Among the hardiest of beans, broad beans are reliable, popular and easy to grow. However, they can be attacked by fungal diseases, some commonly occurring such as rust and chocolate spot. To a certain extent these can be controlled by cultural techniques
Hosts
Broad beans (Vicia faba) are attacked by two important fungal foliage diseases: rust (Uromyces viciae-fabae) (left) and chocolate spot (Botrytis fabae) (right). The rust has several races, which collectively infect a wide range of legumes, but the races affecting broad beans also attack peas (on which the disease is rare in the UK) and some other species of Vicia. Chocolate spot occurs only on Vicia species. Broad beans are also attacked by some less important fungal problems and viruses, but these are seldom problems for gardeners.
Symptoms
The spores of the rust that are most obvious to gardeners are the uredospores. These brown, dusty spores are produced in pustules on both sides of the leaves during summer. Severely attacked leaves shrivel prematurely. The rust may also attack the stems and pods.
The chocolate spot pathogen causes small, circular brown spots of dead tissue on the leaves. As long as the weather remains dry these do not enlarge, but under wet conditions the fungus enters an aggressive phase, the spots enlarge rapidly and the leaf collapses.
Biology
The rust produces resting spores late in the season on stems, carrying the fungus through winter. Early in the season these lead to additional spore stages uncommon on broad beans, so it is only when the fungus begins to produce uredospores that the disease builds in severity. The fungus can only live in association with living host tissues, or as the dormant resting stage.
The chocolate spot fungus overwinters in plant debris or soil as a thick-walled resting structure (sclerotium). It produces airborne spores that infect leaves, causing the typical circular spots. The pathogen is similar to Botrytis cinerea (grey mould), which also causes leaf spots on broad bean, but is less severe: the severity of B. fabae is due to its greater ability to break down defence substances (phytoalexins) produced by the plant to fend off attack.
Control
There are no fungicides available to home gardeners to control these diseases. Rust on broad beans often does not reach high levels until late in the season, and so does not usually affect yield severely.
Destroy all plant debris at the end of the season by burning or composting.
Avoid overfeeding and crowded, humid sites.