Vegetable bean anthracnose
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Bean anthracnose is a significant fungal disease that affects kidney beans and is widely distributed across various regions. It also infects other legumes such as broad beans, peas, mung beans, lentils, and cowpeas. This disease can cause severe yield losses and reduce the quality of the harvested crops.
Symptoms of bean anthracnose appear on both seedlings and mature plants. In young seedlings, reddish-brown to dark brown circular spots develop on the cotyledons, often slightly sunken. As the plant matures, lesions appear on the leaves, typically along the veins. These lesions resemble rust-like streaks and may spread in a polygonal pattern. The color ranges from light to dark brown, and the affected areas may become necrotic. When stems or petioles are infected, elongated brown stripes form, sometimes cracking and becoming hollow. On soybean pods, initial symptoms include brown spots that gradually enlarge into nearly circular lesions with raised edges and sunken centers. In severe cases, the infection can reach the seeds, and when wet, a red, sticky exudate may be observed.
The pathogen responsible for bean anthracnose is *Colletotrichum lindemuthianum*, a fungus belonging to the Deuteromycotina division. The fungus produces spiny conidiophores and black, spiny structures called setae. Conidia are oval-shaped and germinate under favorable conditions. The optimal temperature for its growth is between 21–23°C, and it is killed at 45°C after 10 minutes of exposure.
The disease cycle involves overwintering of mycelium in infected seeds or plant debris. Infected seeds can lead to early infections in seedlings. Spores are spread through rain, wind, and direct contact. The disease thrives in warm and humid conditions, particularly when temperatures exceed 20°C and relative humidity is above 95%. However, when temperatures rise above 27°C and humidity drops below 90%, fungal growth is significantly reduced. Factors such as low-lying fields, heavy soils, excessive planting density, frequent rainfall, morning dew, and foggy weather contribute to the spread of the disease.
To manage bean anthracnose, several preventive measures are recommended:
1. **Cultural practices**: Use disease-free seeds and select resistant varieties. Implement crop rotation for 2–3 years to break the disease cycle.
2. **Chemical control**: Apply fungicides such as 50% carbendazim WP diluted 500 times, 70% thiophanate-methyl wettable powder at 1000 times, 65% zineb WP at 500–600 times, 70% mancozeb wettable powder at 400 times, 80% mancozeb wettable powder at 600–800 times, 10% Trifloxystrobin water-dispersible granules at 1000–1200 times, 75% chlorothalonil wettable powder at 600 times, 40% kresoxim-methyl suspension concentrate at 600 times, 25% propiconazole emulsifiable concentrate at 1000–1500 times, 80% iprodione wettable powder at 600–800 times, 62% pyraclostrobin wettable powder at 600–800 times, and 80% fludioxonil wettable powder at 800 times.
By combining cultural and chemical methods, farmers can effectively reduce the impact of bean anthracnose and protect their crops from significant damage.