Broad bean rust is one of the most common fungal diseases of broad bean leaves. The fungus causes small, dusty, dark brown spots surrounded by a pale yellow halo on leaves and stems.
What is broad bean rust?
Broad bean rust is caused by the fungus Uromyces viciae-fabae var. viciae-fabae. It is not as damaging as another common broad bean disease, chocolate spot, but severe attacks can cause defoliation. Expect to see significant levels of this rust from mid-summer.
It attacks broad beans, peas and other related legumes and has several races, but in the UK is only important on broad beans. Runner and French beans are affected by a more serious, but unrelated rust disease.
You may see the following symptoms:
- On leaves and stems: Small, dusty, dark brown spots, often surrounded by a pale yellow halo, appear
- Maximise air flow around the plants by wider spacing and avoid damp, humid sites
- Destroy infected plant material because spores produced at the end of the season can survive in a semi-dormant state in crop residues and on seed
- Avoid growth of other hosts, such as vetch and sweet pea
There are no effective fungicides available to gardeners for rust control on broad beans.
Uromyces viciae-fabae var. viciae-fabae is a typical example of a rust fungus that carries out its entire life cycle on one host, which, in the UK, is the broad bean. Unlike chocolate spot, the infected leaf tissues remain alive, providing nutrients for the fungus while it produces dusty brown airborne dispersal spores. A semi-dormant resting spore carries it over the winter.
Another fungal pathogen, Didymella fabae (syn. Ascochyta fabae), causes spots on leaves and pods, but is usually less damaging than broad bean rust.