2011 Downy Mildew Control Strategy for Cucurbits

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. While infections may occur in squash and pumpkins, the disease tends to be less virulent in these crops. Due to the varying susceptibility, the management strategies will differ, depending on the type of cucurbit being grown.

Cucumbers

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.
• Forecasts predict strong weather fronts originating in the Southern US.
• 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:

Refer to Table 1, Recommended Fungicides for Cucurbit Downy Mildew Control.

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.

Muskmelons

While downy mildew is a concern in muskmelon crops, some of the commonly used downy mildew fungicides may not offer sufficient protection against other common diseases such as scab, anthracnose or alternaria. Scout fields regularly and consider the full range of foliar diseases when planning a fungicide program for this crop. Table 2, Activity of Fungicides on Cucurbit Diseases provides an overview of the diseases controlled by each registered fungicide.

Pumpkins, Squash and Watermelon

Downy mildew infections are less common in pumpkin, squash and watermelon crops. A preventative downy mildew management program is not usually needed for these crops. Scout these crops regularly for symptoms of foliar diseases. Follow a broad spectrum disease management program, based on the presence of other foliar diseases such as powdery mildew, scab, alternaria, anthracnose and septoria. Table 2, Activity of Fungicides on Cucurbit Diseases provides an overview of the diseases controlled by each registered fungicide.

Cucurbit Transplants

• Ensure all transplants are free from disease and only plant healthy transplants.
• Produce vegetable transplants in greenhouses used solely for vegetable transplant production. If transplants are grown in the same greenhouse as commercial greenhouse vegetables, the mature plants may act as a reservoir for crop pests.
• Where cucumbers are grown under row covers or tunnels, apply a targeted downy mildew fungicide after field-setting, just prior to covering plants. Apply a second targeted downy mildew fungicide immediately after the row covers or tunnels are removed. Melon growers may also choose to apply a preventative fungicide at planting.
• After the row covers are removed follow a standard fungicide program based on the type of crop and risk assessments described on page 1.

Downy Mildew Management Tips

• 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
• Apply a preventative fungicide prior to overhead irrigation.
• 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.

PHI¹= Pre-Harvest Interval

√= control + = suppression ( ) = on label but not considered effective or resistance documented