PAWPAW (Carica papaya)

The pawpaw is a native of tropical America. It belongs to the family Caricaceae and is a widely cultivated fruit tree in the tropics and subtropics. The tree is 2-10 metres in height. Stem suckers often develop but branch only when the terminal bud is damaged. The trunk contains soft fibrous wood. The bark is pale grey, smooth, well marked with leaf scars. The sweet edible flesh bears many black seed on the inside leaving the centre hollow.

Pawpaw tree with fruits

A pawpaw tree with fruits

Tree types

The pawpaw has three types based on the flowers are arranged; female, male and hermaphrodite. These can only be differentiated after it flowers. The characteristics are described below:

Pistillate (Female plants)

They produce female flowers only. These are large, situated singly or in clusters near the trunk of the tree and close to the base of the leaf stalk. When the female flower is opened it will be found that each petal is distinct as it rises from the base of the ovary. There are no stamens, so cross pollination is necessary for the fruit to set. They produce good fruits.

Staminate (Male plants)

They produce only male flowers, which are borne in large numbers in long, drooping branches of up to 1m in length. The true male flower produces pollen only and it is essential for the development of fruit from the female flower. Occasionally male plants will produce 2 hermaphrodite flowers and set fruit on the ends of the branches; however, these fruits are usually of poor quality or no fruit production.

Hermaphrodite plants

They produce flowers which have both male and female parts and are capable of self- pollination. The flower closely resembles the female flower, except that the petals are fused together at the base of the ovary and there are short stamens present. Bisexual fruits are usually long and narrow, and although the eating quality may be excellent, the shape presents some problems for commercial handling.

Varieties

Most of commercial varieties grown in Kenya are derived from Hawaii. A few are from India and some known as ‘Mountain varieties’.

Name Description
·         Honey Dew This is an Indian variety of medium height that produces oval juicy medium size fruit.
·         Kiru

·

This is a Tanzanian variety that produces large fruits.

It is a high yielder of papain.

·         Mountain A variety that grows at high altitudes with very small fruits only suitable for jam and preserves.
·         Solo

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This is a Hawaiian variety that produces small round very sweet fruits with uniform size and shape.

It is hermaphroditic. It is popular for both export and local markets.

·         Solo Sunrise

·

Hawaiian variety that produces smooth pear shaped fruit of high quality, weighing 400 to 650 grams.

The flesh is reddish orange. This variety is high yielding.

·         Sunset

·

·

Hawaiian variety is a dwarf high yielding plant with red flesh and having same characteristics as ‘Solo’ variety. It is very sweet.
·         Solo sunset The fruit is red/pink in color and very sweet. Average fruit size is 425 grams. Hermaphrodite fruit is pear shaped, with a small neck at the stem end. The quality of the fruit and firmness of the shell is similar to Sunrise. Fruit sets at average 1 meter above the ground.
·         Sunrise (Strawberry pawpaw). Has a deep red-flesh. Variety is resistant to the ring spot virus disease. Fruit has a freckled greenish-yellow skin that turns yellow as the fruit ripens.
·         Waimanalo

·

Another Hawaiian variety that produces smooth, shiny round fruits with short neck and is of high quality.

The flesh is orange yellow, thick, sweet and firm. It is recommended for fresh market & processing.

·         Kapoho Solo type with yellow to orange flesh colour.

Fruits are smaller than that of sunrise.

·         Mexican Red A rose-fleshed pawpaw that is lighter in flavor.

It is a medium to very large fruit.

Generally it is not as sweet as Hawaiian types.

 

 

 

Ecological Requirement

Altitude

They grow best in areas below 1500m asl. The quality and yield are low at higher altitudes.

Rainfall

Evenly distributed annual rainfall of above 1000mm. Pawpaws cannot withstand prolonged drought. In low rainfall areas irrigation is necessary to facilitate vigorous growth.

Soils

Pawpaw grows best in light, well-drained soils rich in organic matter with soil pH of 6.0-7.0. It can tolerate any kind of soil provided it is well drained and not too dry. The roots are very sensitive to water logging and even short periods of flooding can kill the plants.

Temperature

Pawpaw thrives in warm to hot areas with adequate rainfall and a temperature range of 21-33°C. Fruit tastes much better during warm sunny seasons. Cold temperatures below 120 C cause the tree to grow poorly. Frost can kill the plant; Cool and overcast weather delays fruit ripening and depresses fruit quality.

Land preparation

The field should be cleared of bushes, tree stumps and leveled out. Deep soil cultivation is recommended thus ploughing and harrowing should be done before planting.

 

 

Propagation

Pawpaw is propagated by seed. To reproduce the desired characteristics it is best to get seeds through controlled pollination. Seeds are extracted from healthy fruits selected from healthy plants. The fleshy outer layer of the seed coat (sarcotesta) enveloping the seed is removed because it inhibits germination. This is achieved by rubbing the seed together against a fine-meshed screen under running water.

Thoroughly dried seeds stored in air-tight containers remain viable for several years. The seed may be sown directly in the field or nursery beds, seed boxes or polythene bags.

Raising seedlings in seed bed

  • Prepare either raised or sunken beds 1m wide and of convenient length.
  • Water the beds thoroughly before and after sowing until seeds germinate.
  • Sow seeds at a depth of 1cm in rows that are 15 cm apart.
  • Seeds germinate in 2-3 weeks.
  • Continue watering until they are ready for transplanting in containers.
  • Prick out at the 2-3-leaf stage, transferring 3-4 seedlings to each container 1 week after emergence.

Raising seedlings in containers

Plants propagated in containers are less susceptible to stress and subsequent loss than those produced in seed-beds.

  • Seeds are sown in small containers (tin cans, plastic bags or paper cups) at the rate of 3-4 seeds per container.
  • Use of sterilized soil minimizes losses resulting from nematodes and damping-off disease.
  • Germination takes 2-3 weeks.
  • Seedlings are transplanted about 2 months after sowing when they reach the 3-4-leaf stage or 20cm height, preferably at the onset of the rainy season. During transplanting, care must be taken not to disturb the roots. Older seedlings recover poorly after planting out.

Direct Planting

  • Sow 6 seeds per planting hole.
  • Thin to 4 plants after germination and later to 1 plant per hole after it is possible to establish the sex of the plant.
  •  For hermaphrodite varieties, plant 2 seeds per hole and thin to 1 seedling per hole after 1 month.

Field establishment

Dig Planting holes of 60 x 60 x 60cm.

Spacing

Spacing    No. of plants/hectare
2.5 m x 2.5 m 1,600
2.5 m x 3 m 1,332
3 m x 3 m 1,110

 

Transplanting in the field

Transplant seedlings to the field 4-5 weeks, when they are 10-20cm in height. Young plants have higher survival rates than older plants.

  • Mix 40kg of well rotten manure with 100-120g of DAP and the top soil and fill the hole with the mixture.
  • Plant 4 seedlings per hole for varieties that have male and female flowers on different plants.
  • Thin to one female or one hermaphrodite plant per hill when the plants reach the flowering stage (after 6-8 months). In the absence of hermaphrodite plants, 1 male plant per 25-100 female plants is retained as pollinator. Most of the male plants should be removed since they bear no fruit.
  • For hermaphrodite varieties plant 1 seedling per hole.
  • Transplanting should be at the same depth as the seedlings were previously growing; deeper planting may cause some loss from stem rots.
  • Plants that have been grown in seed beds should, in addition to being hardened off (reduce irrigation), be provided with some simple cover when transplanted to the growing site.
  • It is good practice to reduce the leaf area of the plant at transplanting, so that transpiration is reduced while the root system is re-established. If possible planting should be done in the late afternoon.
  • Plants propagated in containers are less susceptible to stress and subsequent loss than those produced in seed-beds. However, these will still require attention until they are established.

Whatever method is used, seedlings should be planted into moist soil and watered as soon as possible afterwards. Daily watering will be required for some time.

Wind break

Paw paws do not tolerate strong winds and the establishment of wind breaks hedges is necessary.

Mulching

This is recommended after sowing or transplanting. The mulching should not be so close to the stem of the seedlings, give an allowance of about 10cm from the stem. The mulch should be removed so that the development of the young plant is not hindered.

 

 

Weed control

Frequent hand weeding is essential and care need to be taken not to damage the roots of the young plants. Keep the area around the stem weed free. Slashing is not enough as the seedlings will still suffer from weed competition.

Intercropping

Pawpaw grows best when planted under full sunlight. Intercropping can be done when paw paws are young. Low growing annual crops such as capsicums, beans, onions and cabbages are suitable intercrops.

Manure and fertilizer

Apply manure at the rate of 40kg per tree before the rains begin every year.  It should be applied around the plant basin and incorporated well into the soil.

Apply 40g of CAN per tree two months after transplanting in the first year. If possible apply a split application of 60g CAN per tree at the beginning of the long and short rains. After which 200g of compound fertilizer can be applied per tree per year at the beginning of the rains.

Major Pests

Pests Description and damage Control
Fruit flies (Bactrocera invadens) , (Toxotrypana curvicauda) and (Ceratitis rosa) 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 pawpaw importing countries.

·       Fruit should be harvested at the mature green stage. Over ripe and infested fruit should be buried.

Use Recommended insecticides e.g.  Decis 2.5EC,

Red Spider Mites

(Tetranychus spp.)

The mites are tiny and brownish-red in colour.

They suck the plant sap, leading to poor plant growth and blemishes on the fruit.

Infested leaves show yellow patches on the upper surface, particularly between main veins and midrib.

Feeding by mites causes scarring and discoloration 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.

·       Natural pests normally keep the mites under control.

·       Indiscriminate use of chemicals results in mite build up.

·       Burn all previously infested plants before planting a new crop.

·       Spray Omite, Talstar 50EC, Kelthane,  Dynamec 1.8 EC

Systates weevil (Systates spp.)  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 characteristic indentations.

During the day it hides in the mulch, at the base of plants or in loose soil near plants.

They can be a problem to young pawpaw plants when present in large numbers.

·       Effectively controlled by natural enemies. Can also be controlled by Hand picking

Use recommended insecticide e.g. Actara,  Confidor

Root-knot nematodes (Meloidogyne incognita)

 

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. Affected roots rot.

·         Establish pawpaws in nematode free land,

·         Do not replant pawpaw in the same field.

Thrips

(Scirtothrips spp.)

 

They pierce and suck the sap from the leaves.

Infested leaves become distorted and show silvery, whitish, sunken areas which turn brown and dry up.

·         Chemicals to use may include Decis, Dynamec, Bulldock Star
Birds

 

They feed on the ripe fruit. ·         Harvest when the fruits are physiological mature or starts to show signs of ripening. Leave fruits to ripen under shade.
Aphids (Aphis spp.)

 

These are small pear shaped soft insects that are dark grey, dull black or light to dark green in colour.

They occur in small colonies sucking sap of young tissue.

·         Use recommended insecticides e.g. Atom 2.5EC, Decis, Evisect S, Karate, Achook EC Avoid planting cucurbits, potatoes and tobacco near pawpaw fields.

 

Major diseases

Diseases and causal agent Description and symptoms Control
Damping-off and Foot rot (Phytophthora parasitica, P. palmivora and Pythium aphanidermatum. Phytophthora  and Pythium)

 

These are caused by soil-borne fungi, and result to rotting of roots, stem and fruits.

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 fail to develop and large ones show a soft, wet decay extending towards the trunk.  At this 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.

Good drainage and field hygiene are important for control in the orchard as well as in the nursery.

Do not replant pawpaw on the same land.

Use of Copper based fungicide treatments at beginning of first symptoms can reduce fruit rots.

Avoid damaging trunks during cultivation.

Raise seedlings in disease free seed beds.

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

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 rotten 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.

·         A 20-minute hot-water dip at 45°C reduces post-harvest decay.

·         Destroy rotten fruit.

·         Early harvesting.

Powdery mildew

(Oidium caricae; Sphaerotheca humili)

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.

·         Collect and destroy fallen diseased leaves. Spray wettable sulphur. However, sulphur should not be applied when it is very hot as it may cause leaf scorch. Alternatives to the use of sulphur are baking powder, neem extracts
Papaya ring spot virus (PRSV)

 

 

The virus is spread by aphids and it is also mechanically transmitted.

It is a devastating viral disease.

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 they die young.

Infected fruits develop characteristic line patterns, which form rings and remain green when fruits ripen.

·         Destroy infected plants.

·         Avoid interplanting with cucurbits, which are also a host of PRSV.

·         Use tolerant cultivars e.g Sunrise. Control aphids.

·         Avoid mechanical injury to pawpaw trees.

Anthracnose

(Colletotrichum spp.)

A fungal disease that attacks fruits and stalks of lower leaves.

The attack becomes evident by small, water-soaked areas on ripening portions of the fruit.

More infestation causes fruits to rot and makes them unmarketable.

·         The problem can be reduced if the fruits are immersed in warm water at

·         400 C for 20 minutes.

Papaya mosaic/papaya decline disease This is a viral disease that stunts the plant and leaves it with few leaves. Young leaves may have discoloured veins.

When the infection is severe the leaves become distorted or wrinkled and occasionally have yellow spots.

The main stem becomes distinctively pointed at the tip, fruits drop and eventually the plant dies.

·         Destroy plant residues.
Black speckle This is caused by fungus.

It develops as small black spots on both fruit and mature leaves.

Young leaves are not affected.

Diseased leaves wilt and drop.

·         Field sanitation.

·         Keep the orchard weed free.

·         Watering is necessary in early months of development. Mulching is also recommended.

·         The trees can be inter-cropped with low annual crops like capsicum, beans and onions.

Black rust

(Asperisporium caricae)

It appears as black spots on underside of leaves and on fruits. ·         Control by use of Antracol, Amistar Mancozeb alternated the fungicides.
Rhizopus  (Rhizopus stolonifer)

 

It is a common postharvest disease of the pawpaws and is important only during storage and transit.

The spores are air borne and can be found throughout the orchard and packing houses.

The spores can only enter through damaged skin that may be caused by poor postharvest handling

It is characterized by a soft and watery rot that quickly collapses the entire fruit but leaves the cuticle intact. The fungus can grow out through any break in the cuticle and spread rapidly to adjacent fruits, often destroying the entire contents of a box within a few days. The infected fruit is often covered by coarse, gray, hairy mycelia that form a mass of black sporangia at their tips. The affected fruit becomes quickly colonized by yeasts and bacteria and emanates a sour odor

·         The most important control measure is sanitation in and around the packing area.

·

·

Black spot (Cercospora papaya)

Fruit spots start as tiny black dots that eventually enlarge to about 3 mm in diameter. The spots are superficial, slightly raised, a result of the tissue beneath the epidermis becoming corky, and do not develop into a fruit rot. The spots are somewhat obscure on green fruits but become readily visible when the skin color turns yellow as the fruit ripens. Actual damage to fruits is minor except its impact on their appearance and marketability. ·         Fruit spots start as tiny black dots that eventually enlarge to about 3 mm in diameter. The spots are superficial, slightly raised, a result of the tissue beneath the epidermis becoming corky, and do not develop into a fruit rot. The spots are somewhat obscure on green fruits but become readily visible when the skin color turns yellow as the fruit ripens. Actual damage to fruits is minor except its impact on their appearance and marketability.

Harvesting

  • Pawpaw starts flowering after 6-8 months and fruits are ready for harvesting 8-10 months after trees have been planted.
  • The physiological development stage of fruit at the time of harvest determines the flavour and taste when the fruit is ripened.
  • Fruits harvested too early have longer post harvest life, but give abnormal taste and flavour. Fruits also tend to shrivel and suffer chilling injury when refrigerated.
  • Harvest when at least 2 yellow strips appear between the ridges of the fruit.
  • Harvest by cutting fruits from the tree, leave a stem 0.5-1 cm long on the fruit. The stem can later be reduced when packing the fruits.
  • Avoid latex flowing on the fruit by placing the stalks of harvested fruits facing downwards.
  •  Place the fruits gently into the harvesting crates.
  • It is better to harvest the fruits early and leave them to ripen in shade.

 

 

Stages of fruit ripening

1/4 ripe              1/2 ripe              3/4 ripe             fully ripe

Yields

Under good conditions fruiting starts within a year from planting and continues with a yield of 30-150 fruits per tree during its life span. A yield of 35-50 ton/ha can be realized per year. The fruits are available throughout the year provided water is adequately available. A pawpaw plantation can be productive for 4-5 years but the economical period is 3-4 years.

Post harvest handling and marketing

The fruits should be handled carefully and should not be stored for many days.

They should be sorted, cleaned and graded for the market.

Utilization

The fruit can be eaten whole or mixed in a fruit salad as it has considerable nutrient value, being rich in vitamins A, B1, B2 & C, iron and potassium.

  • It can be used to make ice-cream flavour, refreshing drinks, jam, jelly, wine, marmalade, candies and crystallized fruits.
  • Green fruits are pickled or cooked as a vegetable.
  • Young leaves are sometimes eaten as vegetables.
  • In some countries pawpaw is grown in sizeable plantations for the extraction of papain (a proteinase) an enzyme present in the latex, collected mainly from the green fruit.
  • Papain has varied uses in the beverage, food and pharmaceutical  industries: in chill-proofing beer, tenderizing meat, and drug preparations

 

Papain production

Latex is collected by tapping the green unripe fruit which are 10cm or more in diameter. Fruits may be tapped once a week, until they show signs of ripening. The operation is best done early in the morning because the latex flows slowly in hot weather. Tapping results in ugly scars on the fruit, although quality is unaffected. Tapped fruit can be processed or used as animal feed. The papain producing trees are productive for 2 to 3 years, with the first 2 years being the most productive. If kept longer production is uneconomical.

Marketing

Fruits can be sold in both local and export markets