Bacterial wilt

Initially one of the youngest three leaves turns pale-green or yellow in colour and breaks down at the petiole and the pseudostem. Later all the other leaves collapse around the pseudostem. An infected finger or fruit shows dry and rotted pulp that is coloured brown or black, and the presence of bacterial discharges.

The bunchy top disease

It is a virus disease transmitted by aphids (for more information on aphids click here .) The typical symptoms of bunchy top of banana are very distinctive and readily distinguished from those caused by other viruses of banana. Plants can become infected at any stage of growth and there are some initial differences between the symptoms produced in aphid-infected plants and those grown from infected planting material.

In aphid-infected plants, a few dark-green streaks or dots usually appear on the minor veins and the midrib of the second leaf to emerge after inoculation. They are best seen from the underside of the leaf in transmitted light. The 'dot-dash' symptoms can sometimes also be seen on the petiole. The following leaf may display whitish streaks along the secondary veins when it is still rolled. These streaks become dark green as the leaf unfurls. Successive leaves become smaller, both in length and in width of the lamina, and often have chlorotic, upturned margins. The leaves become dry and brittle and stand more erect than normal giving the plant a rosetted 'bunchy top' appearance.
Suckers from an infected stool can show severe symptoms in the first leaf to emerge. The leaves are rosetted and small with very chlorotic margins that tend to turn necrotic. Dark-green streaks are usually evident in the leaves.
Infected plants rarely produce a fruit bunch after infection and do not fruit in subsequent years. Plants infected late in the growing cycle may fruit once, but the bunch stalk and the fruit will be small and distorted. On plants infected very late, the only symptoms present may be a few dark green streaks on the tips of the flower bracts.

Black leaf streak (Mycosphaerella fijiensis) (also called black Sigatoka)

It is a very serious disease that affects the leaves of banana. The spores of this fungus are carried in the wind. The spores germinate in moisture and infect the leaves. The lesions gradually grow larger and kill large areas of the leaf. This results in lower yields and causes the premature ripening of the fruit.

Fusarium wilt (Fusarium oxysporum f.sp. cubense) (also called Panama disease)

This is a soil-borne fungus that attacks the roots and blocks the vascular system in the banana, so that the plant wilts. Diseased leaves turn yellow from the margins, dry up and collapse leaving a skirt of dead leaves draped around the plant. There is no effective control for Fusarium wilt, which is spread on infected suckers and in ground water.

The cigar end rot disease (Trachysphaera fructigena)

It is a fungal disease that can attack the ripening fruit of banana, causing a dry rot of the flower end that produces an ash grey wrinkled lesion similar to the burnt end of a cigar. In storage or during transport the disease may progress to involve the whole fruit. Symptoms are: lesions, abnormal shape, visible mould.
The disease is usually of minor significance and seldom requires targeted control.

Anthracnose

It is an important post-harvest problem of bananas especially during transport and storage. On green fruit, pin-size brown or black sunken spots develop. Infection in young fruit is not always manifested until the fruit ripens, when black, round, slightly sunken spots appear. The centres of the spots become dark because of the formation of small black fruiting bodies of the fungus. Under moist conditions, masses of spores are produced having a characteristic salmon (pinkish) colour. Pulp of diseased fruit is usually not affected unless the fruit is over-ripe.

The burrowing nematode (Radopholus similis)

The burrowing nematode is the most destructive nematode species attacking bananas. Dark patches or spots on the roots indicate nematode infestation. Severely infected plants may show only stubs of rotted roots and may fall down when the bunch has formed. Bananas are also attacked by other nematode species such as Pratylenchus spp., Helicotylencus spp. and Meloidogyne spp.

Banana weevil borer (Cosmopolites sordidus)

The banana weevil is the most important insect pest of banana. It is about 1-1.5 cm long. The larva (grub) is most destructive: It bores irregular tunnels in the rhizome/corm and pseudostems at ground level. A large proportion of the tissue is destroyed, this reduces the amount of water and nutrients the plants can take up as well as lessening their anchorage. Heavy infestation may kill young plants. Older plants are easily blown over by the wind.

Banana silvering thrips (Hercinothrips bicinctus)

Banana-silvering thrips are small (1.5 mm long), slender, brown insects with pale yellow hind wings that appear as a yellow line down the back of the body when the insect is at rest. Adult thrips have characteristic wings; the transparent wings have a fringe of hairs around the outside edge standing out in the same plane as the wing. The tiny eggs are laid just into the plant tissue on the pseudostem or where 2 fruits touch. The nymphs hatch after 7 or 8 days, are pale yellow or white in colour and often have a black globule of excrement at the end of the abdomen. The larval stage lasts about 10 days after which the nymphs move down into the soil to pupate. Adults emerge after a further 7 to 10 days.

The banana silvering thrips is found in tropical and subtropical regions, they are polyphagous and particularly found on bananas. They feed on the skin of the fruit causing silvering patches (hence the common name of the insect), which turn brown and may cover the whole fruit and deep longitudinal cracks may appear as a result.

Fruit flies (Bactrocera invadens and Ceratitis rosa)

Two species of fruit flies (Bactrocera invadens and Ceratitis rosa) attack banana in Kenya. Bactrocera invadens is a new species recently discovered in Africa, its 2-3mm in size. This fruit fly is reported attacking banana in Sudan and Kenya and it is a major threat since it leads to rejection of banana in the export market.

One of the most effective control techniques against fruit flies in general is to wrap fruit, either in newspaper, a paper bag, or in the case of long/thin fruits, a polythene sleeve. This is a simple physical barrier to oviposition but it has to be applied well before the fruit is attacked. Little information is available on the attack time for most fruits, but few Bactrocera spp. attack prior to ripening.

Infected fruit on the ground will act as reservoir for re-infestation. If infested fruit reaches the market then maggot infested fruit will be discarded, so allowing emerging adults access to new crop areas.

The banana aphid (Pentalonia nigronervosa)

It is a small aphid about 1-2 mm long and blackish-brown in colour. Colonies are usually present on the base of young leaves. The direct damage caused by aphids sucking the plant sap is negligible. However, they are important pests as vectors of the virus causing the bunchy-top disease.

Large colonies of aphids can occur around the base of pseudostems, down to 7-8 cm below the soil surface. Dense colonies can also occur between the sheath of outer leaf and pseudostem. During drought, aphids seek sheltered locations on the plant. In dull and humid weather, however, aphids may spread to foliage generally, and to the bases of maturing hands of fruit and all over the hands of young fruit.

Colonies of P. nigronervosa are attended by ants, which feed on the large quantity of honeydew produced. Many species of ant are involved worldwide. Stechmann et al. (1996) described how ant-attendance reduced the density of indigenous predators of P. nigronervosa considerably, which has implications for biological control (Wellings et al., 1994). Ants also transport aphids from plant to plant, establishing new infestations.

P. nigronervosa is not normally found on plantains, suggesting that they may have resistance to the aphid.