Why Is There Grey Mould on the Underside of My Beetroot Leaves?

A grey to purple-grey, powdery or fuzzy coating on the underside of beetroot leaves — with corresponding pale yellow or bleached patches on the upper surface above each affected area — is beetroot downy mildew, caused by Peronospora farinosa f. sp. betae. This is a water mould (oomycete) rather than a true fungus, and it behaves differently from powdery mildew: it requires cool, wet, humid conditions to sporulate and spread, and is most damaging in wet spring and early summer weather on young plants.

How to confirm downy mildew

Turn a leaf over — the grey, powdery or soft furry growth of sporangiophores on the underside is the key diagnostic feature. The corresponding upper-surface symptom is yellow to pale-brown, irregular patches (not well-defined spots with clear margins, as in Cercospora). Young, central leaves are most commonly affected first. In severe early-season infections, the growing centre of the plant may be completely taken over, stunting or killing young plants. Older plants with established root systems tolerate the same infection with much less overall impact.

Why spacing and airflow matter

Downy mildew requires moisture — free water or high humidity — to release and germinate its spores. Dense, closely packed plantings where leaves overlap and the inner canopy stays wet after rain or dew create ideal conditions. Thinning to the correct spacing (10–15 cm between plants) so that air circulates freely reduces the humidity within the crop and slows spread significantly. Avoid watering in the evening, which leaves foliage wet overnight in cool conditions.

Management

Remove and bin (do not compost) affected leaves as soon as they are noticed. There are no fungicides approved for downy mildew on home-grown beetroot. Improve airflow through thinning. In very wet seasons on susceptible varieties, some defoliation may be unavoidable — check root development and harvest promptly if the plants seem to be deteriorating. The roots from downy-mildew-affected plants are entirely safe to eat as long as the roots themselves appear normal.