MAIN PESTS AND DISEASES OF FRENCH BEANS (Phaseolus vulgaris)

How to grow french beans is quite straightforward. French beans, also known as snap or green beans, and locally in Kenya as mishiri, are a major export crop. Interest in french beans farming is fast-growing for both fresh consumption and processing (mainly canning and freezing). French beans contain protein, fat, calcium, iron, phosphorus, vitamins A, B, D and starch.

Bean plants are annual vegetables that grow quickly and are best planted in the spring. The flowers appear about two months after planting. Harvest time varies greatly, depending on the type of bean. Note that the seeds of raw or undercooked beans can be toxic to people1 and animals2.

BACTERIA OF FRENCH BEANS

• Leaf spot or leaf blight: Symptoms are easily identified in the field and generally only occur on the primary leaves, appearing as round brown spots surrounded with a darker ring, or many small regular black spots, depending on the pathogen. Under high humidity conditions, these symptoms can occur in older trifoliate leaves or ripening pods.

• Halo blight of beans: It may attack the entire plant leaves, buds, and seeds. After entry through the leaves, the bacteria multiply at an exponential rate within the intracellular spaces.
• Anthracnose of bean: Symptoms can also be noted on all the above-ground plant parts. It induces brown lesions, first on the underside of leaf veins. The disease then spreads laterally as lenticular pale brown spots with darker brown borders that are visible on the upper surface of the leaves.
• Damping-off, Rhizoctonia black scurf: Early infection by this disease leads to seed degeneration and post-emergence decline. Later infections induce tissue necrosis and yellowing of leaves because of tissue alteration.
• Collar rot, collar necrosis, pod rot, southern blight: At a quite early stage, symptoms appear as yellowing of the edges of the lowest leaves along with wet rot at the base of the collar just above the soil line. The top leaves then begin yellowing and fall. The pathogen spreads to the stem and roots and destroys the cortex. It sometimes also attacks the vascular tissues and develops in the upper branches and the tissues darken.
• Angular leaf spot disease: This disease develops on the lower leaves of the plant as angular spots that are initially grey, then brown and limited by the leaf veins. The lesions can also be surrounded by a chlorotic halo with no colored border.
• Bean rust: Bean rust damage mainly occurs on the leaves, with the appearance of small (1-2 mm diameter) yellow pustules that soon turn into brownish-red spore masses in the middle of a yellow spot. The leaves wither and fall in highly infected plants.

 

FUNGUS OF FRENCH BEANS

• Ash stem blight or stem rot of bean: The entire plant, including the leaves, buds, roots, and seeds—but mainly the stems and roots—can be attacked by this fungus. Dry rot symptoms appear on the cotyledons as blackening of growing points and the collar. Small black spots develop in the infected area.
• Fusarium root rot of bean: Longitudinal reddish lesions appear at the base of the stem and the roots become necrotic and the taproot turns reddish.
• Damping-off, blackleg and Pythium root rot: When plants are infected by these diseases, emergency after sowing is hampered and the seedlings that managed to emerge have black rotting roots. The plants suddenly wilt and soft and wet rot are noted on the roots, collar and sometimes the stems.

 

PESTS OF FRENCH BEANS

• Aphids: Aphids causes direct damage to bean plants during the vegetative and flowering phases. Aphid colonies can attack young shoots, the underside of leaves, petioles, and seedlings.
• Whiteflies: Direct feeding damage from whitefly adults and nymphs causes chlorotic leaf spots, which can be seen on the upper surface of leaves. Depending on the extent of whitefly colonization, these spots can merge until the entire leaf is yellow. When whiteflies are not controlled, and their feeding is excessive, leaves can become brittle and eventually dry up and fall off.
• Corn Ear Worm, Tomato Grub, Tobacco Budworm, Cotton Bollworm: The bollworm nibbles on the pods and leaves and eats the seeds. Adult females lay their eggs in the bean pods. These eggs and young caterpillars are hard to detect during bean sorting operations, so the pods can subsequently be damaged in the export packing boxes.
• Leafminers: Feeding damage caused by leafminer appears as tunneling (0.13-0.15 mm diameter) on the upper side of leaves. The shapes of these tunnels vary according to the attacked plant, but they are long, linear, and not very wide when sufficient leaf area is available. They are generally greenish to white.
• Legume pod borer: The damage caused by this pest includes round boreholes in the corolla, which can turn the flowers into a brownish mass within 24 h.
• Mylabris beetles: Adult beetles are 25-35 mm long. They are blackish with wide yellow or reddish transversal stripes on the elytra. The tips of the antennae are also yellow or orange. They consume the flowers (which leads to abortive pod set) and sometimes bore wide holes in the leaf blades. Entire fields can be quickly defoliated by these beetles since they often massively colonize crop fields.
• Red spider mite or two-spotted spider mite: Yellow spots are noted on the upper side of leaves, while small mobile mites (0.5 mm) are found on the lower side. A very fine web may be noted when these mites occur in high numbers.
• Armyworm: Young larvae feed on the superficial layer of leaves, often leaving the epidermis and large veins intact. Later instar larvae pierce irregular holes in the leaves and fully developed larvae can consume all of the leaves, leaving only the main leaf ribs.
• Thrips: Punctures caused by thrips induce tissue discoloration and metabolic disorders in the plant, which thus weakens and wilts.
• Looper caterpillar: The appearance of large irregular holes on the surface of leaves is a sign of looper caterpillar feeding.
• Bean fly: Adult flies emerge from the pupae in the stem. The stem tunneling causes the death of young plants and heavy and early infestation often results in complete crop loss.
• Bean seed fly: The larvae feed on the germinating seed, often hollowing it out completely. The seedlings may fail to emerge or be very stunted and gaps will appear in the planting.
• Root-knot nematodes: The larvae penetrate the roots and settle in the vascular area, inducing swellings or galls. The shape, size, and appearance of the galls vary with their age, number, the host plant, extent of the attack, and environmental conditions. Under heavy infestations, the roots can become swelled and stunted.
  Migratory endoparasitic nematode: Attacked plants show halted growth, reduced vigor, leaf necrosis and chlorosis, defoliation, brown, reddish or blackish root rot, and a reduction in yield.

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