2015 Downy Mildew Control Strategy for Cucumber Crops

Downy mildew is a serious disease of cucurbit crops grown in Ontario. The relative importance of this disease depends on the type of cucurbit. Cucumbers are extremely susceptible, followed by muskmelons and watermelons.

Downy mildew must be managed preventatively. Do not wait for symptoms to appear before initiating a fungicide program. Apply registered downy mildew fungicides based on the risk of infection.

Any of the following conditions indicate a high risk of downy mildew infection:

  • Downy mildew has been identified in the Great Lakes Region
  • Prolonged periods of cool, wet weather.
  • Cooler night-time temperatures and heavy dew fall followed by warm, windy days.
  • Extended periods of leaf wetness due to dew, rain or overhead irrigation.

Product selection and spray intervals depend on the risk of infection:

Under High Risk Conditions

  • Start applications immediatley, regardless of crop stage.
  • Maintain a 5-7 day spray interval.
  • Use only targeted downy mildew fungicides.

Under Low Risk Conditions

  • Start applications no later than vine development.
  • Maintain a 7-day spray interval.
  • Rotate between broad spectrum and targeted downy mildew fungicides.

Downy Mildew Management Tips

  • Use only recommended downy mildew fungicides (See Table 1 on page 2)
  • Always rotate between fungicides from different chemical families. Do not make sequential applications of any one fungicide.
  • Use a minimum spray volume of 30-60 gallons per acre.
  • Apply a preventative fungicide before a rainfall event or prior to overhead irrigation.
  • Apply a preventative fungicide prior to overhead irrigation.

Cucumbers - Early Crop

Maintain a preventative fungicide program, based on the risk conditions described above, until the final harvest. Do not stop spraying or lengthen the spray intervals before harvest is complete. Destroy the crop by discing or plowing the crop residue into the soil immediately after harvest. Do not rely on a burn-down herbicide to destroy the crop, as this practice may actually increase the level of disease present in the field.

Cucumbers - Late Crop

Be prepared to begin the preventative fungicide program soon after crop emergence. Use the targeted downy mildew fungicides listed in Table 1, Recommended Fungicides for Cucurbit Downy Mildew Control. Use broad spectrum products in rotation with the targeted fungicides only if the risk of infection is considered low, or at the tail end of harvest.

Table 1. Recommended Fungicides for Cucumber Downy Mildew Control

Broadspectrum Preventative Fungicides

Begin applications no later than vine development.

Group Name (Group #) Common Name Trade Name Rate per Hectare Rate per Acre PHI¹ Notes
dithiocarbamate (group M3) mancozeb Dithane DG Rainshield 1.1-3.25 kg 0.4-1.3 kg 14 24-hour re-entry interval.
Penncozeb 75DF
2.25-3.25 kg 0.9-1.3 kg 14 24-hour re-entry interval.
chloronitrile (group M5) chlorothalonil Bravo 500 4.8 L 1.9 L 1 48-hour re-entry interval.

Downy Mildew-Targeted Preventative Fungicides

Begin applications when weather conditions favour downy mildew or if downy mildew is identified anywhere in the Great Lakes region.

Group Name (Group #) Common Name Trade Name Rate per Hectare Rate per Acre PHI¹ Notes
QiL (group 21) cyazofamid Torrent 400SC 150-200 mL 61-81 mL 1 Tank mix with an NIS or Organosilicone surfactant.
Do not make sequential applications.
12-hour re-entry interval.
carbamate (group 28) chloronitrile (group M5) propamacarb/
Tattoo C 1.8-2.7 L 0.7-1.1 L 2 Do not make sequential applications.
48-hour re-entry interval.
QxI (group 45) Carboxylic acid amide (group 40) ametoctradin/
Zampro 0.8-1 L 0.3-0.4 L 1 Use of a spreading/penetrating adjuvant may improve performance. See label for crop rotation restrictions and re-entry intervals.
benzamides (group 43) + chloronitrile (group M5) Fluopicolide + chlorothalonil Presidio + Bravo 500 220-292 mL + 4.8 L 89-118 mL +1.9 L 2 Do not make sequential applications.
48 hr. re-entry interval.
See product label for crop rotational restrictions.

PHI¹ = Pre-Harvest Interval

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca

Author: Elaine Roddy - Vegetable Crop Specialist/OMAFRA
Creation Date: March 2015
Last Reviewed: 03 July 2015