Sweet potato

Aphids suck sap from leaves and stems. They may cause considerable damage during periods of water stress. Aphids are vectors of virus diseases.

Armyworms may damage sweet potatoes. Young caterpillars scrape sweet potato leaves, while the older caterpillars feed producing large irregular holes and may leave only the veins. Mature caterpillars measure up to 4 cm long and are generally black, heads faintly mottled with dark brown spots and with light yellow stripes at their backs (IRRI, 2001).

Predatory bugs, carabid beetles, spiders and wasps attack the caterpillars, and many parasitic wasps are also known to attack armyworms. Fungal diseases have been observed infecting caterpillars in the field.

The disease is caused by the fungus Ceratocystis fimbriata. Dark circular spots appear on infected roots. These spots can expand to cover much of the sweet potato. Small black fungal fruiting bodies (perithecia) develop and are visible at the centre of the spots. The diseased tissue is darkened and bitter in taste. On young sprouts black spots develop eventually encircling the plant causing dwarfing, yellowing of the foliage and finally death.

The fungus survives in the soil in diseased plant debris and also in diseased stored roots. The disease is favoured by temperatures near 25⁰C and moist soil conditions. In storage, decay progresses most rapidly in moist conditions at temperatures of 14 to 27⁰C.

The disease is caused by the fungus Botryodiplodia theobromae (Diplodia tubericola). It affects sweet potatoes in storage. The inner part of the infected edible root becomes black and brittle. Many tiny black fungal bodies (pycnidia) form just below the sweet potato skin giving the root a pimply appearance. Infection occurs through broken ends and abrasions on storage roots. Wet conditions favour disease spread and development.

Domestic animals (e.g. pigs, cows and goats) and wild animals (e.g. wild pigs, porcupines, baboons, monkeys, elephants, hippos, guinea fowl) can cause serious damage to sweet potatoes.

They causes hairiness on sweet potato, a common problem in many parts of East and Southern Africa also known as "erinose". Eriophyid mites are tiny, much smaller than the spider mites (about 0.2 mm long) and look like a speck of dust. They are not visible with the naked eye. The mites feed in the buds and on young foliage of sweet potato plants, injecting growth substances into it, which induce the plant to produce a dense mat of hairs. As a result the stems, leaf petioles, buds and undersides of leaves become covered with a dense layer of white hairs. The leaves and plants are also generally slightly stunted; the leaves and stems thickened, and the plants yield poorly.

Occasionally, whole crops are affected but often the symptoms affect just one or a patch of plants, and often only particular varieties. The mites invade crops by being blown like dust particles in the wind.

It is caused by the fungus Fusarium oxysporum f. sp. batatas. Initial symptoms on sweet potato are yellowing of the leaves. Leaves later wilt and fall off, stunting results and eventually death of the plant. Death of the stem vascular bundles occurs with brown to purple coloured when split; this may be accompanied by cracking of the stem. The vines may turn tan to light brown. Diseased plants may manage to produce storage roots, but these usually have some discoloured, infected vascular tissues. Rot may follow in storage or the disease may be transmitted to the next crop by infected cuttings. Dying vines often have pinkish fungal growth.

It is caused by a potyvirus (sweet potato virus B/ sweet potato virus T/ sweet potato ipomovirus). The virus is transmitted by whiteflies (Bemisia tabaci). The virus has a wide host range including tomato, tobacco and ornamental species. Symptoms of the disease include leaf mottling and plant stunting. The wishbone flower (Torenia fournieri) is a potential wild reservoir host in East Africa.

Apart from insects other pests reported to cause damage to sweet potatoes are millipedes. They are also known as "thousand-legged worms" or "Mombasa train". They have many legs (30-400) with a hard-shelled, round segmented body and are up to 30 cm long. They are brown to blackish brown in colour. They move slowly and curl-up when disturbed. Millipedes lay eggs single or in clusters of 20-100 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.

Millipedes have recently become important pests of sweet potato in some areas of East Africa. Infestation tends to be severe at the beginning of the long rainy season often causing farmers to plant late. According to farmers in Uganda, millipedes generally do not affect the roots until 5 months after planting. They may be a problem when harvesting of sweet potato is delayed, especially if the roots are stored 'in-ground on the plants' during the dry season and harvesting is done at the first rains of the following growing season. Millipedes maybe a problem in nurseries located in shady sites (for example under a tree), especially if the nurseries are used for long time.

Rats and mole rats occasionally feed on sweet potato storage roots either by digging through the ridges or accessing the exposed roots. They often spoil more roots than they actually eat. Rats and mice breed in burrows, destruction of these burrows can help reduce populations. Rodents like to hide in vegetation and rubbish, as they do not like crossing open spaces where they may be seen and exposed to predation, keeping the field and surrounding areas clean should reduce damage.

The nematodes belong to Meloidogyne spp.. Symptoms consist of poor growth, yellowing of foliage and or wilting of plants during dry hot weather. Root swellings or galls develop on feeder roots. Harvested roots often are misshapen, cracked, and rotted on the surface.

It is found in all sweet potato production areas in Eastern Africa, but is only considered an important pest in relatively dry areas. The adults are butterflies with orange wings with black margins. These butterflies are capable of flying distances of several kilometres. They lay small, pale yellow eggs in clusters on leaves. Caterpillars are greenish-black and are covered with short-branched spines. Fully-grown caterpillars are about 25 mm long. Caterpillars feed on leaves of sweet potato. Young caterpillars feed in groups on the upper leaf surface protected by a layer of webbing for the first two weeks.

Older caterpillars become solitary and nocturnal hiding on the ground during the day. They eat the whole leaf leaving only the midribs. Heavy attack may result on complete defoliation. Mature caterpillars crawl up supports such as tall grasses, leaves or walls near the sweet potato field in order to find a site to pupate in vertical position. The pupae are yellowish and hang singly on their support. The total lifecycle takes 27-50 days. Caterpillars are attacked by predatory ants, ladybird beetles, lacewings and dragonflies. The fungus Beauveria bassiana has been observed on caterpillars in the field during the rainy season.

Adults are large grey hawk moths with black lines on the wings and broad incomplete pink bands on the abdomen. The female lays small spherical greenish eggs singly on either surface of the leaves. Caterpillars have a conspicuous posterior horn. They are variable in colour, usually greenish or brownish. Fully-grown caterpillars are large (up to 9.5 cm long and 1.4 cm broad). They pupate in the soil.

Caterpillars feed on leaves, causing irregular holes. They may eat the entire leaf, leaving only the petiole. Insect frass can often be found near the infested plant part. One large caterpillar can defoliate a plant on its own. When older caterpillars are present in large numbers they can defoliate a field overnight. Yield losses can occur if heavy defoliation takes place when the crop is young. But, if the young plants are healthy and growing well, they can recover. However, damage to the leaves may delay harvest, increasing the likelihood of attack by the sweet potato weevil.

The sweet potato moth is a minor pest in West Africa. Damage to sweet potato plants results from the caterpillar boring into the main stem leading to the roots. Vines with severe tunnelling show weak growth and poor foliage development; this foliage later yellows and wilts. The distal part of the vine above the damage site often dies. Such plants show poor storage root formation. In some cases caterpillars may bore directly into storage roots.

This moth is a pest in Asia, but its status in East Africa is unknown.

This disease is caused by a combination of sweet potato feathery mottle virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV).

Symptoms include severe stunting of plants, the production of small distorted leaves, excessive branching, yellowing of vines and dark, brown to blackish corky spots in the roots. SPFMV is a potyvirus transmitted by aphids in a non-persistent manner whereas SPCSV is a crinivirus transmitted in a semi-persistent manner by the whitefly Bemisia tabaci.

The sweet potato weevil is the most destructive insect pest of sweet potato in the tropics and subtropics. No resistant source is available.

Farmer experience: In Kilifi, Kenya, farmers create planting mounds or ridges incorporating a good amount of fresh leaves of Lantana camara before planting sweet potatoes. This improves soil organic matter and at the same time serves as a repellent of the sweet potato weevil, thus improving both yield and quality of harvested tubers. The superior quality of sweet potatoes grown using Lantana leaves have been confirmed by Ministry of Agriculture staff in the area.

Occasionally termites can be a problem by attacking the seed pieces or the roots and stems of established plants. Yield losses can be very high. In Sudan losses of 18 % have been recorded and in Central Africa losses of 5-10 % are common. In Nigeria plant germination failure of up to 28 % has been reported. The most common damage to sugar cane is the destruction of the planting material (setts).

Adults are broadly oval and shield-like, 6-8 mm long, and may be brightly coloured. They lay eggs singly or in batches on the underside of sweet potato leaves; sometimes the eggs are covered by a papery layer. The larvae are oval, flattened and spiny. Some species of tortoiseshell beetle larvae hold their tails up over their back, usually with excreta and previous cast skins. The pupa is less spiny than the larva, and is fixed to the leaf.

Both larvae and adults feed on leaves. The young larvae scrape on the upper surface of the leaves leaving the lower surface intact, while older larvae and adults eat large round holes in the leaves. Severe attacks can sometimes skeletonise the leaves and peel the stems. The damage on leaves is conspicuous, but generally is not of economic importance.

Whiteflies feed on the lower leaf surface. Direct damage by adults and nymphs sucking sap from the plant, is generally not economically important. However, high numbers of whiteflies may affect plant development, particularly during period of water stress and drought. They are more damaging as vector of virus diseases.