Increased Risk of Downy Mildew in Onions

Marion Paibomesai, Vegetable Crops Specialist, OMAFRA

Michael Celetti, Plant Pathology Lead – Horticulture, OMAFRA

Onion downy mildew has been found on onions in Waterloo and York Regions (updated 18 Aug 2014).  In Ontario, this disease is common in July and August and if left uncontrolled, the size, quality and quantity of yield can be significantly reduced. The neck tissue of the affected onions may not ‘cure’ properly at harvest and is thus open to secondary fungal and bacterial pathogens which can further reduce yield quality. When cool, humid conditions are present particularly when the canopy of the crop is advanced, the risk of downy mildew increases. Foliar symptoms first appear as pale light green patches on infected leaves (Figure 1).

A purple-grey downy growth often appears in the mornings after a period of wet weather or when conditions favour dew formation (Figure 1). This purple-grey downy growth contains the sporangium and spores, which are blown or splashed to other onion plants. As the disease progresses, diseased leaves eventually turn pale, yellow and sometimes become bleached before they become necrotic brown and collapse. The lesions are often invaded by other pathogens such as purple blotch which may further reduce yield (Figure 2).

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 high, depending on canopy size of crop and weather forecast (provided by the Muck Crops Research Station Agriphone for August 5th, 8th, 14th and 18th 2014 available at http://www.uoguelph.ca/muckcrop/agriphone.html). DOWNCAST is not available in other areas of Ontario, thus protective fungicides should be applied during cool, humid conditions.

Successful disease management programs must be implemented prior to the appearance of the disease. Managing downy mildew involves the use of cultural practices integrated with fungicide applications. Crop rotation for 3 -4 years will reduce the over wintering oospore populations in soil but will not affect windblown spores entering from other fields. Several properly timed fungicide applications are required to control this disease in season. For best results, applications of a registered product should always be made prior to infection. Consult OMAFRA Publication 838, Vegetable Crop Protection Guide for the most up to date list of registered fungicides that can be applied to prevent and manage downy mildew.