Cause:
Podosphaera aphanis (formerly Sphaerotheca macularis f. sp. fragariae), a fungus that overwinters on infected plant tissue including living leaves. This fungus is favored by conditions that produce high humidity but dry leaves. Conidia are able to begin germination after 6 hours and complete it within 24 hours, irrespective of temperature. Lesion expansion is related to temperature but does not seem to be related to relative humidity. It is a highly specialized pathogen that forms a close association with the host. Conditions that favor the host also favor the pathogen. Much of the fungus remains on the outside of infected plant parts but sends in root-like structures, haustoria, to obtain nutrients. The white growth seen is composed of both mycelium and fungal spores. ‘Hood’, ‘Totem’, and ‘Benton’ are moderately resistant or tolerant to mildew; ‘Shuksan’, ‘Sumas’, and ‘Linn’ are moderately susceptible; ‘Olympus’, ‘Redcrest’, ‘Independence’, ‘Puget Summer’, ‘Firecracker’, ‘Whonnock’ and ‘Northwest’ are very susceptible.
Symptoms:
Edges of infected leaflets curl up, exposing undersides that often are reddened and coated with a grayish white powdery mildew fungus. Diseased leaves later turn purplish or red. In irrigated fields, the fungus also may attack fruit. Some day-neutral cultivars are susceptible to fruit infection in fall even though leaves may appear healthy.
Note the curled leaves and gray patches on underside of leaves.
Powdery mildew symptoms has a grayish-white appearance on underside of leaves.
Cultural
control:
Destroying old leaves by renovating plants after harvest may help reduce inoculum.
Plant resistant cultivars.
Chemical
control: The disease needs to be controlled on highly susceptible cultivars after summer renovation so plants remain vigorous until they cease growth and go dormant in late fall. Many of these products, such as soaps, oils and sulfurs, may influence mite problems in the field.
Abound at 6 to 15.5 fl oz/A. Do not apply more than 2 sequential applications or more than 4 applications per year. May be applied on the day of harvest. 4-hr reentry.
Bicarbonate-based products. Might supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. Easily washed off by rain, so reapplication is necessary.
Armicarb 100 (85% potassium bicarbonate) at 2.5 to 5 lb/100 gal water. Washington and Idaho only. 4-hr reentry.
Kaligreen (82% potassium bicarbonate) at 2.5 to 3 lb/A. 4-hr reentry.
MilStop (85% potassium bicarbonate) at 2 to 3 lb/A. 1-hr reentry.
Monterey Bi-Carb is registered for home use in all states. Remedy can only be used in Washington.
Cabrio EG at 12 to 14 oz/A. Do not use more than twice sequentially or more than five times per year. May be used at harvest. 12-hr reentry.
E-Rase RTU (Jojoba seed oil) is registered for home use. May solidify below 50oF. H O
JMS Stylet Oil at 3 quarts/100 gal water. Do not use during freezing temperatures, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet because good coverage is essential. 4-hr reentry.
Kumulus DF (80% sulfur) at 5 to 10 lb/A. 24-hr reentry.
M-Pede at 1 gal/50 gal water is registered on strawberry for soft-bodied insects and has shown good activity against powdery mildew on several other crops. 12-hr reentry.
Microthiol Disperss (80% sulfur) at 5 to 10 lb/A.
Activity depends on temperature: it may not be as effective below 65°F and may burn plants if applied above 85°F. Do not use a spreader sticker. 24-hr reentry.
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.
Procure 480 SC at 4 to 8 fl oz/A. Do not apply within 1 day of harvest or more than 32 oz/A/year. See label for crop rotation restrictions. 12-hr reentry.
Propiconazole-based fungicides are registered. Do not use more than 16 fl oz/A/season or 2 sequential applications. May be applied day of harvest.
Orbit at 4 oz/A. 12-hr reentry.
Tilt at 4 fl oz/A. 12-hr reentry.
Quintec at 4 to 6 fl oz/A. Do not apply more than four (4) times per year, more than 24 fl oz/A/season, or within 1 day of harvest. May cause leaf spotting and chlorosis under certain conditions. 12-hr reentry.
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.
Safer Garden Fungicide (Ready To Use 0.4% sulfur) thoroughly sprayed over the entire plant. Do not use when the temperature is over 85°F or within a few weeks of an oil spray. Do not use on fruit that will be used for canning or within 1 day of harvest.
Thiolux Jet (80% sulfur) at 5 to 10 lb/A. 24-hr reentry.
Biological
control:
Serenade MAX (Bacillus subtilis strain QST 713) at 1 to 3 lb/A is registered for suppression only. As such it is not recommended for use in the Pacific Northwest. 4-hr reentry.
Sonata (Bacillus pumilis strain QST 2808) at 2 to 4 quarts/A is registered for suppression only. As such it is not recommended for use in the PNW. May be applied up to and including the day of harvest. 4-hr reentry.
References: Nelson, M. D., Gubler, W. D. and Shaw, D. V. 1996. Relative resistance of 47 strawberry cultivars to powdery mildew in California greenhouse and field environments. Plant Disease 80:326-328.
Miller, T. C., Gubler, W.D., Geng, S. and Rizzo, D.M. 2003. Effects of temperature and water vapor pressure on conidial germination and lesion expansion of Sphaerotheca macularis f. sp. fragariae. Plant Disease 87:484-492.
Content edited by:
Jay W. Pscheidt on
January 1, 2009
