Strawberry Cultivar Susceptibility
Cause Ramularia grevilleana (sexual: Mycosphaerella fragariae), 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 susceptible 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.
Back seed disease was found to be a problem in Eastern Canada when greater than 4 inches of rain occurred in May (the month before flowering).
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. Spots can also occur on petioles and calices.
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.
- 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.
- Use drip irrigation if possible. When using overhead irrigation, shorten sets and water at times that promote rapid drying of the foliage.
Chemical control Alternate or tank-mix products from different groups that have different modes of action. Limit the use of any one group during crop production. Selection of products for rotation and/or mixing must consider group 7 fungicides when used through the irrigation as a nematicide. Action thresholds of 15, 25 or 22 lesions per leaf at 21, 14 or 7 days before 10% bloom, respectively, were used to time fungicide applications in Eastern Canada.
- Bonide Captan 50 WP at 4 to 8 Tbsp/gal water can be used in home gardens. H
- Cabrio EG at 12 to 14 oz/A. May be used at harvest. 12-hr reentry.
- Captan 80 WDG at 1.9 to 3.75 lb/A. May be applied up to the day of harvest. Group M4 fungicide. 24-hr reentry.
- Fixed copper. Begin applying when plants establish and continue regularly through the season. Discontinue if signs of phytotoxicity appear. Group M1 fungicide. O
- Champ WG at 2 to 3 lb/A. 48-hr reentry.
- C-O-C-S WDG at 2 to 2.9 lb/100 gal water. 48-hr reentry.
- Copper-Count-N at 1.5 to 2 quart/100 gal water. 48-hr reentry.
- Cueva at 0.5 to 1 gal/100 gal water/A. May be used on day of harvest. 4-hr reentry.
- Cuprofix Ultra 40 Disperss at 1.25 to 2.5 lb/A. 48-hr reentry.
- Kocide 3000 at 0.75 to 1.25 lb/A. 48-hr reentry.
- Monterey Liqui-Cop at 1 to 2 teaspoons/gal water. H
- Nu-Cop 50 DF at 1 to 3 lb/A. 48-hr reentry.
- Hi-Yield Captan 50 W Fungicide may be used in home gardens at 4 to 8 Tbsp/gal water. H
- Luna Sensation at 7.6 fl oz/A. May be use day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Merivon at 4 to 7 fl oz/A. May be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Mettle 125 ME at 3 to 5 fl oz/A. May be applied on the day of harvest. Group 3 fungicide. 12-hr reentry.
- Pristine at 18.5 to 23 oz/A. Can be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- QuiltXcel at 14 fl oz/A. May be used the day of harvest. Group 3 + 11 fungicide. 12-hr reentry.
- Rally 40 WSP at 2.5 to 5 oz/A. Applications may be made up to the day of harvest. Group 3 fungicide. 24-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1.25 fl oz/gal water. May be applied up to the day of har-vest. Do not apply more than six (6) times per season. Group 3 fungicide. H
- Thiram Granuflo at 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.
- Topsin 4.5 FL at 15 to 20 fl oz/A plus another fungicide. Do not apply within 1 day of harvest. Group 1 fungicide. 24-hr reentry.
Note: When used for Botrytis control, iprodione has activity on common leaf spot. Elevate and Switch have little activity against this disease.
Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Luna Sensation, Luna Tranquility, and Vacciplant.
Chemical control for nonbearing nursery production only
- Equus at 1.5 pints/A. Group M5 fungicide. 12-hr reentry.
Reference Carisse, O., and McNealis, V. 2019. Development of Action Threshold to Manage Common Leaf Spot and Black Seed Disease of Strawberry Caused by Mycosphaerella fragariae. Plant disease, 103:563-570.