onion downy mildew

Symptoms

Pale green elongate lesions on leaves with violet-grey downy growth in humid mornings, leaves collapse + bend at lesion, infected bulbs soft + watery in storage.

Plants affected

• onion

Easily confused with

• onion leaf blight

  How to tell them apart: Botrytis squamosa (Botrytis leaf blight) first appears as numerous tiny (1-3 mm) whitish or straw-coloured oval spots, often surrounded by a pale silvery or water-soaked halo, scattered over the upper leaf surfaces; these coalesce and the leaf tips die back and bleach. Under prolonged wetness a fine grey-brown fuzzy mould (the diagnostic Botrytis sporulation) develops on dead tissue. Peronospora destructor (downy mildew) instead produces large, pale-green to yellow elongated patches running lengthwise along the leaf, and on those patches in cool humid mornings a soft violet-grey to greyish furry down forms on the leaf surface, after which the patch turns yellow-brown and the leaf collapses and bends. Key tell: Botrytis = many small discrete halo-ringed flecks plus grey mould on dead tissue; Peronospora = few large lengthwise patches with violet/grey velvety down on still-living tissue.

• pear brown spot

  How to tell them apart: Stemphylium vesicarium (Stemphylium leaf blight) starts as small whitish-to-yellow spots, usually on the side of the leaf facing the wind or after hail/insect injury, that elongate into tan-to-brown spindle-shaped necrotic lesions; in humid weather these turn olive-brown to black and velvety as the fungus sporulates densely, and the leaf tip dies back. Peronospora destructor (onion downy mildew) produces longer, paler, diffuse yellow-green oval patches with indistinct margins, and its diagnostic sign is a fuzzy grey-to-violet/purplish downy growth on the lesion surface, most visible in early morning dew or fog; whole leaves bend, yellow and collapse from the tip. Key tell: a velvety BLACK fuzz means Stemphylium, while a GREY-VIOLET fuzz means downy mildew. Because Stemphylium often invades old downy-mildew lesions, look at the freshest leading edge of a lesion, not the dead centre, to identify the primary pathogen.

Common questions

How do I recognise onion downy mildew on onion?

Pale green elongate lesions on leaves with violet-grey downy growth in humid mornings, leaves collapse + bend at lesion, infected bulbs soft + watery in storage.

Do I have to report onion downy mildew?

No. onion downy mildew is not a notifiable organism, so there is nothing to report to an authority. Treat it as a normal garden problem.

How is onion downy mildew treated?

Mainly with biological and cultural measures — removing affected material, changing the conditions that favour it, and treating with agents approved in your country. The exact steps and their timing depend on the plant, the season and where you live: Dr. Phyto works them out in a free diagnosis.