Field Pests and Disease Management

Bean Pest

Pests

  1. Cutworms – the larvae feed on roots and base of the stem killing off the seedlings.

Control

  • Early planting
  • During primary tillage, dig soil to expose larvae to predators such as birds and ants.
  • Hand pick and destroy the larvae.
  • Apply recommended insecticides such as Thiamethoxam to the ground around the affected plants.
  1. Bean fly – the larvae tunnel into leaves and down petioles to the stem cutting flow of nutrients causing weathering and drying.

Control

  • Earth up soil around the plant to cover the roots.
  • Plant early.
  • Provide adequate fertilization for plant vigor.
  • Avoid planting beans near cowpeas, beans and other leguminous crops.
  • Use resistant varieties developed by researchers.
  • Use treated seed.
  • Use insecticides under severe attack examples include Imidacloprid, Azadirachtin.
  1. Flower Thrips – feed on young leaves, developing flowers and pods causing curling of leaves, flower abortion and scars on pods.

Control

  • Early planting.
  • Fertility management.
  • Inspect flowers and Use recommended insecticide e.g. Dimethoate 40% only when 10 thrips /flower are seen, early detection important.
  • Spray with botanicals.
  1. Aphids – they suck plant sap and infested leaves curl under and inward and become severely distorted.

Control

  • Early planting.
  • Fertility management.
  • Use bio-pesticides not harmful to natural enemies e.g. neem, soapy water.
  • Monitor crop regularly and apply insecticide if at least 1 in 10 plants is heavily infested.
  • Examples of insecticides include Dimethoate 40%, Azadichtin.
  1. Flower beetles – they feed on petals and pollen of flowers.

Control

  • Remove the plant debris from the field.
  • Remove the weeds from the field.
  • Manually pick and destroy the beetles.

Other pests of the bean plant include;

  • Foliage beetles
  • Pod borers
  • Pod sucking bugs
  • Bean bruchid

Diseases

  1. Common Blight

Symptoms

  • Survives in the seeds but may also be carried over in diseased debris.
  • Small water-soaked spots on the underside of the leaves, which enlarge ad merge to form large brown irregular lesions surrounded by a narrow yellow zone.
  • Pods have sunken circular spots which are initially water soaked but later dry with a reddish brown border.
  • Under wet conditions, yellow slimy bacterial exudates ooze out of the lesions and form a crust.

Control

  • Use healthy seed
  • Remove all infected portions of the plant or the entire plant from the garden, as soon as disease is detected.
  • Practice crop rotation.
  • Spray with Mancozeb.
  1. Halo Blight

Symptoms

  • Has a distinct yellowing around the initial leaf spot, which spreads outwards, though generally, the symptoms are similar to those of the common blight disease.

Control

  • Control just like common blight above.
  1. Bean Common Mosaic Virus (BCMV)

Symptoms

  • Symptoms range from puckering of the leaflets along their midribs to leaflet mounting and elongation.
  • Pod, leaf and petiole distortion.
  • Dwarfing of leaves or the entire plant.
  • Affected leaves may show light green or yellow and dark green or yellow and dark green mosaic patterns.

Control

  • Use certified seeds.
  • Rogue any plants infected with the virus.
  • Avoid adjacent planting and overlapping bean crop during the rainy season (only one crop per rainy season).
  1. Bean Anthracnose

Symptoms

  • Small reddish brown, slightly sunken spots form on the pods and rapidly develop into large, dark sunken lesions.
  • In moist weather, masses of pink spores develop on these lesions.
  • Black sunken spots are visible on the stems and leaf stalks.
  • Infection of the leaves causes blackening along the veins, particularly on the underside.

Control

  • Use disease free seed.
  • Avoid cultivating and harvesting an infected crop when wet to prevent the spread of spores.
  • Do not pack lightly diseased pods as anthracnose can develop during transport.
  • Use recommended fungicides.
  1. Bean rust

Symptoms

  • The initial symptom is small, whitish and slightly raised spots that produce reddish pustules and later on turn dark brown.
  • The leaves turn yellow and then brown and dry.

Control

  • Timely application of fungicides.
  • Use tolerant varieties.
  • Practice crop rotation.
  • Intercropping is recommended.
  • Avoid adjacent planting and overlapping bean crop during the rainy season.
  • Destroy all infected farming materials.
  1. Fusarium wilt

Symptoms

  • The fungus causes a reddish discoloration throughout the root, stem and petioles.
  • Infection causes yellowing of lower leaves and may progress to the upper leaves causing premature defoliation.
  • Infected seedlings are stunted, wilt and die.
  • The fungus can also cause water soaked lesions on pods.

Control

  • Crop rotation with non-host crops.
  • Deep ploughing of infected plant debris.
  • Use of organic amendments such as manure to improve the soil fertility.
  • Seed dressing with recommended fungicides.
  1. Fusarium root rot

Symptoms

  • Reddish discoloration of the taproot which gradually intensifies, eventually turning brown.
  • Fissures develop and the rot becomes dry and papery.

Control

  • Long term rotations of 4-5 years with non-legumes.
  • Improve drainage and avoid injury to the root system.
  • Planting in raised beds or ridges and hilling up soil around the stem before flowering.
  • Apply farmyard manure or other soil amendments.
  • Deep plough infected plant material during land preparation.
  1. Pythium

Symptoms

  • Damping-off of seedlings.
  • Attacks the lateral roots causing plant wilt and death.
  • The pathogen can extensively prune roots and destroy much of the hypocotyls and main root system.

Control as fusarium root rot.

  1. Sclerotium root rot (white mold)

Symptoms

  • Symptoms initially appear on the stems just below the soil line as grey water soaked lesions.
  • Later turn dark brown and extend downward to the tap root leading to wilting.
  • Foliage symptoms consist of yellowing and defoliation in the upper plant branches, followed by sudden wilt.
  • A fan of silky white mold and large, round sclerotia appears at the base.

Control as fusarium root rot.

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