Why Do My Onion Leaves Have Grey Furry Mould?

A grey or violet-grey furry coating on onion leaves — particularly on the outer surface in humid weather, sometimes accompanied by yellowing and collapse of the affected leaf — is downy mildew, caused by Peronospora destructor. Unlike rust (which produces orange pustules), downy mildew creates a soft, velvety grey sporulation that appears when conditions are cool and persistently humid or wet. It is one of the most significant onion diseases in cool, damp climates and can cause severe yield losses in a wet season, so recognising and responding to it quickly is important.

How downy mildew develops

The disease cycle begins with overwintering oospores in the soil or infected plant debris, or from airborne spores arriving from neighbouring crops. Infection requires cool temperatures (10–15°C) and high humidity — cloudy, damp spring and early summer weather creates ideal conditions. The pathogen invades the leaf internally before producing the external grey sporulation. Infected leaves yellow and die from the tip downward. The fungus can also move systemically into the bulb, causing the bulb scales to become infected and the plant to be stunted and pale from emergence. This systemic form — called "primary infection" — comes from infected sets or oospores in the soil.

Managing an active infection

Remove and destroy all leaves showing grey sporulation immediately. Do not compost them — remove them from the site entirely. Improve airflow around plants by thinning overcrowded rows and avoiding weed competition. Stop all overhead watering and water only at the base of plants. Apply a copper-based fungicide (Bordeaux mixture or copper oxychloride) to unaffected foliage as a preventive spray — copper does not cure existing infection but can protect healthy tissue from new spore landings. Reapply after rain. In a wet season with persistent downy mildew pressure, covering with a cloche or low polytunnel to keep rain off the foliage can dramatically reduce infection.

Rotation and soil management

The oospores that cause primary infections in young onion plants persist in soil for many years. A rigorous three to four year rotation of allium crops significantly reduces the build-up of soil-borne inoculum. Remove and destroy all onion crop debris at the end of each season — do not leave dead foliage or spent bulbs in the ground. Using certified, disease-free sets rather than home-saved sets reduces the chance of planting already-infected material. In gardens with a consistent downy mildew problem, growing onions from seed started under cover avoids the worst of the early outdoor conditions and produces plants that are well established before the most susceptible growth stages.

Impact on harvest

A severe downy mildew infection that causes widespread leaf collapse before bulbs have fully sized can noticeably reduce yield. However, bulbs from mildew-affected plants are safe to eat if harvested promptly and sound. Do not store bulbs from heavily infected plants alongside healthy ones — the infection can progress into stored bulbs and cause them to deteriorate in storage. Use affected bulbs first.