Papaya

These are the most important aphids in papaya growing. These insects suck sap from young leaves and flowers and may weaken the plants. However, this type of damage is usually of little importance. Their importance as pests is mainly due to their ability to transmit virus diseases, for instance the papaya ring spot virus and the papaya mosaic virus. Few aphids are enough to transmit mosaic virus. These aphids are also found on other crops such as cucurbits, potatoes and tobacco.

Broad mites are tiny (0.1-0.2 mm long) cannot be seen with the naked eye, and are even difficult to detect with a hand lens. In addition, they often disappear before the damage is noticed. Therefore an attack by broad mites is usually detected by the symptoms of damage. Broad mite attacks mainly the growing point and the underside of young leaves causing hardening and distortion. Severe infestations inhibit new stem growth, with consequent reduction in fruit production. Broad mite damage may be confused with injury caused by some herbicides because in both cases the leaves become claw-like with prominent veins. Grey or bronze scar tissue between the veins on the underside of the leaves distinguishes mite from herbicide damage. For early detection of damage inspect the growing point of the tree regularly. This is important to allow control the pest before serious distortion of terminal growth occurs.

These are caused by the soil-borne fungi Phytophthora parasitica, P. palmivora and Pythium aphanidermatum. Phytophthora and Pythium also occur in the orchard, causing root rot.

Stem and fruit rots are produced on papaya. Infected young fruits develop water-soaked lesions that exude milky latex. These fruits may eventually shrivel and fall off the trunk. Infected trees show yellowing of leaves, which later collapse and hang limply around the trunk before falling. Small roots are generally absent and large ones show a soft, wet decay extending towards the trunk. At that stage, the lateral roots and taproots are entirely destroyed and a foul odour often emanates from diseased trees. Stem cankers, which develop most frequently in the top of the stem where the fruit is borne, induce fruit and leaf fall. These fungi may also cause trunk rot of mature trees. Infected trees eventually die.

Plants are susceptible at all ages but roots of young seedlings are most susceptible.

The adults are about 0.1 mm. It usually feeds on the trunk below the level where the bottom whorl of leaves is attached. The mites move upward on the trunk and outward onto the leaves and fruit as the population increases, leaving a large, conspicuous, damaged area behind them. The affected area becomes raised and blister-like. Later the affected tissue dries up, dies and becomes discoloured, forming a large and continuous callous area, light brown and scaly and/or scabby.

Damage by feeding on young papaya fruits is manifested by sunken areas. Sometimes feeding by the mite causes a copious outflow of a milky white liquid that mars the appearance of the fruits. Under heavy mite infestations, the papaya stem, which normally remains green for a long time, becomes brownish and corky in appearance, and has a spindly growth (Martin Kessing and Mau, 1992; CABI Compendium, 2000).

Two species of fruit flies have been recorded from papaya in East Africa, namely Bactrocera invadens and Ceratitis rosa (personal communication S. Ekesi, AFFI, icipe). The flies usually deposit their eggs in ripe fruit. Some fruit flies lay eggs on green pawpaw, but most of the eggs die due to the latex secreted when fruits are punctured by females while laying eggs. Developing larvae cause rotting of ripening fruits. Fruit flies are a major concern of papaya-importing countries.

Mealybugs are small, flat, soft bodied insects covered with a distinctive segmentation. Their body is covered with a white woolly secretion. They suck sap from tender leaves, petioles and fruits. Seriously attacked leaves turn yellow and eventually dry. This can lead to shedding of leaves, inflorescences, and young fruit. Mealybugs excrete honeydew on which sooty mould developed. Heavy coating with honeydew blackens the leaves, branches and fruit. This reduces photosynthesis, can cause leaf drop and affect the market value of the fruit.

A wide range of natural enemies attacks mealybugs. The most important are ladybird beetles, hover flies, lacewings, and parasitic wasps. These natural enemies usually control mealybugs. However, mealybugs can cause economic damage to mango when natural enemies are disturbed (for instance by ants feeding on honeydew produced by mealybugs or other insects) or killed by broad-spectrum pesticides, or when mealybugs are introduced to new areas, where there are no efficient natural enemies.

The latter is the case of two serious mealybug pests on mangoes in Africa: Rastrococcus invadens in West and Central Africa and Rastrococcus iceryoides in East Africa. These mealybugs, of Asian origin, were introduced into Africa, where they developed into serious pests since the natural enemies present were not able to control them. They cause shedding of leaves, inflorescences and young fruits. In addition, sooty moulds growing on honeydews excreted by the insects render the fruits unmarketable and the trees unsuitable for shading. They cause direct damage to fruits leading to 40 to 80% losses depending on locality, variety and season. Rastrococcus invadens was brought under control in West and Central Africa by two parasitic wasps (Gyranusoidea tebygi and Anagyrus mangicola) introduced from India (Neuenschwander, 2003).

Rastrococcus iceryoides is a major pest of mango in East Africa, mainly Tanzania and coastal Kenya. Although several natural enemies are known to attack this mealybug in its aboriginal home of southern Asia (Tandon and Lal, 1978; CABI, 2000), none have been introduced so far into Africa (ICIPE).

Insecticides do not generally provide adequate control of mealybugs owing to their wax coating.

It is a devastating virus disease. In Africa, it occurs in Kenya, Nigeria, Tanzania and Uganda. Initially, the disease appears as oil streaks on stems and petioles and as it progresses, mottling of leaves becomes evident. Severely infected plants do not flower and die young. Infected fruits develop characteristic line patterns, which form rings and remain green when fruits ripen. The virus is spread by aphids and it is also mechanically transmitted.

Young crown leaves show light green patches covered by a white powdery growth. Fruits develop circular, white patches on the surface. As the fruits develop, the white mould disappears leaving grey-scarred areas. The disease is particularly severe on immature tissue. The white mildew produced on leaves and fruits, contains large numbers of spores, which are spread to adjacent trees by wind and rain.

Several fungal pathogens are involved in fruit decay. They include Colletotrichum gloeosporioides (Glomorella cingulata), Alternaria tenuis, Phomopsis caricae-papayae and Ascochyta caricae.

Symptoms first appear as brown, superficial discolourations of the skin. These develop into circular, more or less sunken spots and tend to occur in a group on the outer exposed side of the fruit and often join to form a large rotted area extending deep into the flesh. The fungi causing ripe fruit rots live on dying leaf stalks and produce spores, which spread to the fruit particularly during wet weather.

Several of these fungi, especially, Colletotrichum gloeosporioides, may infect green fruit and remain dormant in the tissues until ripening when they develop rapidly. They constitute a big post-harvest problem especially during transport and storage.

Feeding root-knot nematodes cause root swellings or root galls, resulting in yellowing and premature abscission of the leaves. Infestation by nematodes reduces growth and yield. In nurseries, severely infested seedlings wilt and die.

Several species of mites damage papaya (personal communication, M. Knapp, ICIPE):

• Spider mites (Tetranychus spp., Eutetranychus spp. and Oligonychus gossypii)
• The false spider mite (Brevipalpus phoenicis)
• The broad mite (Polyphagotarsonemus latus)

The spider mites suck the plant sap, leading to poor plant growth and blemish on the fruit. Infested leaves show yellow patches on the upper surface, particularly between main veins and midrib. Feeding by mites causes scarring and discolouration of fruit, and reduced fruit size affecting its market value. Infestations usually begin on the older leaves and the spreads to the younger growth. Serious infestations occur during long dry periods. Broad mites attack mainly the terminal buds; they feed on the young leaves as they emerge from the growing point. Affected leaves are thick and brittle, with down curled edges. Severe infestations inhibit new stem growth, with consequent reduction in fruit production.

This is a very common weevil in East Africa. It attacks many crops and ornamental plants. The adult is a black weevil, about 12 mm long with a swollen, rounded abdomen, and long, thin, elbowed antenna. It is active at night, feeding on the edges of leaves producing a characteristic indentations. During the day it hides in the mulch, at the base of plants or in loose soil near plants. They feed on a wide range of crops and wild plants. They can be a problem to young papaya plants when present in large numbers.

Several species of whiteflies are found on cassava in Africa. Feeding causes direct damage, which may cause considerable reduction in root yield if prolonged feeding occurs. Some whiteflies cause major damage to cassava as vectors of cassava viruses. The spiralling whitefly (Aleurodicus dispersus) was reported as a new pest of cassava in West Africa in the early 90s.

The adults and nymphs of this whitefly occur in large numbers on the lower surfaces of leaves covered with large amount of white waxy material. Females lay eggs on the lower leaf surface in spiral patterns (like fingerprints) of white material secreted by the female. This whitefly sucks sap from cassava leaves. It excretes large amounts of honeydew, which supports the growth of black sooty mould on the plant, causing premature fall of older leaves.

The tobacco whitefly (Bemisia tabaci) transmits the African cassava mosaic virus, one of the most important factors limiting production in Africa. The adults and nymphs of the tobacco whitefly occur on the lower surface of young leaves. They are not covered with white material. The nymphs appear as pale yellow oval specks to the naked eye.