Onion downy mildew has been reported in Ontario. This disease appears as a purple-grey, velvet-like growth on the leaves usually in patches within the field. Affected leaf tissue will turn pale-green to yellow and eventually collapse, which compromises yield and storability. Ideally, this disease develops during cool, wet seasons when the foliage is wet for 2-6 hours at 3-14ºC. Even under moderate to high temperatures, high humidity (95% or more) may promote further infections. Fungicides can help if applied preventatively. There are several fungicides registered for downy mildew control including, Ridomil Gold MZ 68 WP, Cabrio EG, Aliette WDG, Revus and mancozeb-based fungicides (Manzate Pro-Stick). Serenade Max is registered for downy mildew suppression. Be sure to follow directions for use on the label and practice resistance management strategies when using fungicides. According to DOWNCAST, the risk of downy mildew on onions is moderate in the Holland Marsh.
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