Alliums Disease Fungicides Pest Management

Onion Downy Mildew Detected in Ontario

Downy mildew of onions has been reported this year in Ontario. Downy mildew has been detected in parts of the Holland Marsh. See the August 19th, 2013 report from the Muck Crops Research Station at:, for specific information on the areas in which it has been detected. There have also been reports of downy mildew in the Waterloo Region.

In Ontario, this disease is common in July and August and can affect yield and bulb quality. 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.

Downy mildew infection on onion leaf.
Downy mildew infection on onion leaf.

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. DOWNCAST, which helps predict downy mildew outbreaks in onions in the Holland Marsh, indicated that the risk of downy mildew on onions is high particularly because it has been detected in the area.

Growers should make applications for downy mildew control on a regular basis. Rotate fungicides with different chemical groups. See the table below for products registered on dry bulb onions for downy mildew management. Refer to labels for Re-Entry Interval Restrictions, Rotational Crop Restrictions and labelled directions for use.  This table is provided for general information only.

Product name


active ingredient

Chemical Group

(FRAC #)

Rate Pre-harvest Interval Notes
Manzate Pro-Stick



Group M3 0.9-1.3 kg/ac 10 days  
Ridomil Gold MZ 68WG



Group 4/M3 1 kg/ac 7 days Apply before first outbreak of disease.
Pristine WG



Group 11/7 0.4-0.5 kg/ac 7 days Suppression only at high rates.
Cabrio EG



Group 11 226-340 g/ac 7 days  
Quadris Top



Group 11/3 287-404 mL/ac 7 days  



Group 2 303 g/ac 15 days As per labels, tank-mix with recommended rates of Manzate Pro-Stick for downy mildew control. Apply this tank-mix as a preventative treatment. Follow most restrictive label.
Aliette WDG



Group 33 1.1 kg/ac 7 days Aliette should not be tank-mixed with other products, particularly micronutrients.



Group 40 161 mL/ac 7 days The use of a non-ionic adjuvant (0.2% v/v) is recommended.



Group 45/40 0.4 L/ac 0 days Downy mildew control. The addition of a spreading/penetrating adjuvant is recommended to improve performance.
Acrobat 50 WP



Group 40 182 g/ac 0 days Suppression only. Must be tank-mixed with another fungicide with activity against downy mildew.
Serenade Max


Bacillus subtilis

Group 44 1.2-2.4 kg/ac 0 days Suppression only.

For more information see: Ontario CropIPM – Onion Downy Mildew


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