Downy mildew on onions — symptoms and treatment

Severity: high

What is Downy mildew

Onion downy mildew is caused by Peronospora destructor, and it thrives in cool, humid conditions. It attacks the leaves, and since the leaves build the bulb, foliage lost means bulbs that never size up. Severity is rated high, and the reason extends past the field: infection carries through to bulb rot in storage, so a crop that looked acceptable at harvest can break down in the store later. That makes it more expensive than the leaf damage alone suggests.

Symptoms

The first sign is subtle — pale oval spots on the leaves, elongated along the leaf, easy to walk past. What confirms it is what grows on them in humid weather: a grayish-purple fuzzy sporulation, a velvety bloom over the pale patches, clearest in the morning while the dew is still on the crop. That violet-grey fuzz is the diagnosis. Once the spots establish, the leaves collapse from the tips downward, folding over and dying back. Then comes the part you find later: bulb rot in storage, from infection that moved into the neck.

Key signs:

• Early: pale oval spots on the leaves
• Early: grayish-purple fuzzy sporulation on the spots, clearest in morning dew
• Advanced: leaves collapsing from the tips downward
• Advanced: bulb rot appearing in storage

Botrytis leaf blight looks different: small white oval lesions with light green halos and no fuzz. If there is grayish-purple fuzz, it is downy mildew.

Causes and conditions

Peronospora destructor needs cool, humid conditions and free water on the leaf to infect. Long dew periods and mild damp weather are ideal — it sporulates overnight in high humidity and the spores blow to fresh leaves on the wind. This is why it can appear across a whole planting at once rather than spreading from one plant. Overhead irrigation, dense stands and still air all lengthen leaf wetness. Between crops it survives in onion debris, volunteers and infected sets — which is why a rotation that leaves cull piles standing does not work.

Treatment

Rotation and airflow first — they decide how much pressure the sprays must hold.

Crop rotation and spacing — cultural

Pre-planting and throughout. Rotate with non-allium crops for 3+ years. Ensure wide spacing for air circulation. Avoid overhead irrigation. The pathogen is specialised to alliums, so a 3-year-plus rotation starves it out — but only if volunteers and cull piles go too, since those keep it alive through any rotation. Wide spacing and base irrigation cut the wetness spores need.

Metalaxyl + Mancozeb — chemical

When conditions favor disease. Apply preventive fungicide sprays when conditions favor disease. Repeat every 7-10 days. Preventive is the whole point: these sprays protect healthy leaf, and cool humid weather with long dews is the signal to start — not the fuzz, which means infection already happened. Onion leaves are upright and waxy and shed spray, so coverage needs care. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rotate with non-allium crops for 3 or more years, and remove volunteers and cull piles.
• Plant clean sets and bulbs — infected planting material introduces the pathogen directly.
• Space plants widely so air moves through and leaves dry after dew.
• Irrigate at the base rather than overhead.
• Cure and store bulbs properly and inspect the store — this disease shows up there.

Frequently asked questions

Is it contagious to my garlic and leeks? Yes — it is an allium pathogen and the spores are wind-borne, so nearby alliums are at risk. That is why the rotation must be to non-allium crops.

Can I store onions from an infected crop? Be careful. Infection carries into the bulb and shows as rot in storage, so a crop hit by downy mildew is a poor candidate for long storage — use it early.

When should I spray? When conditions favour the disease — cool, humid weather with long dews — rather than waiting for symptoms. The grayish-purple fuzz means infection already took hold; a preventive spray protects leaves that are still clean.