Increased Risk of Downy Mildew on Onions

Marion Paibomesai, Vegetable Crops Specialist, OMAFRA
Michael Celetti, Plant Pathology Lead – Horticulture, OMAFRA

In Ontario, this disease is common in July and August and if left uncontrolled, quality and quantity of yield may be reduced. 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).

Figure 1. Foliar symptoms first appear as light green patches on the leaves.
Figure 1. Foliar symptoms first appear as light green patches on the leaves.

A purple-grey downy growth often appears in the mornings after a period of wet weather or when conditions favour dew formation (Figure 2). 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 impact yield (Figure 3).

Figure 2. A purple-grey downy growth often appears in the lesions after a period of wet weather or when conditions favour dew formation.
Figure 2. A purple-grey downy growth often appears in the lesions after a period of wet weather or when conditions favour dew formation.

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 is moderate to high in seeded onions and high in transplanted onions, depending on canopy size of crop and weather forecast (provided by the Muck Crops Research Station Agriphone for July 16th, 2015 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 relatively cool, humid conditions.

Figure 3. Purple blotch often invades downy mildew lesions.
Figure 3. Purple blotch often invades downy mildew lesions.

Successful disease management programs shold 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 on onions.

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