Bean angular leaf spot is caused by the fungus Phaeoisariopsis griseola, and is a serious disease of beans in many bean growing regions.
The disease develops rapidly in warm temperatures, about 25 degrees centigrade but can also occur over a range of moderate to warm temperatures of approximately 16-28 degrees centigrade, when accompanied by wet weather or high humidity alternating with dry, windy conditions.
The most diagnostic feature of Phaeoisariopsis griseola is the striking angularity of the spots.
Yield reduction attributed to infection by angular leaf spot is majorly as a result of reduction in photosynthetic area. However, lesions on pods greatly reduce quality of the produce.
Disease Cycle
Infection is solely through spores which are produced by the fungus on infested debris or seed. They are dispersed by wind or rain-splashed onto healthy tissue. When spores land on susceptible bean tissue, they germinate and initiate infections.
Identification & Symptoms
The disease infects leaves, petioles, stems and pods. However, symptoms are most recognizable on leaves.
On leaves, the lesions appear as brown spots with a tan or silvery centre which are initially confined to the leaf tissue between major veins, giving it an angular appearance. The lesions can also be seen on the underside of the leaf but are delimited by the veins and veinlets. They appear slightly paler than those on the upper surface.
Lesions may be surrounded by a chlorotic halo but do not have a coloured border. They may be so numerous leading to premature defoliation.
Stem lesions caused by Phaeoisariopsis griseola are usually elongate and brown in color. Compared to leaves, pods are less infected therefore the lesions are not very many. They are oval or circular and initially superficial with margins that are almost black and reddish-brown centres, which are sharply defined. The spots/lesions vary in size and coalesce covering the pod in cases of severe infection.
Management & Control
Considering the losses caused by infection with angular leaf spot, it is of high requirement to manage and/control this disease.
Several methods can be employed. However, the use of chemicals (fungicides) is the most preferred because it is highly reliable.
The following fungicides are effective against Angular leaf spot. They can be used to prevent or eradicate this disease.[vc_column width=”1/2″]
- CHARIOT 500SC 20ml/20l
- MEGAPRODE LOCK 525 WP 25g/20l
- MILLIONAIRE 690 WDG 40g/20l
- PYRAMID 700WP 50g/20l
- RANSOM 600WP 15g/20l
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- RIMETA GOLD 300SC 40ml/20l
- EXEMPO CURVE 250SC 15ml/20l
- TOWER 720WP 50g/20l
- TRINITY GOLD 50G/20L
- ABSOLUTE 375SC 15ml/20l
- COLONIZER 440WP 50g/20l
Note;
- Whenever spraying any fungicide, mix it with INTEGRA 3ml/20l. This is a sticker, spreader and penetrant which improves the effectiveness of the fungicide.
- In order to prevent the pathogen from gaining resistance or tolerance on any of the fungicides, it is advisable that alternating the chemicals be done. This is possible because the chemicals have different active ingredients.
There are also other practices which can be used to help manage this disease. These include;
- Planting resistant varieties
- Seed dressing
- Crop rotation with non-host crops (non leguminous crops)
- Maintenance of field hygiene
- Proper weed management
