Much has changed in the world of powdery mildew control.  Five years ago, we had three products from which to choose (two conventional and one organic). Today the cucurbit powdery mildew control list contains 15 different active ingredients (10 conventional and five organic.) The conventional products come from five different chemical groups, which is excellent news for product rotation. 
    

It is nice to have these many options, but sometimes decisions can be overwhelming.  Optimum powdery mildew control is a combination of variety selection, fungicide timing and fungicide selection.
  

Consider using powdery mildew-tolerant varieties to reduce disease pressure in the crop.  In heterozygous varieties, the resistance comes from one parent.  These varieties are powdery mildew-tolerant, but will still develop symptoms under heavy pest pressure.  Homozygous resistance comes from both parents. These plants show a higher level of resistance.
    

Whether using regular, tolerant or resistant varieties, scouting is the key to effectively managing this disease. As soon as the disease is identified in the crop, apply the most effective fungicide products first. Disease management is not a case where you want to “save the best for last.” Follow up applications may be required on seven to 10 day intervals, depending on when the disease arrives in Ontario and the weather conditions during fruit-sizing and ripening, 
    

The control threshold for powdery mildew is one disease lesion per 50 leaves. Lesions initially develop on the lower leaf surface or petioles of older leaves.  Fungicide sprays will not be effective once the 
disease is established in the crop and the lesions are readily apparent on the upper leaf surfaces. Not only is it ineffective, spraying after the disease is established in the crop also increases the risk of developing fungicide resistance.
    

Cheryl Trueman, a vegetable pest management researcher at the University of Guelph’s Ridgetown Campus, has been conducting downy mildew 
efficacy trials since 2009. In these trials, several products consistently provided good control of powdery mildew. These products are powdery mildew targeted, and have a single site mode of action. To prevent the development of resistance, it is essential to always rotate between different fungicide groups and/or tank mix with a broad spectrum fungicide.

Powdery Mildew Targeted Fungicides Showing Consistent Control in the Ridgetown Field Trials:
Group 13  Quintec (quinoxyfen)        

Group 71  Fontelis (penthiopyrad)        

Group U8   Vivando (metrafenone)

1 Note: Aprovia, Sercadis and Pristine are also group 7 fungicides, however they were not tested in the Ridgetown Campus trials.

Several group 3 fungicides are labelled for powdery mildew in cucurbits including: Inspire (difenoconazole), Proline (prothioconazole) and Quadris Top (azoxystrobin/difenoconazole). The efficacy data for these products is not as strong as the ones listed above, however they may be useful for product rotation. Fungicides containing chlorothalonil (Bravo ZN and Echo) provided a similar level of powdery mildew control.
    

Research in Ontario and other jurisdictions indicates that the group 11 (QoI) fungicides no longer control powdery mildew. However, they may provide control of other cucurbit diseases such as anthracnose and alternaria.
    

The powdery mildew targeted fungicides listed above will not provide control of other foliar cucurbit diseases unless tank-mixed or alternated with a broad spectrum fungicide such as chorothalonil (Bravo ZN, Echo) or mancozeb (Dithane, Manzate, Penncozeb). These broad spectrum fungicides are also valuable for resistance management.

Scouting protocol for all cucurbit diseases:

• In fields less than 10 acres in size, inspect a minimum of 100 plants
• Inspect 200 (10 at 20 locations) in larger fields. 
• Look at upper and lower leaf surfaces and leaf petioles.
• Be sure to include field edges and low lying areas.
• Don’t underestimate the impact of dew.
• Know the period of activity and environmental conditions for each disease.
• Start scouting well in advance of the typical period of activity.