activity on common leaf spot. Elevate and Switch have little activity against this disease.
References:
Maas, J. L. 1998. Compendium of Strawberry Diseases. St. Paul, MN: American Phytopathological Society.

Carisse, O., Bourgeois, G. and Duthie, J. A. 2000. Influence of temperature and leaf wetness duration on infection of strawberry leaves by Mycosphaerella fragariae. Phythopathology 90:1120-1125.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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