Chocolate Spot in Faba Beans: Identification and Management

Chocolate Spot, caused by the fungi Botrytis fabae and Botrytis cinerea, is a significant disease affecting faba bean (Vicia faba) crops. This disease can lead to substantial yield losses and reduced seed quality if not properly managed.

Understanding Chocolate Spot

Chocolate Spot manifests as small, dark brown lesions on the leaves, stems, and flowers of faba bean plants. Under conducive conditions, these spots can enlarge, coalesce, and lead to extensive tissue damage. Severe infections may result in defoliation, flower abortion, and stem collapse, severely impacting plant health and productivity.

Disease Cycle

The pathogens responsible for Chocolate Spot can survive between growing seasons as sclerotia in soil or crop debris, and can also be seed-borne. Infection typically begins when spores from these sources are dispersed by wind or rain to new plants. Once established, the disease can spread rapidly within a crop, especially under cool (15–22°C) and humid conditions, with high relative humidity (above 90%) favouring disease development.

Impact on Crop Yield and Quality

Chocolate Spot can cause yield reductions ranging from minor losses to complete crop failure, depending on the severity and timing of infection. Early infections, particularly during the flowering stage, can lead to significant flower drop and reduced pod set. Additionally, infected seeds may exhibit discoloration, diminishing their market value and suitability for planting.

Management Strategies

Effective management of Chocolate Spot involves an integrated approach:

• Resistant Varieties: Selecting faba bean cultivars with resistance to Chocolate Spot can reduce disease incidence.
• Crop Rotation: Implementing a rotation system with non-host crops for at least four years can help decrease pathogen levels in the soil.
• Seed Health: Using certified disease-free seed and treating seed with appropriate fungicides can prevent initial infections.
• Field Hygiene: Removing and destroying crop residues post-harvest reduces the survival of the pathogens.
• Fungicide Application: Applying foliar fungicides at the first sign of disease, especially during flowering, can help control the spread. Follow-up applications may be necessary under persistent conducive conditions. Chitosan sold by Eutrema will help dry off any spores and activate the plants defences against the pathogen.
• Potassium Bicarbonate is used by some farmers against chocolate spot.

Conclusion

Chocolate Spot is a pervasive disease that poses a significant threat to faba bean production. Understanding its life cycle and implementing comprehensive management practices are essential for minimizing its impact. Regular monitoring, combined with cultural and chemical control measures, can effectively manage the disease and safeguard crop yields.

Article by Dr Russell Sharp

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