Powdery Mildew in Cucurbits

Powdery mildew appears to have made an early appearance in Ontario this year.  We typically look for this disease in late-summer.  However, I am already seeing symptoms in our research plots in pumpkins, squash and cantaloupe.

Powdery mildew first appears as a dense, white (powdery) fungal mass on the lower leaf surface of the older leaves.  There is often a light yellow patch on the corresponding upper leaf surface.  If left unchecked, it will completely cover and kill the infected leaves and petioles.  Premature death of the canopy results in lower sugar content and early-ripening of infected fruit, before it properly sizes.  Infected stems also become brittle and break off the fruit during harvest.  This is a particular problem in jack-o-lanterns and decorative pumpkins or gourds.

In order to be effective, fungicide spray programs must start at the first sign of infection.  Once the symptoms are readily visible in the crop canopy, fungicides will have little (if any) impact on the spread of the disease.  Scout all cucurbit fields regularly and apply preventative fungicides before the levels of powdery mildew in the canopy exceed 2% of the leaves with one lesion.

Registered powdery mildew fungicides include: chlorothalonil (Bravo 500, Echo 720), pyraclostrobin (Cabrio EG), quinoxyfen (Quintec), and difenoconazole (Inspire).  For Organic growers, potassium bicarbonate (MilStop) will provide suppression of this disease.  Always consult with your organic certifier before applying any crop protection product.

Powdery mildew is an airborne pathogen and, unlike many crop diseases, it is able to infect plants at a comparatively low relative humidity.  Dry weather conditions will not necessarily stop the spread of this disease.  Fortunately, it does not grow well at temperatures above 30 C.  For more information on the biology and management of powdery mildew, visit the Ontario CropIPM website at: http://www.omafra.gov.on.ca/IPM/english/cucurbits/diseases-and-disorders/powdery-mildew.html