Why Do My Radish Leaves Have White Pustules Underneath?

Small, raised white or cream-coloured pustules on the underside of radish leaves — often with corresponding yellow or pale patches on the upper leaf surface above each pustule — are the hallmark of white rust or white blister (Albugo candida). This is a common disease of brassica family plants including radish, and while it rarely kills plants outright, a heavy infection causes significant leaf stress during the growing period.

Identifying white rust

The pustules are white and have a powdery appearance when fresh; as they mature they may turn beige or cream. They are typically clustered on the underside of the leaf in irregular groups, not in rows or rings. The upper surface shows pale, slightly raised yellow-green areas corresponding to each cluster below. In severe infections, the leaf surface becomes heavily distorted. White rust is a different disease from powdery mildew, which forms on the upper surface and has no pustules.

Conditions that favour white rust

White rust spreads via airborne spores that germinate in free water on leaf surfaces. Wet, humid, cool conditions — particularly autumn and early spring when overnight moisture is common — favour the disease. Overcrowded plantings where leaves overlap and air movement between plants is poor create the microclimate the pathogen needs.

Managing white rust in radish

Because radish matures so quickly, white rust that appears in the final week before harvest rarely affects root quality — harvest promptly and use the radishes immediately. If white rust appears in week one or two, it reduces photosynthesis during the critical root-development period. Remove the most badly affected leaves to reduce spore spread. Apply a copper-based fungicide if the infection is progressing rapidly and the crop is early in its development.

Cultural prevention

Thin radish seedlings to the correct spacing for good airflow. Avoid overhead watering, particularly in the evening. In autumn, where conditions are regularly wet, growing radish under row cover or a cloche can significantly reduce leaf wetness and disease pressure. Rotate away from brassicas in badly affected beds for one or two seasons.