This disease has not been reported this year in Ontario. When cool, humid conditions are present the risk of downy mildew increases. Downy mildew infections can occur when the leaves are wet for 2-6 hours at 3-14ºC. Accordingly, DOWNCAST, which helps predict downy mildew outbreaks in onions in the Holland Marsh, indicated that the risk of downy mildew on onions is moderate to high, depending on canopy size of crop (14 August 2012). DOWNCAST is not available in other areas of Ontario, thus protective fungicides should be applied during cool, humid conditions.
In Ontario, this disease is common in July and August and can affect yield and bulb quality. Last year we had an isolated report of downy mildew in mid-July (See Downy Mildew in Onions 2011) and so far this year we’ve had no reports. The neck tissue of the affected onions may not ‘cure’ properly at harvest and is thus open to secondary fungal and bacterial pathogens. Initial symptoms are elongated, pale patches on the leaves that may have a purple-grey velvety growth present during moist conditions. The affected leaves eventually turn pale green to yellow and collapse and die.
For more information see: Ontario CropIPM – Onion Downy Mildew