By Charei Munene
Bean anthracnose is a seed-borne disease caused by the fungus Colletotrichum lindemuthianum. This disease causes severe losses to bean farmers in Kenya. The fungus infects leaves, stems, and pods of common bean plants, especially in the cold season, often killing the bean plant at a young age. This disease reduces yields by up to 100 percent in some cases.
Life Cycle of the disease:
Bean anthracnose survives between crops in infected crop residue and can be spread in seeds, air, and water. Infected points (which appear as characteristic sunken spots) begin producing spores 2-4 days after infection. New plant infections are favored by temperatures of 13-26º C and relative humidity greater than 92 percent (or free moisture). Frequent, moderate rainfall, particularly if accompanied by winds or splashing rain favours disease development.
Symptoms and Signs
The most characteristic symptom of the disease is the black-red sunken spots that develop on infected pods.
On seedlings grown from infected seed, symptoms start as small dark brown to black spots on the seed leaves. Light brown-colored specks appear and enlarge along, and sometimes around, the stem. If the spots grow completely around the stem, the seedling is killed. The spots become sunken and are filled with a characteristic ooze which consists of millions of spores.
On established plants, the veins on the underside of the leaves and the petiole can become infected with brick-red to purple spots that soon turn dark-brown to black. Spots may sometimes occur on the upper sides of leaves.
On the pods, symptoms begin with small light brown colored spots. The spots progress to black-red sunken lesions surrounded by a slightly raised black ring with cinnamon-colored border. In the center of the spots, there is often a tan colored ooze, which consists of millions of spores. Often the ooze dries to gray-brown to black granules. These are still very infective. The pods may then shrivel and die if infection is severe.
Figure 1: Pods displaying bean anthracnose symptoms
On the seeds, large or small yellowish-brown sunken spots may form, which turn brown to black.
Figure 2: Seeds displaying bean anthracnose symptoms
How do you prevent anthracnose in beans?
- Always allow beans to dry completely before touching or bean planting.
- Plant cultivars of bean that are resistant to anthracnose.
- Always clean the tools you’re using in the bean farm before using them elsewhere.
- Do not plant beans for at least 2 years in land that has carried an infected crop. Rotate with non-host crops such as maize.
- Uproot diseased plants to reduce the spread of the disease.
- Avoid cultivating and harvesting an affected crop when wet, to prevent the spread of spores.
- Use furrow rather than sprinkler irrigation because of the importance of water for disease development.
- Do not pack lightly diseased pods as anthracnose can develop during transport.
How to control bean anthracnose?
- Do not compost infected leaves, fruit or stems, incorporate clean crop debris into soil after harvest to hasten decomposition.
- To avoid spreading the disease, keep out of farm when plants are wet and make sure to disinfect all garden tools.
- Neem oil can help prevent this fungal disease from developing on the surface of leaves or stems. Neem oil is an organic, multi-purpose fungicide/insecticide/miticide that kills eggs, larvae and adult stages of insects as well as prevents fungal attack on plants. Apply early, at the first sign of budding, every 7-14 days as a preventative measure or on a 7-day schedule until existing problem is eliminated. Neem oil also reduces the population of aphids and other pests which may carry spores to your plants.
- Another good product that can help prevent/control anthracnose disease is Serenade Garden. This product is an organic fungicide that uses Bacillus subtilis to kill off fungal growth. It’s quite effective against most strains of anthracnose. For best results, treat prior to disease development or at the first sign of disease infection. Repeat at 7-day intervals or as needed.