(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, which are round or oval in shape. Over time, these spots turn brown and the surface becomes rough. 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. In severe cases, the small lesions can merge together, causing more pronounced swelling and depression on the leaves, leading to curling and distortion. In the later stages, the lesions turn reddish-brown, and the affected leaves may show poor growth or even wither prematurely. Similar symptoms are observed on the stems, primarily on the middle and lower parts. Infected stems often become twisted and stunted, which reduces the number of effective pods, the size of individual pods, and the overall yield of beans.
(2) Disease Onset Conditions: This is a fungal disease caused by a pathogen that survives in the form of dormant sporangia within infected plant tissues. These structures serve as the initial source of infection during the following growing season. Once the spores germinate into zoospores, they can infect young roots and stems of broad bean plants, spreading through the soil as the seedlings grow. This becomes the primary source of infection in the field. The disease spreads rapidly through rain splash and direct contact between diseased and healthy plants. Wind also plays a role in spreading the pathogen. The disease tends to be more severe in shaded slopes, low-lying areas, and during rainy and humid seasons.
(3) Control Methods:
1. Seed treatment with 0.2% triadimefon before sowing to prevent early infection.
2. When the first signs of disease appear in the field, apply a fungicide such as 25% carbendazim diluted at 600 times, or 300 times the concentration of a copper-based fungicide. Spraying should be done once every 7 to 10 days, and it may be necessary to repeat the application up to three times depending on the severity of the outbreak. It is important to monitor the crop regularly and take action promptly to minimize damage.
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