Cause The fungus-like oomycete, Pseudoperonospora cubensis, was fairly widespread in commercial fields in certain areas of the PNW during 2009 after a new race entered the US. This oomycete survives on living cucurbit hosts and is limited to the cucurbit family. It is dependent on living hosts to survive and reproduce; plants in greenhouses and plastic tunnels can function as local reservoirs for infections further afield. Disease is believed to be spread by wind-blown sporangia. There may be potential for seedborne transmission since this pathogen has been reported inside of cucurbit seeds. Free moisture and moderate temperatures favor infection and high relative humidity favors sporulation.
Symptoms Angular yellow lesions will form on upper leaf surfaces and grayish-purplish fluffy growth on the underside of the affected leaf area is a sign of downy mildew sporulation. As lesions expand, the affected tissues may turn brown.
Cultural control
- Cucumber varieties with resistance to the older races are available.
- Provide adequate aeration within, and especially between, rows by increasing plant and row spacing. This is not always economically feasible.
- Reduce the relative humidity and periods of leaf wetness inside greenhouses by heating and venting.
- Overhead irrigation or time irrigations to allow drying of plant canopy before night fall.
Chemical control
- Actigard 50WG at 0.5 to 1 oz/A on 7-day intervals. May be applied the day of harvest. 12-hr reentry.
- Chlorothalonil (Group M5) formulations are labeled.
- Bonide Fung-onil is available for home gardens. H
- Bravo Ultrex at 1.4 to 1.8 lb/A on 7-day intervals. May apply through sprinkler irrigation. May be applied the day of harvest. 12-hr reentry.
- Echo 720 at 1.5 to 2 pints/A on 7-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Copper formulations (Group M1) offer limited control.
- Badge SC at 0.5 to 2.5 pints/A on 5- to 7-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
- Bonide Liquid Copper is available for home gardens. H
- Kocide 2000-O at 1 to 2.25 lb/A or Kocide 3000-O at 0.5 to 1.25 lb/A on 5- to 7-day intervals. 48-hr reentry. O
- Previsto at 0.75 to 2 quarts/A on 5- to 7-day intervals. 48-hr reentry. O
- Curzate 60DF (Group 27) at 3.2 to 5 oz/A on 5- to 7-day intervals. Use only in combination with another protective fungicide. Do not apply within 3 days of harvest. 12-hr reentry.
- Elumin (Group 22) at 8 fl oz/A on 7- to 10-day intervals. Do not make more than one (1) application before alternating to a fungicide in different FRAC groups (non-Group 22). Preharvest interval is 2 days. 12-hr reentry.
- Forum (Group 40) at 6 fl oz/A on no shorter than 5-day intervals in combination with another fungicide that has a different mode of action. Do not apply more than once before alternating with another fungicide with a different mode of action. Can apply the day of harvest. 12-hr reentry.
- Gavel 75 DF (Group 22 + M3) at 1.5 to 2.0 lb/A on 7- to 10-day intervals. Preharvest interval is 5 days. 48-hr reentry.
- Omega 500F (Group 29) at 12 to 24 fl oz /A on a 7- to 10-day interval. Preharvest interval is 7 days. 12-hr reentry.
- Orondis Opti (Group M5 + 49) at 1.75 to 5 pints/A on 7- to 14-day intervals. Do not apply more than two (2) sequential applications. Preharvest interval is 0 day. 12-hr reentry.
- Orondis Ultra (Group 49 + 40) at 5.5 to 8 fl oz/A on 7- to 10-day intervals. Do not apply more than two (2) sequential applications. Preharvest interval is 0 day. 4-hr reentry.
- Phosphonates formulations (Group P7; formerly Group 33) are effective.
- Aliette WDG at 2 to 5 lb/A on a 7- to 14-day intervals. Preharvest interval is 0 days. Preharvest interval is 12 hr. 24-hr reentry.
- Presidio (Group 43) at 4 fl oz/A on 7- to 10-day intervals in combination with another fungicide that has a different mode of action. Preharvest interval is 2 days. 12-hr reentry.
- Ranman 400SC (Group 21) at 2.1 to 2.75 fl oz/A on 7- to 10-day intervals. Do not make more than three (3) applications before alternating for three (3) intervals to a labeled fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
- Regalia Biofungicide (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Ridomil Gold Copper (Group 4 + M1) at 2 lb/A on 14-day intervals. Preharvest 5 days. 48-hr reentry.
- Strobilurin (Group 11) formulations are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Evito 480 SC at 3.0 to 5.7 fl oz/A on 7- to 14-day intervals. Preharvest interval is 1 day. 12-hr reentry.
- Reason 500 SC at 5.5 fl oz/A on no shorter than 5-day intervals. Do not apply within 14 days of harvest. 12-hr reentry.
- Quadris Top (Group 11 + 3) at 12 to 14 fl oz/A on 7- to 14-day intervals. Preharvest interval is 1 day. 12-hr reentry.
- Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand-alone product. 4-hr reentry. O
- Vacres (potassium bicarbonate) at 2.5 to 5 lb/A on 7- to 14-day intervals. Preharvest interval is zero (0) days. 4-hr reentry.
- Zampro (Group 40 + 45) at 14 fl oz/A on 5- to 7-day intervals. Do not apply more than two (2) applications before alternating to a fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
Biological control Efficacy in the Pacific Northwest is unknown.
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 4-hr reentry. O
- Double Nickel LC at 1 to 6 quarts/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- LifeGard WG at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
- Romeo at 0.45 lb/A on 7- to 10-day intervals starting prior to infection. Preharvest interval is 0 days. 4-hr reentry. O
- Stargus at 2 to 4 quarts/A plus a nonionic surfactant on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
Reference Keinath, A.P., Wintermantel, W.M., and Zitter, T.A.. 2017. Compendium of Cucurbit Diseases and Pests, 2nd Ed. St. Paul, MN: APS Press.