2009 Downy Mildew Control Strategy for Cucurbits

Downy mildew is a serious disease of cucurbit crops grown in Ontario. Cucumbers are particularly susceptible to this disease. Once established in a region, downy mildew can spread rapidly causing significant loss of fruit quality and yield.

Downy mildew must be managed preventatively. Do not wait for symptoms to appear before initiating a fungicide program. Use only recommended downy mildew fungicides. These products have performed consistently in research trials across North America.

Follow the following fungicide spray intervals:

Under High Risk Conditions:

• Maintain a 5-7 day spray interval.
• Use only targeted downy mildew fungicides.
• Start applications as soon as the risk conditions
  indicate a need.
• Consider a banded application on small plants.

Under Low Risk Conditions:

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

Refer to Table 1 and Table 2, Recommended Fungicides for Cucurbit Downy Mildew Control for a list of recommended fungicides.

Always rotate between fungicides from different chemical families. Use a minimum spray volume of 30-60 gallons per acre. Apply a preventative fungicide before a rainfall event or prior to overhead irrigation. Do not rely upon "kick back" or curative fungicide activity. Downy mildew must be managed consistently and preventatively.

Foliar fungicides in the strobilurin or QoI family (group # 11) pose a high risk of developing resistance and have not performed consistently in research trials. Due to the highly infectious nature of this disease, group 11 fungicides are not recommended for downy mildew control in Ontario.

Any of the Following Conditions Indicate a High Risk of Cucurbit Downy Mildew:

• Downy mildew has been identified in the Great Lakes Region.
• Strong weather fronts originating in the Gulf of Mexico are predicted.
• 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 overhead irrigation.
• At a temperature of 20°C infection will occur with only 2 hours of leaf wetness.

Downy mildew conditions will be communicated to Ontario growers via the weekly OMAFRA Vegetable Crop Update. For a copy of this update, contact OMAFRA at 519-674-1616.

Cucurbit Transplants, Row Covers and Tunnels

• Ensure all transplants are free from disease.
• Produce vegetable transplants in greenhouses used solely for vegetable transplant production.
• Where row covers are used, apply either Ranman 400SC or Tattoo C immediately after field-setting, just prior to covering plants with row covers or tunnels.
• Immediately after the row covers or tunnels are removed, apply either Ranman 400SC or Tattoo C.
  Do not make sequential applications of either of these products.
• After the row covers are removed follow a standard preventative program using targeted downy mildew products on a 7-day spray interval.
  

Broad Spectrum Fungicides

These products have multiple modes of action and are applied preventatively to protect the crop against a wide variety of leaf diseases. Thorough spray coverage is essential to the performance of these fungicides.

Targeted Fungicides

These products are used to prevent the germination of downy mildew spores on the leaf surface. They must also be used preventatively. Targeted fungicides have a single mode of action. The risk of developing resistance to these products is quite high if they are not rotated properly, or if they are overused. Thorough spray coverage is also essential to the performance of these fungicides.

For more information on the biology and management of downy mildew, refer to the OMAFRA Infosheet Downy Mildew in Cucurbits: http://www.omafra.gov.on.ca/english/crops/facts/downy-mildew-a.htm