Why Does My Rhubarb Have Orange Powdery Spots on the Leaves?

Orange, rust-brown or yellow powdery pustules on the undersides or upper surfaces of rhubarb leaves — often with corresponding yellow patches on the leaf surface above each pustule — are caused by rhubarb rust, a fungal disease caused by Phragmidium species. Rust is a common and visually striking disease, but in most cases it is more alarming in appearance than in actual impact on the plant.

Is rust harmful to the plant?

Mild rust infections on a well-established rhubarb crown are not seriously damaging. The infection reduces photosynthetic efficiency in affected leaf areas, but a large, vigorous plant can sustain moderate leaf damage without any noticeable impact on stalk production. A severe infection that covers a high proportion of the leaf surface area on all leaves simultaneously, late in the season, may reduce reserve accumulation slightly — but this is unusual.

How rust spreads

Rust spores are wind-borne and highly mobile. Infection is favoured by wet conditions and warm nights, typically in mid-to-late summer. Spores overwinter on plant debris, including old rhubarb leaves, stems and in the soil. Removing and disposing of (burning or binning — not composting) all plant debris at the end of the season reduces the overwintering spore load for the following year.

Management without fungicides

Remove the most severely affected leaves promptly and bin or burn them. Improve air circulation by ensuring crowns are not overcrowded. Keep weeds down around plants to reduce overall humidity at the base. A sulphur-based spray applied at the first sign of rust has moderate efficacy and is acceptable in organic growing. There is no resistant variety widely available for rhubarb rust.