Pomology/Papayas

FAMILY

Caricaceae

SCIENTIFIC NAME

Carica papaya L.

ORIGIN

American tropical lowlands

PAPAYA

Papaya, (Carica papaya), also called papaw or pawpaw, succulent fruit of a large plant of the family Caricaceae. Though its origin is rather obscure, the papaya may represent the fusion of two or more species of Carica native to Mexico and Central America. Today it is cultivated throughout the tropical world and into the warmest parts of the subtropics. The papaya fruit is slightly sweet, with an agreeable musky tang, which is more pronounced in some varieties and in some climates than in others. It is a popular breakfast fruit in many countries and is also used in salads, pies, sherbets, juices, and confections. The unripe fruit can be cooked like squash.

PHYSICAL DESCRIPTION

The papaya plant is considered a tree, though its palmlike trunk, up to 8 metres (26 feet) tall, is not as woody as the designation generally implies. The plant is crowned by deeply lobed leaves, sometimes 60 cm (2 feet) across, borne on hollow petioles (leaf stalks) 60 cm long. Normally, the species is dioecious, male and female flowers being produced on separate plants, but hermaphroditic forms are known, and numerous irregularities in the distribution of the sexes are common. Male flowers are borne in clusters on stalks 90 cm long; the flowers are funnel-shaped, about 2.5 mm (0.1 inch) long, and whitish, with 10 stamens in the throat. The female flowers are considerably larger, on very short stalks, and often solitary in the leaf axils; they have five fleshy petals that are united toward the base and a large cylindrical or globose superior ovary that is crowned by five fan-shaped sessile stigmas. The fruit is commonly spherical to cylindrical in form, is 75 to 500 mm (3 to 20 inches) or even more in length, and sometimes weighs as much as 9 to 11.5 kg (20 to 25.5 pounds). The very juicy flesh is deep yellow or orange to salmon-coloured. Along the walls of the large central cavity are attached the numerous round, wrinkled black seeds. The unripe fruit contains a milky juice in which is present a protein-digesting enzyme known as papain, which greatly resembles the animal enzyme pepsin in its digestive action. This juice is used in the preparation of various remedies for indigestion and in the manufacture of meat tenderizers. Papaya is a short-lived perennial growing to 30 ft (9.14 m) high. Its hollow, herbaceous stem is usually unbranched. The deeply lobed, palmate leaves are borne on long, hollow petioles emerging from the stem apex. Flowers occur in leaf axils. Older leaves die and fall as the tree grows. Papaya flowers are fragrant and have five cream-white to yellow-orange petals 1 to 2 in (2.5 to 5.1 cm) long. The stigmatic surfaces are pale green, and the stamens are bright yellow. Papaya fruits are smooth skinned. They vary widely in size and shape, depending on variety and type of plant. Hermaphrodite plants of commercial 'Solo' varieties in Hawaii usually produce fruits that are pear shaped and weigh approximately 12 to 30 oz (340 to 851 g). Female plants of 'Solo' varieties produce round fruits. Other papaya varieties produce variously shaped fruits, which may weigh up to 20 lb (9.1 kg). The fruits usually contain many seeds surrounded by a smooth yellow to orange-red flesh that is sweet in good varieties. Flower type. Flower type is determined by the presence or absence of functional stamens (male parts) and stigma and ovary (female parts). Within varieties, flower type is usually identified by flower size and shape. Female flowers are relatively large and rounded at the base. They have a stigma but lack stamens. They generally must receive pollen in order to set fruit. Pollen can be carried by wind or by insects. Male flowers are thin and tubular. They have perfect structure (i.e., they contain both male and female organs), but the small, vestigial ovary is nonfunctional. Male flowers are usually borne on a long flower stalk (peduncle). Hermaphrodite flowers are intermediate between female and male flowers in size and shape. They are less bulbous than female flowers, but not as thin as male flowers. They have perfect structure with functional stigma and stamens and usually are self-pollinating. Plant type. Three types of plants are recognized based on flower type: female, hermaphrodite, and male. Female plants always produce female flowers. If no male or hermaphrodite plants are nearby to provide pollen, female plants usually fail to set fruit. Unpollinated female plants occasionally set parthenocarpic fruits, lacking seeds. Male plants are distinguished by their long flower stalks bearing many flowers. Usually they do not produce fruit, but on rare occasions there is female expression in the flowers, and they may set fruits . Hermaphrodite plants may have male flowers, hermaphrodite flowers, or both, depending on environmental conditions and the time of year. Hot, dry weather may cause suppression of the ovary and the production of female-sterile (i.e., male) flowers. This accounts for occasional seasonal failure of hermaphrodite plants to set fruit. Male flowers on hermaphrodite plants are borne on short peduncles. Hermaphrodite plants tend to produce self pollinated seeds, which result in relatively uniform progenies. Seeds from hermaphrodite plants of 'Solo' varieties characteristically produce one-third female and two-thirds hermaphrodite plants, but no male plants are produced. Although hermaphrodite and female plants have similar fruit texture and quality, female plants may be less productive, and the round female fruits are marketed only in small quantities in Hawaii as direct sales from growers to consumers. Female plants are removed from commercial orchards as soon as they can be distinguished at flowering. In home gardens, female plants may be kept if hermaphrodite or male plants are nearby to serve as a pollen source.

CULTIVATION

Papayas are usually grown from seed. Their development is rapid, with fruit being produced before the end of the first year. Under favourable conditions, a plant may live five years or more. The papaya ringspot virus nearly wiped out papaya crops around the world, first hitting Hawaiian plantations in the 1940s and soon spreading. A genetically modified (GMO) variety named the Rainbow papaya was developed in the early 2000s with resistance to the virus. It was one of the first GMO fruits in commercial production, and the majority of exported papayas are now GMO crops.

VARIETIES

The 'Solo' variety is valued for its productivity, uniform fruit shape and size, and excellent fruit quality. 'Solo' strains are predominantly self-pollinated and thus are highly inbred and uniform. Three 'Solo'-type varieties are grown commercially in Hawaii. The most important is 'Kapoho', which has yellow-orange flesh and fruits that weigh 12 to 22 oz, considered an ideal size for export. 'Kapoho' is adapted to the Puna district of the island of Hawaii, where approximately 90 percent of the state's papayas are grown. The 'Sunrise' variety, commercially grown primarily on Kauai, has reddish-orange flesh and larger fruit than 'Kapoho'. 'Sunrise' is grown and marketed on a large scale overseas, but in Hawaii its production and export are small compared to those of 'Kapoho'. The 'Waimanalo' variety, which has yellow-orange flesh and somewhat larger fruit than other 'Solo' papayas, is grown and marketed almost entirely on Oahu.

USES

Ripe papaya is usually consumed fresh as a breakfast or dessert fruit; it can also be processed and used in a variety of products such as jams, fruit juices, and ice cream. Papaya is also consumed as a dried fruit. Culled fruits can be fed to pigs and cattle. Papaya is an ingredient in a variety of cuisines throughout the world. Unripe fruits and leaves are consumed as vegetables. Papaya seeds are also used as an ingredient in salad dressings. Papain is a milky latex collected by making incisions in unripe papayas. The latex is either sun-dried or oven-dried and sold in powdered form to be used in beer clarifiers, meat tenderizers, digestion aids, wound debridement aids, tooth-cleaning powders, and other products. The ‘solo’ papaya is not a good variety for papain production due to its low yield of papain. The papaya fruit is about 88.8% water, 9.8 % carbohydrate, 0.8% fiber, 0.6% protein, 0.6% ash and 0.1% fat. A 100g (3.5 oz.) serving of papaya has 39 calories, compared to banana's 92 calories. Papayas also contain 16% more vitamin C than oranges and are a good source of vitamin A (about half of that contained in mango). Consumption of the fruit is reported to aid digestion because of the papain content.

PROPAGATION

Papaya is grown from seed. Dry seed may be stored for a year or more in airtight refrigerated containers. Fresh seeds will usually germinate in 10 to 14 days. Germination can be improved by removing the gelatinous outer seed coat (sarcotesta) before drying. Seeds are sown either in containers or directly in the ground. Transplanting container-grown plants is usually limited to areas where there is dependable rainfall or supplemental irrigation. When direct-sowing, 10 to 15 seeds are sown 1/4 to 1/2 in (63 to 127 mm) deep in each planting hole. To ensure adequate stands in lava lands, approximately 0.5 cubic feet (14.16 cubic dm) of soil should be placed in each planting hole. The soil helps to retain fertilizer nutrients and moisture.

SOIL TYPES AND LOCATION

Papaya grows well on many types of soil, but they must be adequately drained. Restricted soil drainage promotes root diseases. Most commercial production in Hawaii is on porous a lava. Production on other soil types is limited to low rainfall areas where restricted drainage is less likely to cause problems. Heavy clay and pahoe lava soils should be avoided. Soil pH near neutral (pH 6.0 to 7.0) is preferred. Soil categories used for commercial production vary from island to island. On Hawaii, both a lava and soils are used. On a lava lands, additional soil is usually brought in and placed in the planting hole. The soils used for papaya on the island of Hawaii have a high preplant phosphorus fertilizer requirement. These soils, as well as the organic soils derived from lava, are usually acidic, and liming may be necessary. On Kauai the soils used for papaya usually require liming and high levels of phosphorus fertilization. On Oahu the soils on which papaya is grown are often poorly drained. Papaya grows best in warm areas below 500 ft (152 m) elevation. Fruit production and quality decline at higher elevations, where cooler temperatures cause flower drop and cat-faced (carpelloidic) fruits. (Carpelloidy is the abnormal development of stamens into fleshy structures.) Papaya can tolerate moderate winds if well rooted. Forty to 60 in (102 to 152 cm) of rainfall evenly distributed throughout the year is adequate for growth. With higher rainfall, soils should be porous and well drained. Planting on slopes or on raised hills helps to prevent waterlogging.

CULTURAL PRACTICES

Seedlings are thinned to five or six plants per hole six weeks after planting. Six weeks later, three vigorous seedlings per hole are selected, and the others are removed by cutting them off at ground level. When flower buds appear five to seven months after planting, the female plants can be distinguished and removed, keeping one hermaphroditic plant per hole. Tree spacing varies from 5 to 7 ft (1.52 to 2.13 m) between plants in rows 8 to 11 ft (2.44 to 3.35 m) apart. Fertilizer schedules vary with soil type, climate, and season. The general suggestions for fertilizer applications given here are based on commercial practices but can be used by the home gardener. In commercial plantings, fertilizers are either broadcast onto the soil surface within the leaf drip zone or applied through drip irrigation lines. In home gardens, fertilizers are usually broadcast on the soil surface. Organic matter mixed with the soil before planting and applied afterward to the soil surface as mulch helps to ensure good rooting conditions and a supply of micronutrients. Preplant fertilizer applications. On acid soils with a pH of less than 5.5, a preplant lime application is usually recommended to raise the pH and provide calcium. Dolomite, which provides both calcium and magnesium, may be substituted for part of the lime requirement. These soil amendments should be thoroughly mixed with the soil in the rooting zone before planting Phosphorus applied before planting should be thoroughly mixed with the soil or placed in a band away from the seeds. Application rates depend on soil type. For soils that do not have a high phosphorus requirement, a preplant application of 4 oz (113 g) of 0-46-0 per planting hole is recommended. Preplant applications of complete fertilizers are usually not necessary when planting papaya, because the developing seedlings can initially use nutrients stored in the seeds. Soluble fertilizers placed in the planting hole may burn the roots and make them susceptible to root rots. Slow-release fertilizers generally do not burn roots when mixed with the soil, and in commercial practice they are applied on the soil surface when the seeds are planted. Postplant fertilizer applications. Moderate applications of slow-release fertilizers are recommended during the first few months of growth. Commercial growers apply 0.5 oz (14 g) of slow release fertilizer per hole at planting and again six weeks later. Postplant fertilizer applications usually begin three months after planting with rates increasing with plant size. Maximum rates are applied at five- to eight-week intervals after the plants begin to flower. A typical application schedule with a fertilizer such as 14-14-14 begins with 4 oz (113 g) per plant per month during the third through fifth months after planting. This rate increases to 6 oz (170 g) every five weeks after the sixth month. During winter months, when growth and fruit production are slower, quantities of fertilizer applied should be reduced. In the absence of rain or irrigation, repeated fertilizer applications can lead to buildup of undissolved fertilizers, which may release high concentrations of nutrients into the root zone with the next rainfall. Excessive N will cause excessive vegetative growth and is also believed to contribute to soft-fruit problems. Papayas planted in replant fields often succumb to diseases caused by accumulation of soil-borne pathogens. A fallow period of three to five years may help to avoid the replant problem. Preplant soil fumigation is an alternative to the fallow period. On lava soils, the "virgin soil" technique can be used. Soil is collected from land where papayas have never been grown, and approximately 0.5 cubic feet (14.16 cubic dm) of soil is placed in holes that are at least 1 ft (30.5 cm) in diameter and 4 to 6 in (10.2 to 15.2 cm) deep. For best growth of young plants and good yields of bearing plants, papayas should be irrigated as necessary to supplement rainfall. When rainfall is limited, commercial growers may apply up to 10 gal (37.851) of water per tree per day to bearing trees. Yields can also be increased by good weed control practices.

HARVEST AND POST-HARVEST

Papaya fruit should be harvested after color break - when some yellow shows on the fruit - but before fully yellow (about 9 to 14 months after planting). Fruit for home consumption is best harvested when half yellow. If left to ripen on the tree, the fruit is often damaged by fruit flies and birds. When papaya plants are short, fruits can be harvested by hand while one is standing on the ground. As the plants grow taller, harvesting aids are required. One technique uses a modified plumber’s helper to snap the papaya from the stem. The fruit is caught before it falls to the ground. One person can harvest about 800 to 1000 pounds per day with this technique. Another technique involves a platform rigged to a tractor, which lifts the workers. This method requires flat terrain. To be certified for shipment, fruit for export must be harvested and packed in strict compliance with quarantine regulations. Harvested fruit is ripened at room temperature and refrigerated when fully ripe. When ready to eat, fruit is usually partially to fully yellow and slightly soft; refrigeration can extend its storage life for several days. In commercial situations, the storage life of partially ripe fruit can be extended for up to two weeks by holding it at about 50 F (10 C), but lower temperatures may interrupt the ripening processes and cause injury; the optimal storage temperature for fully ripe fruit is about 36 F (2.2 C). Papayas must be treated to kill any eggs or larvae of fruit flies that may be present in the fruit. Treatments include double-dip hot water treatments and vapor heat treatments. The double-dip treatment may lose certification for papayas destined for the U.S. mainland. A dry heat treatment has been approved and awaits certification.

DISEASES

• Anthracnose and chocolate spot, Colletotrichum gloeosporioides (fruits, petioles)
• Phytophthora, Phytophthora palmivora (fruit, stem, roots)
• Powdery mildew, Oidium caricae (leaves)
• Black spot, Cercospora papayae (fruit)
• Damping off, Pythium, Phytophthora, Rhizoctonia (seedlings)
• Wet rot, Phomopsis sp. (fruit)
• Dry rot, Mycosphaerella sp. (fruit)
• Watery fruit rot, Rhizopus stolonifer
• Stem-end rot, Botryodiplodia theobromae, Mycosphaerella sp., Rhizopus stolonifer, Phomopsis sp. (mature fruit)
• Papaya ring spot virus (formerly referred to as papaya mosaic)
• Reniform nematodes, Rotylenchulus reniformis
• Root-knot nematodes, Meloidogyne spp

INSECTS

• Stevens leafhopper, Empoasca stevensi
• Mediterranean fruit fly, Ceratitis capitata
• Melon fly, Bactrocera cucurbitae
• Oriental fruit fly, B. dorsalis

MITES

• Broad mite, Polyphagotarsonemus latus (seedlings, young plants, lower surface of young leaves)
• Red and black flat mite, Brevipalpus phoenicis (fruit)
• Tuckerellid mites, Tuckerella ornata, T. pavoniformis (trunks of old plants)
• Carmine spider mite, Tetranychus cinnabarinus (lower surface of mature leaves)
• Citrus red mite, Panonychus citri (upper surface of mature leaves)
• Texas citrus mite, Eutetranychus banksi (upper surface of mature leaves)

PRODUCTION

Papaya trees bear fruit throughout the year. "Skips" in the fruit column caused by temporary sterility may occur during hot, dry summer periods and may cause fruit shortages in winter. The plants will continue to bear for many years, but yields usually decline as the trees age, and picking becomes difficult. In commercial production, fields are usually replanted or abandoned after three years. Papayas marketed in or exported from Hawaii are regulated by a Federal Marketing Order that sets fruit size and quality requirements. In the United States, Hawaii is the major producer and supplier of papaya, with 2500 acres in 1989. It is also estimated that there were 350 acres of papaya grown in Dade County, Florida in 1987-1988. The United States also imports papaya from foreign countries. In 1989, the United States imported 6 million lb of fresh papaya at a CIF (cost, insurance, and freight) value of $1.1 million. Mexico was by far the largest supplier, with 76% of the total. Other major suppliers were the Bahamas (9.7%) and the Dominican Republic (7.5%). Production of papaya (fresh and processed) in Hawaii in 1989 was almost three times that in 1972. Storms affected the production in 1979 and 1985. Poor weather, disease, and Hurricane Iwa contributed to the decline in production in 1982. Most of the commercial papaya production is in Puna on the Big Island. In 1989, about 97.4% of the papaya was produced on the Big Island, followed by Kauai (1.3%), Oahu (1.1%), and Maui and Molokai (0.2%). The 1989 market supply of fresh papaya for the state of Hawaii was 18.8 million lb. The major market was Honolulu (78.2%), followed by the Big Island (10.1%), Maui and Molokai (8.5%), and Kauai (3.2%). During 1989, 44.6 million lb, or 70% of Hawaii's production of papaya for the fresh market, was exported out of state. The primary overseas destinations were the US mainland (26.6 million lb), Japan (14 million lb), and Canada (4 million lb). The major markets on the US mainland were Los Angeles (61.4%), San Francisco (10%), and New York City (9.4%). The average wholesale prices for Hawaii papaya in these markets were $0.975/lb, $1.15/lb, and $1.18./lb, respectively. In 1992, the total farm gate value of papaya in Hawaii was $14.4 million for the 71,300,000 lb harvested from 259 farms. A total of 55,800,000 pounds of the harvested papaya was consumed as fresh fruit, while 15,500,000 pounds was processed. The average farm gate price of papaya for the fresh market was 25.0 cents per pound. The average farm gate price for processed papaya was considerable lower at 3.0 cents per pound. The total value of utilized production was $14,415,000. Currently, papaya ringspot virus is a serious problem for growers on Oahu and the Big Island. There is no cure for the disease at this time and the only method of control is prevention. This disease may affect papaya production in the State of Hawaii detrimentally.