Anthracnose and rust of garden beans
Anthracnose and rust are two common fungal diseases of garden beans. These diseases can cause considerable losses when environmental conditions are favorable, but the severity of disease is often minimized through cultural practices and host resistance. Identifying and differentiating between these two diseases can be relatively simple since they are caused by different fungi and produce different symptoms. Although these diseases are caused by different fungi, they are both controlled with similar management practices.
Fig 1: Anthracnose on bean pods.
Photo by M.A. Hansen VAPISU, bugwood.org
Fig 2: Anthracnose on bean leaf.
Photo by H.Schwartz, CSU, bugwood.org
Fig 3: Rust on bean pods.
Photo by H. Schwartz, CSU, bugwood.org
Fig 4: Rust on bean leaves.
Photo by Michelle Grabowski.
Anthracnose, caused by the fungus Colletotrichum lindemuthianum, primarily infects snap beans and dry beans such as pinto, kidney, and navy beans. Anthracnose survives winter in infected plant debris and produces spores in the spring that are rain splashed or wind blown to healthy plants. Anthracnose may also be introduced into the garden on infected seed.
Like many fungal diseases, anthracnose develops quickly during cool, wet conditions. Symptoms are often visible shortly after emergence, but are usually first noticed as small, reddish brown spots on the pods. These spots later enlarge, becoming brown to black and sunken. Occasionally, spots contain a pink jelly-like material which consists of spores. The leaves, petioles, stems, and seeds may all be infected. Leaf lesions are typically observed along the veins as brown lesions, while yellow to brown sunken lesions develop on infected seedpods.
Bean rust, caused by the fungus Uromyces appendiculatus, unlike anthracnose, is not seed-borne. The fungus survives winter in infected plant debris and produces spores in the spring that are wind blown, often for great distances, to healthy plants. Bean rust typically appears as reddish-brown, raised pustules on the bottom of leaves and on pods. These pustules are often surrounded by a yellow halo. Powdery, rust-colored spores, called urediniospores, are released from these pustules throughout the growing season. The spores are wind blown to healthy plants where they start new infections. Severe infections cause leaves to brown, dry, and drop from the plant. Later in the summer, black pustules called teliospores form, which are the overwintering stage of the fungus.
These diseases are best prevented by planting clean, disease-free seed. Gardeners who save their own seed should avoid planting seed from diseased plants, since anthracnose can survive on infected seed. Plant resistant cultivars whenever possible. Avoid working in the garden when plants are wet. Water at the base of the plants early in the day to allow the leaves to dry quickly. Plant in widely spaced rows that run parallel to the prevailing wind. Remove and dispose of all plant material at the end of the season. Do not place diseased plants in compost piles, since compost piles do not reach high enough temperature over the winter to kill fungal material. Practice crop rotation and avoid planting beans in the same location more than three years in a row. Remove infected portions of the plant or the entire plant from the garden as soon as disease is noticed. Fungicides are of limited effectiveness for anthracnose and generally not necessary.
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ReferencesHall, Robert, ed. 1991. Compendium of Bean Diseases. APS Press, St. Paul, MN.
Howard, R. J., J.A. Garland and W.L. Seaman, eds. 1994. Diseases and Pests of Vegetable Crops in Canada. Canadian Phytopathological Society and Entomological Society of Canada.
Sherf, A.F. and A.A. MacNab. 1986. Vegetable Diseases and their Control, 2nd edition. John Wiley & Sons. New York.
Chad Behrendt, Crystal Floyd