Why Do My Rhubarb Leaves Have Brown or Black Spots?

Spots on rhubarb leaves are common, and in most cases are not a serious problem. The leaves of rhubarb are naturally not eaten — only the stalks are used in the kitchen — so a spotted leaf canopy does not directly affect the harvest. However, understanding the cause can tell you whether any action is warranted, and some causes do affect the plant's vigour even if the leaf symptoms look similar.

Ramularia leaf spot

The most common cause of rhubarb leaf spotting is Ramularia rhei — a fungal disease that produces pale brown or cream spots with a darker border, often with a white or greyish centre as the lesion ages. It appears most commonly in summer, particularly after warm wet periods. It is rarely damaging enough to merit treatment — affected leaves still function photosynthetically except where lesions are very dense. Removing the most badly affected leaves and improving air circulation by not overcrowding plants is usually sufficient management.

Bacterial leaf spot

Angular, water-soaked dark spots that do not have the pale centre of Ramularia, and that spread rapidly in wet conditions, may be bacterial leaf spot. This is more aggressive but still primarily affects the leaves rather than the crown or stalks. Remove affected leaves, avoid overhead watering, and ensure plant spacing allows adequate airflow.

Frost damage

Newly emerged rhubarb leaves caught by a late frost in spring develop brown, translucent patches that look alarming but are simply dead cells. The leaf does not recover, but the crown itself is usually undamaged and will produce new growth. Remove frost-damaged leaves and wait for new unaffected growth to develop.