Why Are There Grey Patches on My Cabbage Leaves?

Yellow or pale patches on the upper surface of cabbage leaves, corresponding to a grey, purple-grey, or white fuzzy or downy growth on the underside — particularly visible in cool, wet weather — is downy mildew, caused by Peronospora brassicae (and related Peronospora species). Downy mildew is an oomycete pathogen — related to the potato blight pathogen — that thrives in cool, humid, wet conditions and is a very common problem on brassicas in spring and autumn when mornings are dewy and temperatures are low.

What the disease looks like

The characteristic symptom on cabbage is angular, pale yellow lesions on the upper surface of the outer leaves — the angularity is caused by the lesions being bounded by the leaf veins. On the underside of these lesions, a grey-white to purple-grey fuzzy growth (the sporangiophore masses of the pathogen) is visible, particularly in the morning before leaves dry. Young seedlings can be severely damaged by downy mildew; established plants usually sustain cosmetic damage to outer leaves without significant impact on head quality.

Conditions that favour the disease

Peronospora brassicae needs free moisture on leaf surfaces for spore germination and infection — dew, rain, or overhead watering that leaves leaves wet overnight are the key enabling conditions. Temperatures between 8–15°C are optimal for the pathogen. Cool, moist autumn conditions and dewy spring mornings are the peak infection periods. Dense plantings with restricted air movement increase the duration of surface moisture and infection risk.

Management

Remove affected outer leaves and dispose of them. Improve spacing between plants to increase air circulation. Water at the base of plants in the morning rather than using overhead irrigation, and avoid watering in the evening. Copper-based fungicides (e.g., copper oxychloride) provide some control applied preventively but are not a substitute for improved growing conditions. Resistant varieties are not widely available for cabbage, but robust varieties grown in well-drained, open sites with good airflow are less susceptible.