Protected cultivation of Papaya

A. Background

• Protected cultivation of papaya is practiced in Japan, Israel and Spain
• The papaya ringspot virus (PRV) is a serious commercial threat, against which only transgenic papaya cultivars have shown acceptable resistance.
• Greenhouse cultivation provides both adequate temperatures and exclusion of PRV vector and is thus the system of choice in subtropical countries like the Canary Islands (Spain), Israel and Japan.
• In the case of the papaya, after a brief juvenile phase, vegetative growth coincides with flower development -an inflorescence emerges continuously in the axil of each leaf, provided climatic and cultural factors are adequate.
• When environmental conditions are poor, flowering and fruiting are severely affected.
• Temperatures below 20°C have a very negative effect, causing among other problems, carpelloidy (a plant disorder that results in misshapen fruits caused by abnormal development of the ovule-bearing part of the flower in angiosperms), sex changes, reduced pollen viability, and low sugar content of the fruit.
• Furthermore, if the temperature falls below 12-14°C for several hours, growth and production are severely affected, particularly in dioecious cultivars.
• Thus, in theory, the papaya can only thrive in a stable tropical climate, and even then, cultural practices should be at an optimum level to maintain a high constant growth rate throughout the entire life of the plant in order to achieve maximum yield.

B. Greenhouse types for Papaya cultivation

• A frame of galvanized steel pipe (∅ 5 to 10 cm and 6 to 7 m in length) embedded in concrete bases can be used (mostly practiced in Spain).
• In the case of papaya, particularly with low height cultivars, 3 m high typical vegetable and ornamental greenhouses, with a similar structure, are also used
• Cladding is usually polyethylene film sandwiched between a double-weave wire network.
• Net cladding, occasionally used for the roof alone, is frequently used for the laterals in the warmer areas
• Similar prefabricated galvanized steel frame structures are being used elsewhere, with a height of 3 to 6 m in Japan and about 4 m in Israel.
• The Israelis tend to prefer polyethylene films for covering, while the Japanese use net, vinyl chloride, or polyethylene.

C. Cultivars

• Low-height bearing cultivars of ‘Solo’ are best adapted to greenhouse cultivation.
• While ‘Sunrise’ used to be the cultivar of choice in Japan and the Canary Islands (Spain),
• ‘Baixinho de Santa Amalia’, a Brazilian dwarf mutant of ‘Sunrise’, the South African selection‘BH-65’ (another dwarf ‘Solo’ type), both of which are very similar to ‘Sunrise’ in fruit characteristics
• ‘Maradol’ is also cultivated on a small scale. In Japan, locally selected female plants of ‘Wonder Flare’, a Female plants are prevalent in Israel, where the local selection ‘Paradise’ is the planting cultivar of choice.

D. Cultivation techniques

• Cultural practices for greenhouse papayas do not in general differ from those under open air where cultivation without protection is possible.
• Planting materials in the Canary Islands come from true-to-type seeds obtained through self-pollination, while in Israel, rooted cuttings are currently used.
• Both seed and tissue culture plants are planted in Japan.
• In all cases, nursery propagation is done in structures isolated from PRV host plants and vectors.

E. Planting

• Planting densities are variable, but can be as high as 2,666 plants/ha, capable of yielding up to 200 t/ha over the three-year period before replanting.
• Usually 2-3 seeds are planted together and later thinned to leave one hermaphrodite seedling per hole.
• Many farmers replant between tree rows on the third year, to achieve continuity once the previous crop has been removed.
• Plantings in Japan, under drip irrigation, are spaced at 1.5 x 1.5 m, while in Israel it varies from 1.5- 2.0 m between trees within rows and 2.0-3.0 m between rows.

F. Irrigation and fertigation

• Drip irrigated greenhouse papayas are fertilized much the same as those in open-air plantations, applying 150 g of N, 120 g of P2O5, 200 g of K, and, where necessary, 37 g of borax per plant per year
• Organic matter, around 5 kg of manure/plant, is usually incorporated at planting
• Drip-irrigated papayas start with 4 L/plant/day until flowering and increase to 12 L once full production is achieved. All of these figures show a substantial savings compared to open-air plantations
• Hand pollination is a common practice in Israel, but is not necessary in the areas where fruit set is good all year around.
• Fruit or flower thinning to retain 1-2 fruit per node is manually done in the Canary Islands in ‘Baixinho’ and ‘BH-65’ cultivars, which produce more than 10 flowers per node.
• Other agrotechniques are similar to those used elsewhere for open-air cultivation.
• A practice currently used in Japan is to lean the shoot obliquely, nearly parallel to the ground, in order to reduce plant height, but dwarf cultivars are preferred for new greenhouse plantations.

G. Pest and disease incidence and control

• The banana moth (Opogona sacchari Bojer) and the red or carmine spider mite (Tetranychus cinnabarinus Boisduval) are the only relevant pests,
• Oidium caricae Noack and occasionally Botrytis cinerea Pers. are the main fungal pathogens affecting greenhouse papayas
• Mites are also common pests in Israel as well as Japan where aphids are also serious pests and Fusarium fruit rots are of importance