Why Do My Brussels Sprout Leaves Have Grey Mildew Underneath?

A grey or grey-purple furry mould on the underside of Brussels sprout leaves, accompanied by yellow patches on the upper surface of the leaves, is the characteristic symptom of downy mildew. This fungal-like disease (technically caused by an oomycete organism) is common on brassicas in cool, humid conditions, particularly in autumn when the weather is damp and the plants are maturing. It rarely kills established plants but can significantly weaken seedlings and reduces the appearance and quality of the harvest.

Identifying downy mildew

The diagnostic feature is the combination of yellowing on the upper leaf surface and a grey, downy, felt-like growth on the lower surface directly beneath the yellowed areas. The yellow patches are angular, bounded by leaf veins. In wet weather the lower surface mould can become quite pronounced. On seedlings, downy mildew can cause collapse and damping-off; on established plants it is more of a cosmetic and stress issue.

Conditions that encourage downy mildew

Downy mildew thrives in cool, moist conditions with poor airflow — the kind of weather typical of autumn in northern Europe. Overcrowded plants, excessive overhead watering, and persistently wet foliage all increase risk. Seedlings raised under cover in humid propagation conditions are also vulnerable. The disease spreads by spores that are released in wet conditions and land on wet leaf surfaces.

Managing downy mildew

Remove and dispose of heavily infected leaves. Improve airflow by thinning out any plants that are very close together — Brussels sprouts should be spaced at least 60 cm apart. Avoid watering overhead; water at the base of the plants instead. Do not compost infected material — bin it or burn it to prevent spores persisting in the garden. In severe cases, copper-based fungicides can be applied as preventative sprays, though these are less effective once infection is established.

Growing resistant varieties

Some Brussels sprout varieties have better resistance to downy mildew than others. Checking variety descriptions when choosing seeds or plants can help reduce the incidence of the disease, particularly in areas with reliably wet autumns. Crop rotation — not growing brassicas in the same ground for at least three years — reduces the buildup of disease spores in the soil.