Field Pests and Disease Management

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.

2. 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.

3. 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.

4. 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.

5. 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.

2. 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.

3. 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).

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.