Strawberry (Fragaria spp.)-Powdery Mildew

Note the curled leaves and gray patches on underside of leaves.
Note the curled leaves and gray patches on underside of leaves.
Powdery mildew symptoms has a grayish-white appearance on underside of leaves.
Powdery mildew symptoms has a grayish-white appearance on underside of leaves.
PNW Plant Disease Image

Powdery mildew on both leaves and fruit of the strawberry cultivar Sweet Sunrise.

See also:

Cause Podosphaera aphanis var. 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. Only the youngest leaf or fruit tissue is susceptible to infection. As leaves and fruit age they rapidly become resistant to infection. Cultivars with leaves that emerge folded generally have more disease on the underside of the leaf as it is exposed to inoculum while susceptible. The upper surfaces are exposed to inoculum as they become more resistant to infection. Fruit becomes resistant to infection 10 to 15 days after bloom (about white stage).

Powdery mildew fungi are highly specialized pathogens that form a close association with the host. Conditions that favor the host also favor the pathogen. Much of the fungus remains outside infected plant parts where it grows on the surface but sinks root-like structures called haustoria into plant cells to obtain nutrients. The white growth seen is composed of both mycelium and fungal spores. Cleistothecia may form within the colonies late in the growing season, stay associated with the overwintering leaves and shed mature sexual spores in the spring when plant growth resumes.

'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. The powdery mildew of raspberry does not infect strawberry and that of strawberry does not infect raspberry despite the similar name.

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.

Cultural control

  • Destroying old leaves by renovating plants after harvest may help reduce inoculum.
  • Plant resistant cultivars.
  • Frequent overhead irrigation for short periods may help remove conidia and reduce disease pressure. Do not irrigate such that leaves are wet for extended periods of time.

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. Powdery mildew on most cultivars grown in the PNW does not seem to impact yield the next year. Therefore, growers have not typically managed this disease after harvest. Many of these products, such as soaps, oils and sulfurs, may influence mite problems in the field. Alternate or tank-mix products from different groups that have different modes of action. Resistance to group 3 fungicides has been reported.

  • Abound at 6 to 15.5 fl oz/A. Do not apply more than two (2) sequential applications or more than four (4) applications per year or with silicone-based surfactants. May be applied on the day of harvest. Group 11 fungicide. 4-hr reentry.
  • Aftershock at 2 to 5.7 fl oz/A. Do not apply more than twice consecutively, or more than four (4) applications total per season, use more than 22.8 fl oz/A/season, or within 1 day of harvest. Group 11 fungicide. 12-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. O
    • Armicarb "O" (85% potassium bicarbonate) at 2.5 to 5 lb/100 gal water. 4-hr reentry.
    • Kaligreen (82% potassium bicarbonate) at 2.5 to 3 lb/A. 4-hr reentry.
    • MilStop (85% potassium bicarbonate) at 2.5 to 5 lb/A.
    • Oregon and Washington only. 1-hr reentry.
    • Monterey Bi-Carb Old Fashioned Fungicide is registered for home use in all states. H
  • Cabrio EG at 12 to 14 oz/A. Do not use more than twice sequentially or more than five (5) times per year. May be used at harvest. 12-hr reentry.
  • Cosavet-DF (80% sulfur) at 5 to 10 lb/A. Group M2 fungicide. 24-hr reentry. O
  • E-Rase RTU (Jojoba seed oil) is registered for home use. May solidify below 50°F. H O
  • Evito 480 SC at 2 to 5.7 fl oz/A. Do not apply more than twice consecutively, or more than four (4) applications total per season, use more than 22.8 fl oz/A/season, or within 1 day of harvest. Group 11 fungicide. 12-hr reentry.
  • Flint 50 WG at 2 to 3.2 oz/A. Do not apply more than twice consecutively, or more than six (6) applications total per season, or use more than 19.2 oz/A/season. May be applied up to the day of harvest. Group 11 fungicide. 12-hr reentry.
  • Fontelis at 16 to 24 fl oz/A. Can be used day of harvest. Foliar reddening or speckling may occur in some cultivars. Group 7 fungicide. 12-hr reentry.
  • Fracture (BLAD) at 20.5 to 24.4 fl oz/A. Reapply if rain occurs within 12 hours of original application. Do not use within one day of harvest. 4-hr reentry.
  • Horticultural Mineral Oils. Generally effective at 1% to 2% volume to volume. Necrotic foliage may result if applied within 10 days of any sulfur application. Do not tank-mix with copper-based products when fruit is present. 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. O
    • JMS Stylet Oil at 3 quarts/100 gal water.
    • SuffOil-X at 1 to 2 gal/100 gal water.
  • Inspire Super at 16 to 20 fl oz/A. Can be applied day of harvest. Group 3 + 9 fungicide. 12-hr reentry.
  • Kenja 400 SC at 13.5 to 15.5 fl oz/A. May be used up to and including day of harvest. Group 7 fungicide. 12-hr reentry.
  • Kumulus DF (80% sulfur) at 5 to 10 lb/A. Group M2 fungicide. 24-hr reentry. O
  • Luna Privilege at 6.84 fl oz/A applied through a drip irrigation system. May be used day of harvest. Group 7 fungicide. 12-hr reentry.
  • Merivon at 4 to 7 fl oz/A. Do not use with EC or oil-based products. May be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
  • Mettle 125 ME at 3 to 5 fl oz/A. Do not apply more than 20 oz/A/season. May be applied on the day of harvest. Group 3 fungicide. 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.
  • M-Pede at 1 to 2 gal/100 gal water. Good coverage is essential. Do not use within 3 days of applying sulfur. Do not mix with hard water. 12-hr reentry. O
  • Oso SC at 3.75 to 13 fl oz/A. May be applied on the day of harvest. Group 19 fungicide. 4-hr reentry.
  • Prev-Am Ultra at 50 fl oz/100 gal water. Do not use above 90°F or when plants are under heat or moisture stress. 12-hr reentry.
  • Pristine at 18.5 to 23 oz/A. Do not use more than two (2) consecutive applications. Can be used day of harvest. Do not use if used for gray mold control. Group 7 + 11 fungicide. 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. Group 3 fungicide. 12-hr reentry.
  • Propiconazole-based fungicides are registered. Do not use more than 16 fl oz/A/season or two (2) sequential applications. May be applied day of harvest. Group 3 fungicides. 12-hr reentry.
    • Bumper 41.8 EC at 4 fl oz/A.
    • Tilt at 4 fl oz/A.
  • Quadris Top at 12 to 14 fl oz/A. May be applied on the day of harvest. Group 3 + 11 fungicide. 12-hr reentry.
  • QuiltXcel at 14 fl oz/A. Do not use more than 56 fl oz/A/season or more than two (2) sequential sprays. May be used the day of harvest. Group 3 + 11 fungicide. 12-hr reentry.
  • Quintec at 4 to 6 fl oz/A. A surfactant is not required when used alone but a nonionic surfactant is preferred if needed for tank-mixes. 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 2.5 to 5 oz/A. Can apply up to the day of harvest. Group 3 fungicide. 24-hr reentry.
  • Regalia at 1 to 3 quarts/A. Use on 7-day intervals. May be used day of harvest. Group P5 fungicide. 4-hr reentry. O
  • Torino at 3.4 oz/A. Do not make more than two (2) applications per year. May be used day of harvest. Group U6 fungicide. 4-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. H
  • Spectracide Immunox at 1.25 fl oz/gal water. May be applied up to the day of harvest. Do not apply more than six (6) times per season. H

Notes Although registered, Distinguish is unavailable for purchase or use.

Some registered products offer only suppression of this disease and thus are not recommended for use. These products include DoubleNickel 55, Ph-D, Serenade, and Sonata.

Forecasting The Gubler-Thomas (UC-Davis) program modified by Broome uses only temperature during the summer to predict conidial infection periods. This has been useful in California and may be of use in the PNW.

References Asalf, B., Gadoury, D.M., Tronsmo, A.M., Seem, R.C., Dobson, A., Peres, N.A., and Stensvand, A. 2014. Ontogenic resistance of leaves and fruit, and how leaf folding influences the distribution of powdery mildew on strawberry plants colonized by Podosphaera aphanis. Phytopathology 104:954-963

Carisse, O., Lefebvre, A., Van der Heyden, H., Roberge, L., and Brodeur, L. 2013. Analysis of incidence-severity relationships for strawberry powdery mildew as influenced by cultivar, cultivar type, and production systems. Plant Dis. 97:354-362.