Root-knot nematodes (Meloidogyne spp.)

Potatoes are very susceptible to root-knot nematodes. Root-knot nematodes prefer warm temperatures and are likely to become established in potato crops grown in relatively warm areas. Generally, they are not a major problem in cool climate potato production areas, but can become a problem when potatoes are grown intensively or rotated with other susceptible crops. Infested potato plants may show varying degrees of stunting, yellowing of leaves and a tendency to wilt under moisture stress. Roots have swellings or galls. Affected tubers have blisters or swellings.

On potato tubers, galls may or may not be produced on the tuber surface, depending on cultivars. When galls are produced, they appear as small, raised lumps above giving the skin a rough appearance. Galls may be grouped in a single area or scattered near the tuber eyes. Infestations are difficult to detect in freshly harvested potato tubers.

Symptoms may develop when tubers are stored, particularly when taken to warmer climates where nematode numbers can rapidly increase. Symptoms are most severe when crops are grown on sandy soils and warm climates above 25°C. Nematode attack reduces the quality, size and number of tubers.

Root-knot nematode infested potatoes can become more susceptible to bacterial wilt, and symptoms are more severe when plants are also infected with fungal pathogens such as Verticillium and Rhizoctonia.

Root-knot nematodes are mainly spread in potato tubers and in infested soil. Egg masses may be transported into clean fields via soil adhering to farm machinery. Spread within fields occurs during cultivation and in water, during irrigation or natural drainage.

Aphids (Myzus persicae, Macrosiphum euphorbiae, Aulacorthum solani, Aphis gossypii)

Many aphid species attack potatoes. The most important are the green peach aphid (Myzus persicae) the potato aphids (Macrosiphum euphorbiae, Aulacorthum solani) and the cotton aphid (Aphis gossypii).
Aphids are mainly found on young shoots on the underside of leaves. Feeding by aphids causes irregular curling of young potato leaflets and hinders growth of the leaflet. Potato aphids can also attack potato sprouts in stores.
Direct damage caused by aphids sucking sap from the plant is usually of little importance. 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. Seed potatoes are particularly susceptible to this virus and even low aphid populations can be very damaging.

Cutworms (Agrotis spp)

Cutworms are caterpillars of some moths. The caterpillars live in the soil and eat into tubers boring a wide shallow hole. They are also serious pests of newly sprouted potato plants, and can leave empty patches in a potato field.

Epilachna beetles (Epilachna spp.)

Several species of Epilachna beetles feed on the leaf tissue in between the veins leaving a network of veins intact. The adult beetles are oval, about 6 mm in length and reddish brown to brownish yellow in colour with black spots on their backs. They look very similar to the beneficial ladybird beetles (predators), but the body of this pest ladybird beetle is covered with short, light coloured hairs, which give them a non-glossy or matt appearance.

They lay eggs in clusters (20 to 50 eggs), usually on the underside of the leaves and placed vertically. The larvae (grubs) are pale yellow and easily recognisable by the strong branched spines covering their body. Generally, these beetles are minor pests of potatoes, but occasionally the infestation is so severe that control measures are needed.

Millipedes or thousand-legged worms

Millipedes are not insects, but are related to them. They have many legs (30 to 400) with a hard-shelled, round segmented body. They are brown to blackish brown in colour. They move slowly and curl up when disturbed. Millipedes lay eggs singly or in clusters of 20 to 100 in the soil. They live in moist soil and congregate around the plants in soil that is rich in organic content. They dry out easily and die. Thus, they seek wet places, such as compost piles, leaves and other plant debris, to hide under during the day. They tunnel into potato tubers.

Weeds

The major weeds in potato crops are annuals. Weeds can reduce yields through direct competition for light, moisture and nutrients. They may harbour pests and diseases that attack potatoes. Early season competition of weeds is very critical. When properly grown, the crop normally covers the ground and smothers any weed competition. Weeds present at harvest increase mechanical damage to the tubers, and reduce harvesting efficiency by slowing down harvesting operations and leaving tubers in the ground. The most important weed in potatoes worldwide is pigweed or fat hen (Chenopodium album).

Bacterial wilt (Ralstonia solanacearum)

Bacterial wilt can be very destructive at the lower altitude, warmer extreme of the potato's range in Kenya. This disease causes rapid wilting and death of the entire plant without any yellowing or spotting of leaves. All branches wilt at about the same time. The pathogen is transmitted through tuber seed into the soil. Also infested soil can be important source of disease inoculum (infection).

Bacterial soft rot (Erwinia carotovora pv. carotovora , E.c. pv. atroseptica)

The bacteria enters through wounds and cause extensive rotting of potatoes due to degradation of tuber cell walls, often reducing tubers to a smelly pulp. They may also be carried over on tubers, and shoots (stems) emerging from infected tubers. These are blackened and commonly referred as blackleg, and the affected stems subsequently die.

Late blight (Phytophthora infestans)

This disease is favoured by cool, cloudy, wet conditions. Symptoms of late blight are irregular, greenish-black, water soaked patches, which appear on the leaves. The spots soon turn brown and many of the affected leaves wither, yet frequently remain attached to the stem.

Early blight (Alternaria solani)

Leaf spots of early blight are circular, up to 12 mm in diameter, brown, and often show a circular pattern.

On the potato tuber early blight results in surface lesions that appear a little darker than adjacent healthy skin. Lesions are usually slightly sunken, circular or irregular, brown and vary in size up to 1-2 cm in diameter. There is usually a well defined and sometimes slightly raised margin between healthy and diseased tissue. Internally, the tissue shows a brown to black corky, dry rot, usually not more than 6 mm. Deep cracks may form in older lesions.

Early blight thrives best under warm wet conditions.

Black scurf and Scab

Black scurf (Rhizoctonia solani), powdery scab (Spongospora subterranea) and common scab caused by the bacterium (Streptomyces scabies).

These diseases may cause similar symptoms. Black scurf and scab cause skin blemishes (russeting; rough corky tissue) on tubers.

These lesions may be so numerous as to involve the entire surface of affected tubers. Such lesions spoil the appearance of the tubers and cause waste in peeling and reduction in grade. These pathogens live in the soil and survive in infected tubers. In most cases infection occurs whilst the tubers are still in the ground.

Viral diseases

Viral diseases of importance include:

Potato Leaf Roll Virus (PLRV)
Potato X Potexvirus (PVX)
Potato Virus Y Potyvirus (PVY)

Under field conditions, there is often a composite infection with the 3 viruses (PLRV; PVX; PVY) and it becomes very difficult to distinguish the three on the basis of symptoms.
These viruses are transmitted by aphids. PVX causes distinct leaf mottling and crinkling. PVY causes dwarfing of plants, with a rough, mosaic appearance to the leaves and affected plants give a poor crop of small tubers. PVY is also tuber-borne. Potatoes infected with PLRV show an upward and inward rolling of leaflet margins and plants are dwarfed. Affected leaflets become thick and dry, and the lower leaves may become completely brown. Infected plants produce tubers smaller in size and numbers. PLRV also infects other solanaceous crops and weeds (e.g. tomato, tobacco, jimson weed).

Potato Spindle Tuber Viroid (PSTVd)

It is particularly destructive for seed production. Potato plants severely infected are upright, stunted and much thinner than normal plants. Leaves of infected plants are smaller and may be grey or distorted. The affected stems are often more branched and branches are at very sharp angles to the stem. Symptoms are more obvious on tubers. Affected tubers are small, narrow and spindle shaped (oblong).
In some varieties tubers develop knobs and swellings. In other varieties eyes on tubers are numerous, shallow and prominent, and affected tubers are often cracked. This viroid is easily mechanically transmitted. An infection can be introduced by sowing infected seed tubers, by insects such as aphids, grasshoppers and flea beetles, and through pollen. Tomato and capsicum are additional hosts for PSTVd.