Organic farming scorch spot disease of garden bean colletotrichum lindemuthianum

Burn spot causes roundish brown spots with dark margins on leaves (left) and pods (center) and leads to seedling damage (right). Photos: E. Nega, JKI

Damage to beans

Photo: JKI

The disease is manifested by roundish brown leaf spots with dark margins and light interior. They can already appear on Cotyledons occur, the young plants then often die. on parts of the plant. Usually originate from the leaf veins. They are usually sunken and grow from a width of one to 5 – 10 millimeters in diameter. Shaded rings may form inside them. As infestation increases, the spots enlarge, their shape becomes more irregular, and they fuse and cover larger leaf areas. Small spherical, brown to black spore containers form on the infested areas, which release reddish spore slime under moist conditions.

Infestation of the Pods leads to similar spots that penetrate the pod wall and cause fruit set and developed Seeds can damage. At Stems Initially, smaller oval spots appear, which spread along the stem during the infection. The plants may die above the infected area.

Burn spot disease also occurs on pea (Ascochyta pisi), field bean and vetch (Ascochyta fabae) with similar symptoms, as well as on lentil (Ascochyta lentis) and chickpea (Ascochyta rabiei). However, these are different species that are specialized on the respective host plants.

Harmful effect in vegetable growing

Bean scorch disease particularly affects bush and pole beans and fire beans, but can also occur on rarer species such as lima bean, mung bean, black-eyed pea, helmet bean and pigeon pea. It occurs especially in humid summer weather conditions. Infested beans are unusable for green harvest; dry bean production is rarely affected. The greatest damage occurs when infection occurs at the seedling stage in cool weather, it can lead to the death of the plant. Yield losses can also be caused by breaking stems. storage of the plants occur. Infected beans are no longer usable as seed, but they are not always identifiable by visible burn spots.

Biology of the fungal disease

Burn spot disease is mainly transmitted by seed. Depending on the climatic conditions of the crop year, from minimal amounts to one-third of all seed samples may be infested. Infectivity is reduced by half in one year with seed storage. can survive for years. The fungus remains viable on plant residues for several months. Can reproduce by fertilization with a particular form of spore. On plant residues, the fungus remains viable for several months. Can reproduce by pollination with a special form of spores. In addition, it is suspected that the fungus can also temporarily persist on the residues of non-host plants.

In wet weather, reddish spore slime forms on infested plants and is transferred to healthy plants by rain splash or cultural operations. This Transfer but only occurs over short distances of about ten meters. Healthy neighboring stands are rarely infected, so an infection spread further in the field indicates infected seed. Spores germinate under mild spring temperatures and high humidity and at least four hours of leaf moisture at 20°C, or ten hours of leaf moisture at 10°C. After the first spots appear, the spores produced in them are spread further into the plant and crop by raindrops.

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