PEST AND DISEASE CONTROL             

Crop Rotation

• One of the most important strategies of IPM is crop rotation
• Crop rotation prevents pest and disease build-up and in addition, it replenishes soil nutritional status
• It is recommended to rotate crops with those from different families; an effective rotation program should last for 3 – 4 years

1. Major Pests

Pests’ damage causes a reduction in quality and quantity of produce. The following are the major pests of Potato in Malawi:

 

 

Potato Tuber Moth

 

Potato Moth on a tuber   

Potato tuber damaged by Potato Moth

Identification:

• The moth is small, brownish grey in color with narrow fringed wings
• The moths are active mainly at dusk
• The caterpillars are up to 12 mm long, whitish to pale greenish in color

Damages:

• Caterpillars burrow in the tubers  making long irregular tunnels filled with excreta  exposing tubers to secondary bacterial and fungal infection
• These tunnels make the potatoes unfit for human consumption
• The pest is transferred with the harvested tubers to the potato store, where it can reproduce and infest other tubers

Control:

• Use healthy and clean seed, since infested seed tubers are the main cause of re-infestation in the field
• Plant as deeply as possible (10 cm deep) and ridge at least 3 times during the growing season
• Deep ridging: very important to prevent moths reaching the tubers to lay eggs
• Store all harvested tubers before dusk to avoid moths laying eggs on them
• Don’t leave harvested tubers in the field overnight during dry season
• Use of repellants e.g.) Lantana camara and Eucalyptus leave

Aphids

Aphid on the leaf of a Potato Plant

Identification:

• Many aphid species attack the potato including the green peach aphid, potato aphid & cotton aphid
• Aphids are mainly found on plant terminals and on the underside of leaves

Damages:

• Feeding by aphids causes irregular curling of young potato leaflets and hinders growth of the leaflets
• Most damage is caused by honeydew production on foliage and virus transmission
• Aphids are important pests as vectors of potato viruses, such as the Potato Leaf Roll Virus, a serious disease affecting potatoes

Control:

• Use of pesticides, such as
  • Nuprid 200 SC® (a.i. Imidacloprid)
  • Karate 2.5WG® (a.i. Lambda Cyhalothrin)
• Incorporate a sticker/spreader, such as Agral 90®
• Control aphids in potato planted for seed production
• Grow the crop at the right (high) altitude

 

Root-knot Nematode

Identification:

• Root-knot nematodes are microscopic slender, transparent worms that live in the soil
• However, they can be identified by the symptoms they produce in the potato crop
• Root-knot nematodes prefer warm temperatures and are likely to become established in potato crops grown in relatively warm areas

Damages:

• Varying degrees of stunting, yellowing of leaves and a tendency to wilt under moisture stress
• Knotty swelling (galls) on roots
• Rough warty patches on the outside of the tuber
• Golden brown spots on the flesh of the potato are seen when a thin strip of the skin is removed
• Root-knot nematodes are mainly spread in potato tubers and in infested soil
• Nematodes increase incidence of infection by bacterial wilt and other pathogens

Control:

• Practice proper Crop Rotation
• Maintain high levels of organic matter in the soil (manure, compost & “BOKASHI” etc.)
• Destroy crop residues
• Use of nematicides, such as
  • Bio nematon (a biological nematicide)

 

Spider Mites

Identification:

• To the naked eye, spider mites look like tiny moving dots but can be seen using a hand lens
• They live in colonies, mostly on the under-surface of the leaves and spin a silk-like web
• Adults have 8 legs and an oval body with two eye spots on the head end of the body
• Eggs are spherical & translucent becoming cream colored before hatching
• The pest is destructive during dry weather

Damages:

• Mites cause damage by sucking cell content from leaves
• At first, the damage shows up as light dots on the leaves which at times take a bronze colour
• As the feeding continues, the leaves turn yellow and drop off
• Often, leaves and stems are covered by a large amount of webbing
• Damage is usually worse when compounded by water stress

Control:

• Spray appropriate miticides which kill different stages of the mite (egg to adults) such as

Hattrick EC® (a.i. Oxydemeton-Methyl)

• To prevent the pest from building resistance, alternate chemicals with different active ingredients

 

Cutworms

Identification:

• The larvae of cutworms stay buried in the soil and cut stems during the night
• The pest is destructive during dry weather

Damages:

• Cutworms feed on tubers and roots, boring a wide shallow hole
• They are also serious pests of newly sprouted potato plants, and can leave great empty patches in a potato field

Control:

• Ploughing and hand picking
• Prepare field and destroy vegetation and weeds 10 – 14 days before planting
• Ploughing exposes caterpillars to predators and desiccation by the sun
• Flooding of the field for a few days before planting can help kill cutworm caterpillars in the soil

 

Major Diseases

Disease infestation leads to reduction in quality and quantity of produce. The following are the major diseases of Potato in Malawi:

Late Blight

• This is a fungal disease which is favored by cool, cloudy wet conditions
• It is one of the most destructive disease of potato

Symptoms:

• Water soaked spots on leaves which enlarge and turn brown
• Below the leaf, the fungus produces white mouldy growth seen clearly at the edge of the spot
• The affected leaves wither, yet frequently remain attached to the stem

Control:

• Resistant varieties, such as “Zikomo”, “Chuma”, have some resistance
• Practice Crop Rotation with non-solanaceaous crops
• Practice good field hygiene by rouging
• Select only disease-free seed potatoes
• Spray with appropriate fungicides both protective & curative, such as
  • Antracol WP70® (a.i. Propineb)
  • Dithane M45® (a.i. mancozeb)
  • Nando 500SC® (a.i. Fluazinam) Ridomil Gold MZ68® (a.i. metalaxyl + mancozeb)
  • Milraz WP76® (a.i. Propineb 70 % + Cymoxanil 6 %)
  • Victory 72WP® (a.i. metalaxyl + mancozeb)

Bacterial Wilt

• The disease is caused by a bacteria known as Pseudomonas solanacearum
• It is soil borne and the most serious disease which can destroy an entire field
• The bacteria survives in the soil for a long time and enters into the host plant through wounds on the roots and the base of stems
• The disease is spread by infected tubers, crop residues, contaminated surface water, contaminated soils, and tools 

Symptoms:

• Affected plants wilt even when there is adequate moisture in the soil
• Wilting is rapid and wipe out the entire fields in few days
• Slimy continuous white discharge emanate from the eyes of the affected tubers

Control:

• Use of disease-free seed
• Practice Crop Rotation (2 – 3 years) rogueing volunteer potato plants during rotation
• Destruction of infected plant debris by burning
• Avoid contaminating the field with soil from an affected field
• Avoid cutting of seeds as a way of multiplying seeds when having insufficient seed potato
• Spot treatment with 10 % of Sodium hypochlorite (bleach)

Early Blight

• This is caused by a fungus: Altenaria solani
• The fungus persist in debris of affected plants for several years
• Early Blight thrives best under warm wet conditions

                           Early Blight on Potato Leaf

Symptoms:

• First, oval or angular dark brown to black “target” spots appear on leaflets
• Usually, a narrow chlorotic zone is around the spot which fades into the normal green
• Lowest, oldest leaves are infected first, and they droop and dry as the disease progresses and eventually fall off

Control:

• Use of disease-free seed
• Good field sanitation by rouging
• Practice Crop Rotation
• Destroy all infected crop residue
• Spray with appropriate fungicides, such as

• Antracol WP70® (a.i. Propineb)
• Dithane M45® (a.i. mancozeb)
• Nando® (a.i. Fluazinam)
• Ridomil Gold® (a.i. metalaxyl + mancozeb)
• Milraz WP76® (a.i. Propineb 70 % + Cymoxanil 6 %)
• Victory 72WP® (a.i. metalaxyl + mancozeb)

 

Bacterial Soft Rot

• The disease is caused by the bacteria: Erwinia spp.
• It enters the plant through wounds on leaves or stems near the soil surface and young tubers 
• The disease development is favoured by high temperatures and humidity
• The bacteria is spread by rain splash
• In poorly drained fields, tubers decay

        Potato tubers infected by Bacterial Soft Rot

Symptoms:

• Stems and leaves develop lesions which are water soaked, dark green and eventually develops to a soft rot with a foul odour
• On tubers, reddish brown spots form at the lenticels
• The inner parts of the tubers shows a soft rot and a creamy exudate is seen and may decay during transportation or storage in poorly ventilated, high temperature and humidity

Control:

• Practice crop rotation with crops such as cereals
• Use of healthy seed tubers
• Improved field drainage
• Store and transport tubers in dry, well ventilated conditions
• Field hygiene
• Sorting before storage

Potato Leaf Roll Virus (PLRV)

• This is an important potato disease which occurs in all potato growing areas
• It is transmitted by aphids.
• The virus is also spread through infected  tubers and diseased volunteer plants

     PLRV-infected plant next to an uninfected Potato plant

Symptoms:

• In plants infected through aphid transmission, the apical leaves roll upwards and occasionally become pinkish in colour
• In plants infected through use of infected seed tubers, the lower leaves roll upwards (after sprouting) and becomes spoon-like
• Severely affected leaves develop a tubular shape
• Plants are stunted and plants develop small tubers
• If these tubers are used as seeds, plants are stunted and crop produces very low yields

Control:

• Use of chemicals to control aphids which can transmit the virus to potato plants, such as

• Nuprid 200 SC (a.i. Imidacloprid)
• Karate 2.5WG (a.i. Lambda Cyhalothrin)
• Use of virus-free seed tuber