Broad Bean Chocolate Spot

Broad Bean Chocolate Spot

Broad bean chocolate spot is a fungal disease that thrives in damp and overcrowded conditions. Spores can overwinter on infected plants and plant debris. The chocolate spot disease is more likely to be a problem where soil is short on potassium.

Susceptible Plants- broad beans and field beans

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 broad bean 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.

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Life Cycle- 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 (gray mold), 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 defense substances (phytoalexins) produced by the plant to fend off attack.

Prevention and Control- Provide ample spacing between plants. Grow in well-drained soil. Avoid summer sowing if this disease is a regular problem. Spring sown beans are more likely to recover than plants affected later in the season. Improve potassium levels if low. Destroy all plant debris at the end of the season by burning or composting.

Fungicidal Control- There are no fungicides available to home gardeners to control broad bean chocolate spot. Rust on broad beans often does not reach high levels until late in the season, and so does not usually affect yield severely.

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