Bacterial diseases

Common scab

Scab is a cosmetic disease that results in lowered tuber quality due to scab-like lesions on the tuber surface.

It occurs in all potato production areas of South Africa and is most severe in sandy soils with a pH above 5,5. Scab symptoms are quite variable. Usually, roughly circular, raised, tan to brown, corky lesions of varying size develop randomly across tuber surfaces. Scab is most severe when tubers develop under warm, dry soil conditions.

 Control

• Chemicals: Soil treatments (quintozene) and tuber treatments.
• Resistant cultivars.
• Plant scab free tubers.
• Crop rotation (a 4 year cycle) with non host crops.
• Green manuring with Brassica crops.

VYNTN (Potato virus YNTN)

A new potato virus Y strain, named the necrotic tuber necrosis (NTN) strain, PVYNTN, has been identified as the causal agent of potato tuber necrotic ringspot disease (PTNRD ). This damaging disease causes ring spots on the surface of tubers thus affecting the cosmetic value of the potatoes.

Potatoes expressing PVYNTN symptoms are rejected at the market and are unsuitable for processing, causing economic losses to growers.

Weeds may act as a reservoir for the virus. Several aphid species, such as the green peach aphid (Myzus persicae), transmit the virus.

The virus is also readily transmitted mechanically and by grafting, not by seeds.

Control

• Avoid planting any solanaceous crops close to potato fi elds to prevent transfer of the virus to potato by aphids.
• Regularly scout fi elds for the first occurrence of symptoms of the disease. If feasible, infected plants should be pulled up and destroyed.
• The use of registered aphicides on a regular basis during the growing season may assist to control aphid populations and prevent spreading of the virus.
• Regularly weed and destroy all annual weeds in and around the field, including those in ditch banks, hedges, fencerows, and other places.

Potato tuber moth

Moths are nearly 1cm long, the wings are grayish, with dark gray spots. The moth is active at night and hides between plants during the day.

Moths live for about a week and lay up to 250 eggs. The larvae are miners and mine through the leaves and tubers. The potato tuber moth larvae damage both foliage in the field as well as tubers beneath the ground and in storage. The moth does not feed, but lays her eggs on or near plants. The resulting larvae tunnel or mine into all plant parts, causing severe yield losses when the tubers are reached.

If the foliage becomes scarce at the end of the season, the larvae will go down cracks in the soil to reach the tubers. They then tunnel into the tuber, usually just beneath the skin. If tuber moths lay eggs on tubers in storage, the damage may be severe. The eyes (where sprouts form) are usually attacked first, resulting in no sprout development. This can lead to severe loss of stored seed potatoes.

Control

• Some insecticides are registered to control this pest.
• Ridging potatoes and regularly check to ensure all cracks are covered (prevent small larvae from reaching the tubers).
• Use seed that is not infected.
• Do not discard old potatoes on nearby dumping sites.
• Remove volunteer plants from harvested fields.

Potato leafminer

The potato leafminer is a small fly that also attacks a wide range of other vegetables. The female fly damages plants by piercing leaves with her ovipositor, causing “stippling”. These “punctures” may also stress plants. In some of the puncture marks, the female lays her eggs. A small cream coloured maggot hatches from an egg and starts to mine between the leaf surfaces.

Severe infestations may give leaves and haulms a burnt appearance and usually result in foliage dying off prematurely. This “removal” of green material before the plant is ready to die off naturally results in yield reduction. Up to 70% yield loss may occur. The larvae do not attack tubers.

Control

• Several insecticides are registered to control this pest.

Aphids

Aphids are small insects with sucking mouthparts. However, they usually do not damage plants by their feeding only. They are virus vectors that may transmit viral diseases to healthy plants after feeding on infected plants elsewhere.

After multiplying in such a fi eld, the virus may be spread to other plants inside the same field by the offspring that have fed on the infected plant. The two most important potato viruses are Potato leafroll virus (PLRV) and PVY.

Tubers infected with viruses may be downgraded under the Seed Certifying Scheme.

Control

• Some insecticides are registered to control this pest.
• Only seed producers need to control this pest.

Nematodes

These are microscopic, slender roundworms that live in the soil. Nematodes can damage tubers and limit the transport of water or nutrients. Locally, the three most important nematodes that attack potatoes are root knot nematodes, lesion nematodes and potato cyst nematodes.

Root knot nematodes are the most common and they prefer a hot climate and sandy soil. They survive in the soil for many years in the form of small eggs. 

Eggs hatch when a host plant grows close to it and the worms move in the soil water to reach the roots. Infected tubers make small galls in which the female worms and egg sacks are found.

Planting such a tuber will infect soils with nematodes. Fallow cultivation, rotation with cereals or grasses and nematicides are some of the control measures. Lesion nematodes are less common but can cause economic losses. Infected tubers look unhealthy, could have purplebrown pimples, pustules or wart-like protuberances.

Potato cyst nematode infections must be reported to the Department of Agriculture as it is a quarantine pest.

Control

• The only two control actions that are effective nematodes are nematicides (applied at planting time) and crop rotation (see text below). 

DID YOU KNOW?

Potatoes belong to the family Solanaceae. Tomatoes, brinjals, peppers, chillies and gooseberries also belong to this family, so they cannot be used in the same rotation as potatoes. These plants are sensitive to the same pests and diseases, so they could have a devastating effect on potatoes if planted with, or near to, them.

Source: The Agricultural Research Council (ARC)