Greenhouse Cultivation Of Papaya
Papaya is a tropical crop which requires high temperatures to produce good quality fruits all year round. 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. A description of greenhouses for papaya cultivation, cultivars, cultural practices and phytopathological problems occurring in these countries are given in this paper.
In single-stemmed crop such as pineapple or banana, in which prioritizes growth over flowering and fruiting, the natural range of climatic tolerance of the plant can be extended to somewhat different less tropical zones without particular risk from an agronomic (and economic) viewpoint. But in the case of papaya, after the adolescence stage, the vegetative growth coincides with flower development. An inflorescence emerges continuously in the Axil of each leaf, provided better climatic and cultural factors are adequate. Surely when environmental conditions are poor, flowering and fruiting is severely affected. Temperatures below 20°C have a very negative effect, causing among other problems, carpelloidy, 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. Modern protected cultivation is the solution of choice for the mild subtropics such as the Canaries, where over 150 hectares of papaya (around 90% of the plantings) are already being grown in greenhouses and further plantings are planned for the near future. Commercial greenhouse cultivation is also done in Japan, with around 10 hectares, some 30% of total plantings and in Israel with 10 hectares (100 %) . Unless otherwise said, all the information about these two countries comes from these two sources
ADVANTAGES AND DISADVANTAGES OF GREENHOUSE CULTIVATION
- The general advantages of greenhouse cultivation in the subtropics have been studied in depth in the Canary Islands and include:
- (1) Increase in the number of hours with temperatures above 20°C;
- (2) Protection against wind and other unfavorable weather conditions;
- (3) Reduction in water consumption, as evapotranspiration is reduced by up to 25%;
- (4) increase of light on the leaf surface leading to higher photosynthetic capacity;
- (5) Reducing the time for growth and the length of developmental phases; and
- (6) Improved production, both in quality and in quantity. Greenhouse papayas grow and fruit better than in the open as a consequence of both higher temperatures and wind protection, and exclusion of PRV as long as clean planting materials are used and Cucurbitaceae or other host plants are not interplanted with the crop
- This last advantage is of particular relevance as it is the only viable alternative to using genetically modified or transgenic cultivars such as ‘UH Rainbow’ or ‘UH SunUp’, commercially cultivated in Hawaii. These fruits are marketed in the Continental United States and, more recently, Canada, but strongly objected to by other consumers, especially in Europe and Japan. Recent studies done in Brazil corroborate the economic interest of cultivating papayas under greenhouse to target these specific markets, recommending this as a commercial practice. Another advantage over open-air plantings in the subtropics is the prolonged harvest period, which can cover the whole year with good quality fruit. Data for the Canary Islands show TSS above 11% for practically the whole year, which commands excellent prices for out-of-season produce. Annual yields are also very high, over 80 t/ha in the Canaries and over 100 t/ha in Japan. Some disadvantages associated with the use of greenhouses include the shortening of the commercial productive life of the plants as a consequence of the greater Internode length, which in turn means that plants are taller when flowering begins, quickly reaching ceiling height. There is also increased incidence of mites and powdery mildew, and little or no natural pollination of the female flowers Under subtropical conditions, however, this last disadvantage is not a serious problem and certainly not in the Canary Islands, as in enclosed plantings no pollination problems occurs. However, in dioecious plantings (males and female trees) parthenocarpic fruits may be produced in the absence of pollination under subtropical conditions and hand pollination may be required to obtain commercial fruits. The cost of the greenhouse, around 8 USD/m2 in the Canaries, the negative visual impact of the structure itself and problems of biodegradability of the cover materials are the chief drawbacks.
TYPES OF GREENHOUSES
Green_ houses for papaya in the Canary Islands are the same type as those used for bananas, as indicated by a frame of galvanized steel pipe (∅ 5 to 10 cm and 6 to 7 m in length) embedded concrete bases. 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 of the Islands. The radiation spectrum of some of these types of covering used in the Canaries for tropical fruits, are given in Fig. 1. While the structure itself can last two decades or more, the weak point of a greenhouse has traditionally been cover replacement due to storm damage or, more importantly, ageing. Recently developed, multi-layered polyethylene films, particularly “Celloclim” clear bubble plastic, a three-layer film that sandwiches an expanded polymer layer with gas micro bubbles between two plain polymer layers, may last up to 5 years without renovation. This is much more durable than those previously utilized in the Canary Islands, which needed to be replaced every 2-to-3 years, while the new netting cover material may last more than 10 years. Trials are underway at the Instituto Canario de Investigaciones Agrarias (ICIA) to evaluate different types of greenhouse covers with emphasis on long-life polyethylenes, preferable to mesh/net covers for papaya, as fruit TSS increases with increased heat even though extra ventilation is needed in the summer as temperatures approach 35 ºC to avoid the tendency of bisexual cultivars to develop male, non functional flowers, usually associated with high temperatures 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