Grey Furry Coating on My Leek Leaves

A grey, lilac-grey, or purple-grey fuzzy or furry coating on the surface of leek leaves — particularly visible on the upper surface but sometimes present on both — is downy mildew, caused by the oomycete pathogen Peronospora destructor (the same organism that causes downy mildew on onions). It is distinct from powdery mildew (which forms white dry patches) and from rust (which forms raised orange pustules). Downy mildew thrives in cool, humid, still conditions and is most common in spring and autumn when these conditions are most frequent.

How to confirm downy mildew

The grey growth on the leaf surface is the sporulating mycelium of the pathogen. Below the grey coating, the leaf tissue is pale green or yellowish, and as the infection advances the affected area turns brown and dies. In severe infections, the grey growth spreads across large areas of the leaf and the plant's photosynthetic capacity is significantly reduced. Young plants may be badly stunted or killed by systemic infection. Older plants typically tolerate moderate downy mildew on the outer leaves while the inner shank remains unaffected.

Conditions that favour it

Downy mildew spores germinate on leaf surfaces when temperatures are cool (below 15°C) and relative humidity is high, particularly when leaves remain wet for extended periods. Dense plantings with poor air circulation between plants create the ideal microclimate. Overhead watering in the evening leaves the foliage wet overnight — the most conducive condition for spore germination. Improving air circulation by widening spacing and avoiding evening overhead watering significantly reduces downy mildew incidence.

Management

Remove heavily affected leaves and dispose off-site. Do not compost them, as spores can survive in compost that does not reach high temperatures. For actively infected plants, a copper-based spray (copper hydroxide or copper oxychloride) applied at the first sign of infection can limit further spread — copper compounds are approved for use in UK organic growing. Remove all crop debris at the end of the season and rotate leeks to a fresh bed the following year to reduce the overwintering spore population.

Effect on the edible leek

Downy mildew primarily affects the outer leaves. The inner shank and tight leaf sheaths are typically clean and unaffected. A leek with downy mildew on its outer leaves can be harvested and used normally — strip the outer leaves, and the inner plant is fine. Only plants with systemic infection (very young plants, usually) are so badly affected that the shank itself is compromised.