Vine Crop Edition
by Elaine Roddy, Vegetable Crop Specialist, OMAFRA, Ridgetown

Downy Mildew Update
Early Identification
Ontario Early Warning Scouting Program

Downy Mildew Update

New downy mildew infections continue to be reported in the Southern United States.  To date there have been no reports of downy mildew in field cucumbers in the Northern United States or Canada.  However, unsettled weather patterns continue to dominate both the short and the long range forecast.  Prolonged periods of leaf-wetness combined with fast moving storm fronts could result in a moderate risk of infection at this time.  Keep in mind over the past 4 years, the initial downy mildew infections have occurred in Ontario in late June.

All processing and fresh market cucumber growers should be on a 7-day preventative fungicide spray program.  Due to the current weather conditions, use only targeted downy mildew fungicides such as Tattoo C and Ranman 400SC.

Early Identification

While the control strategy for downy mildew focuses on preventative fungicide applications, early identification can make a big difference in our ability to curtail the spread of this disease, both in the field and between fields.

Downy mildew symptoms develop 4- 12 days after the initial infection.  They first appear as water-soaked lesions or pale greenish-yellow or tan coloured spots on the upper surface of older leaves.  In cucumbers the lesions are often confined by the leaf veins and take on an angular appearance. See figures 1 and 2.

As the disease progresses, the centre of the lesion will eventually dry up and die.  A greyish-purple downy growth and dark purple spores are often present on the lower leaf surface.  This is particularly noticeable in the mornings after a period of wet weather or heavy dew. 

Figure 1.  Water-soaked lesions on upper leaf surface.

Figure 2.  Early pale yellow-tan lesions. 

Ontario Early Warning Scouting Program

If you suspect you have downy mildew in your field, please report it to OMAFRA (519 674 1616 or elaine.roddy@ontario.ca), your processor or a local agri-business representative.  Good communication across the industry is the cornerstone to managing this virulent disease.