(1) Symptoms: Both the leaves and stems of broad bean plants can be affected by this disease. Initially, the lesions on the leaves appear as pale green spots, round or oval in shape. Over time, these spots turn brown and develop a rough texture. The lesions resemble small tumors, typically 2 to 5 mm in diameter and 1 to 3 mm in height. The back of the lesion is sunken, and each leaf may have between 1 to 20 such lesions. When multiple lesions occur on a single leaf, they may merge together, causing more severe symptoms such as pronounced bulges and depressions, which lead to leaf curling and deformation. In the later stages of the disease, the affected areas turn reddish-brown. Infected leaves often show poor growth, yellowing, or even early withering. Similar symptoms are observed on the stems, primarily in the middle and lower sections. Affected stems may twist, become stunted, and result in fewer effective pods, smaller pod sizes, and reduced bean yield.
(2) Disease Development: This is a fungal disease caused by a pathogen that survives in infected plant tissues as dormant sporangia. These sporangia serve as the primary source of infection during the next growing season. Once the dormant spores germinate into zoospores, they can infect young roots and stems of broad bean plants. As the seedlings grow, the pathogen spreads downward with the sprouts, becoming the initial source of infection in the field. The disease spreads through rain splash and direct contact between diseased and healthy plants. Wind can also aid in the spread of the pathogen. The disease tends to be more severe in shaded slopes, low-lying areas, and during rainy and humid seasons.
(3) Control Measures:
1. Seed treatment with 0.2% triadimefon to prevent early infection.
2. When the first signs of the disease appear in the field, apply a fungicide such as 25% carbendazim diluted at 600 times, or 300 times the concentration of mancozeb (a copper-based fungicide). Treatments should be repeated every 7 to 10 days, and up to three applications may be necessary depending on the severity of the outbreak. Regular monitoring and timely intervention are essential to effectively manage the disease and reduce losses.
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