Angular leaf spots
Angular leaf spot (ALS), caused by the fungus Phaeoisariopsis griseola, is considered a serious disease of beans in many regions. It attacks bean stems, leaves and pods.
Photos which illustrate symptoms of Angular leaf spots on bean leaves and pods.
Spots originating on the lower leaf surface are delimited by the veins and veinlets and develop into grey lesions which later turn light-brown. Lesions may be surrounded by a chlorotic halo but they lack a coloured border. The striking angularity of the spots is a diagnostic feature of P. griseola. The lesions may
be so numerous as to cause premature defoliation.
Stem lesions caused by P. griseola are elongate and brown.
Lesions on pods are less frequent than on leaves. They are oval to circular, superficial at first, with nearly black margins and reddish-brown centres, both of which are sharply defined. The spots vary in size and, ultimately, may become so crowded that they coalesce and occupy the width of the pod.
Prevention and control
Cultural Control
- Use crop rotation, possibly with 2 years between bean crops to allow for decomposition of plant residues,
2. Ascochyta Leaf and Pod Spot
Ascochyta leaf and pod spot is an important disease of beans. Symptoms occur on leaves, stems and pods of infected plants, and can be confused with the early stages of chocolate spot. On leaves, small, circular, dark-brown spots appear first. As the disease develops, lesions enlarge and turn light and then change to dark grey in colour. They become irregular in shape, often zonate, and may coalesce to cover most of the leaf surface. Leaf tissue
next to the lesions may become black and necrotic. Within the lesions, numerous pinhead- sized black fruiting bodies (pycnidia) of the fungus develop. These appear only under moist conditions and are often concentrically arranged. On the stem, lesions are more elongated, sunken and darker than leaf lesions and are usually covered with scattered pycnidia. Stems may split and break at the point of infection causing plants to lodge. On pods, lesions are sunken and have pale centre and dark margins; they can be covered by numerous pycnidia. Well-developed lesions can penetrate the pod and infect developing seeds causing them to be shrunken and discoloured. Badly infected seeds have yellowish brown stains on the outer seed coat, which considerably reduces its market value. Late infection with ascochyta can sometimes cause seed staining when disease levels in the crops have appeared too low to warrant fungicides sprays.
This disease can be managed through an integrated approach including careful paddock selection, the use of resistant varieties, strategic use of foliar fungicides and early harvest to minimize discoloration caused by ascochyta infection.
3. Rust
Photos: Symptoms of beans rust on leaves and pods
Management
4. Beans common mosaic virus disease ( BCMV) and Beans Common Necrosis Virus (BCMNV)
BCMV/BCMNV causes common mosaic and necrosis (black root) in Phaseolus vulgaris. The type of symptom produced is determined by the strain of BCMV/BCMNV, temperature and the host genotype. Symptoms associated with common mosaic include leaf rolling or blistering, light and dark-green patches on the leaf (green mosaic), chlorotic vein banding, yellow mosaic and growth reduction. Mottling and malformation of the primary leaves is an indication that the primary infection occurred through seed. Cultivars which develop common mosaic may have distinct chlorotic or necrotic local lesions.
Photo: Symptoms of BCMV/BCMNV on leaves
Prevention and control
Possible control measures for BCMV/BCMNV include planting healthy seed, improving cultural practices and using appropriate chemicals like Supermithrine or Rocket
5. Beans Aphids
Aphids have soft pear-shaped bodies with long legs and antennae and may be green, yellow, brown, red, or black depending on the species and the plants they feed on. The presence of cornicles distinguishes aphids from all other insects.
Ways that aphids damage plants:
Infestations are frequently localized with heavily infested leaves curled downward.
Photo: Beans aphids on bens stem and leave
Aphids may transmit viruses from plant to plant on certain vegetable and ornamental plants. Squash, cucumber, pumpkin, melon, bean, potato, lettuce, beet and chard are crops that often have aphid-transmitted viruses associated with them. The viruses mottle, yellow, or curl leaves and stunt plant growth. Although losses can be great, they are difficult to prevent by controlling aphids, because infection occurs even when aphid numbers are very low; it takes only a few minutes for the aphid to transmit the virus, while it takes a much longer time to kill the aphid with an insecticide.
Management
Although aphids seldom kill a mature plant, the damage they do and unsightly honeydew they generate sometimes warrant control. Most insecticides will destroy beneficial insects along with the pest.
Check your plants regularly for aphids at least twice a week when plants are growing rapidly in order to catch infestations early, so you can knock or hose them off or prune them out. Many species
of aphids cause the greatest damage in late spring when temperatures are warm but not hot (65°-80°F). For aphids that cause leaves to curl, once aphid numbers are high and they have begun to distort leaves, it is often difficult to control these pests, because the curled leaves shelter aphids from insecticides and natural enemies.
Bacterial diseases of garden beans
Common blight, halo blight, and bacterial brown spot are bacterial leaf spot diseases of beans. The symptoms expressed and the life cycles of these three bacterial diseases are all very similar. Therefore, making an accurate diagnosis can be difficult. Fortunately, all three diseases can be controlled with the same practices.
Most bacterial leaf infections initially appear as a small angular or circular, water-soaked (dark green) spot. As these infections develop, the spots turn brown and dry, while a yellow halo of leaf tissue encircles the brown spot. Severely infected leaves often appear yellow and shredded as the dry, brown spots disintegrate or fall from the leaf.
Management
The best way to control these bacterial diseases is through prevention. Plant commercially grown disease-free seed. Gardeners
who save their own seed should not plant seed from diseased plants. Use resistant bean varieties whenever possible. Remove and dispose of all plant debris at the end of the growing season. Diseased plants should not be composted since the pile does not reach a high enough temperature over winter to kill bacterial organisms. Practice crop rotation and avoid planting beans in the same location for more than three years in a row. To minimize the spread of disease, avoid overhead irrigation and stay out of the garden when plants are wet. Remove all infected portions of the plant or the entire plant from the garden, as soon as disease is detected.
Bean weevils
Descriptin of the pests
Bean weevils are relatively small beetles, 0.13 to 0.2 inch (3.5 to 5 mm) in length, somewhat teardrop or triangular in shape, and dull-colored with white, reddish, or black markings. The eggs may be glued to green pods, or laid loosely among beans or through cracks in the pods. The larval and pupal stages are spent inside the bean. Infestations can begin in the field. Adults move to bean fields from trash beans left in sacks, harvesters, planters, or feed areas.
Damage
The larval stage of the weevil pests of dry beans tunnel and develop within the beans. They may consume nearly the entire bean contents. Pupation occurs in the beans and adults emerge through a round hole in the seed coat. Damage is a combination of the feeding and contamination.
Management
Sanitation offers the most practical means of control. Because field infestations originate from beans, eliminate potential sources of weevils in production areas. Potential sources of weevils include broken sacks of seed beans left over from planting; seed beans left in planting hoppers; cull beans used in animal feed programs in a production area; small collections of beans remaining on or in a harvester following harvest; and small piles of beans remaining in or around the field after harvest or in a warehouse area.
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