Why Do My Blackberry Leaves Have Orange Rust Spots?

Orange or yellow pustules on the underside of blackberry leaves — and the corresponding yellow spots on the upper surface — are the classic symptoms of blackberry rust caused by the fungus Phragmidium violaceum. This rust disease is common in the UK in warm, humid summers and can cause significant defoliation of heavily affected plants, though it rarely kills plants outright. Knowing the disease cycle tells you when and how to intervene most effectively.

Identifying blackberry rust

On the upper leaf surface, yellow to orange spots appear — slightly angular or irregular in outline. On the corresponding underside, orange-yellow pustules (spore-bearing structures) erupt through the leaf surface, releasing orange powder when touched. Later in the season (September–October), the pustules change colour — going through a reddish-orange to eventually producing dark brown-black telial pustules on the undersurface. Severely affected leaves yellow and drop prematurely.

Management: infected leaf removal

Collect and remove all fallen leaves in autumn — they carry the overwintering spores that will reinfect the plant the following spring. Similarly, removing the most heavily infected leaves from the plant during summer reduces the spore load available to spread to uninfected tissue. Do not compost infected leaves — bag and dispose of them or burn them.

Improving air circulation

Rust spreads rapidly in humid conditions. Congested, tangled blackberry canes create the perfect microclimate — warm, still air trapped between densely packed foliage. Training canes on a well-spaced system (see the training guide) and reducing the total number of canes to a manageable count improves air circulation significantly. Well-aired plantings consistently show lower rust severity than untrained tangles.

Fungicide treatment

Where rust is severe and causing significant defoliation, myclobutanil-based fungicides (e.g. Systhane) applied at the first sign of infection and again two to three weeks later provide effective control. Organic-approved sulphur sprays have moderate efficacy against rust if applied preventively.