Citrus originated in Southeast Asia, where it is today one of the most important fruit crops. It is also grown in other parts of the world, in a total of more than 100 countries. In 1995, the Food and Fertilizer Technology Center carried out a regional survey on production methods used by citrus farmers in various countries. The results of the survey showed that yields per unit area in Asia were low compared to Western countries, while production and marketing costs were relatively high. The WTO agreement, which ushers in a new era of global free trade, makes it imperative for the citrus industries of Asian countries to become more competitive. This will enable farmers to hold their place in domestic markets in the face of trade liberalization, and if possible to expand it.
As well as high production costs and low yields, citrus production in the region suffers from serious problems of disease, particularly greening and virus diseases, so that the life span of trees is often relatively short. In view of the long time it takes citrus trees to reach maturity, lengthening the life span of trees would make an important contribution to increasing the percentage of productive orchards, and reducing production costs. Other problems include a lack of suitable improved varieties, a lack of disease-free planting material, and the need for new cultural techniques so that farmers can have more control over the timing of the harvest, thus avoiding seasonal surpluses.
Citrus orchards in the region cover a range of environments. In Japan, Korea and Taiwan ROC they are mainly on slopelands, sometimes on steep slopes, while farms are generally small. This makes it difficult to reduce production costs. Cold-season temperatures in all three are low enough to limit citrus production to a single harvest per year, but also to prevent a continuous build-up of pest populations, so that it is easier to protect trees from pests and insect-borne diseases.
In Thailand, some large commercial farms are found in lowland areas, especially in the central plain which is a major citrus growing area. Trees can be harvested three or four times a year, but the warm temperatures all year round mean that it is difficult to protect trees from insect pests. Some orchards are sprayed thirty times a year. The acid sulfate soil of the central plain, with a pH of around 4, has a high water table, and trees are often infected with phytophthera root rot and other root and trunk fungal diseases. However, it is greening disease which is the main threat to citrus orchards in Thailand and other tropical countries of the region. Thailand, in collaboration with the German Government, has recently established a program for the production of disease free planting materials. In land preparation for Thai citrus orchards, low ridges are built up, separated by ditches or small canals. The trees are planted on the ridges, while the ditches serve as drainage, and also as a transport network for small boats carrying harvested fruit, sprayers and other equipment between the rows.
Citrus orchards in Batangas, Philippines, formerly a major mandarin growing region, have been devastated by greening disease, to the extent that much of the production has shifted south to Mindanao, and to Bicol, Ilocos and the Southern Tagalog region in Luzon. A Budwood Registration Program will soon come into operation, which will provide commercial nurseries, through the Dept. of Agriculture, with foundation stock for disease free planting materials.
Citrus production is difficult in Malaysia, which has not only a tropical climate, but one which is humid throughout most of the year. Whereas Thailand and the Philippines have monsoon climates with pronounced wet and dry seasons, Malaysia has an even distribution of rainfall throughout the year. This promotes the spread of pests and diseases. Greening is a major problem, and Malaysia is producing disease free planting materials as a first step in control. Phytophthera rot does serious damage in high-rainfall areas, but can be effectively controlled by the use of tolerant rootstocks. Tolerant rootstocks can also be used to reduce damage by tristeza, which has infected most orchards but generally does relatively little damage.
In both tropical and temperate countries of the region, there is increasing interest in the control of insect pests by various IPM programs. While the direct damage done to trees by leaf miners etc. is considerable, even more important is the transmission of insect-borne diseases such as tristeza and greening disease. The importance of disease free planting stock is widely appreciated, and the emphasis then is to reduce pest populations to a minimum and protect trees as long as possible from infection. While some chemical control is still essential, IPM promises to give more effective pest control with fewer chemical applications, thus reducing both production costs and the dangers of environmental pollution and pest resistance.
Average yields in most Asian countries are lower than those in Western countries, where 40 mt/ha would not be considered an exceptionally high yield. In contrast, average yields in Korea are 26 mt/ha, in Thailand 15 mt/ha, and in Taiwan only 15 mt/ha.
What are the reasons for this difference? It is important to remember that average yields do not represent the production levels of advanced full-time farmers, who in Taiwan, for example, can obtain yields of 60 mt/ha. However, there are also many part-time citrus farmers in Taiwan who obtain most of their income from off-farm employment. The yields of the part-time farmers are low, reflecting the low level of management and farm inputs they give the trees. This pulls down the average figures. Similarly in Thailand, average yields include non-productive trees and the orchards of farmers with limited education. However, skilled farmers who are well informed and use modern inputs can obtain yields comparable with those of Western countries.
The outlook for increased production seems very good, provided there is sufficient market demand to absorb a higher output without reducing prices. There are two main methods of achieving this, improved cultural practices and the breeding of improved varieties. In practice, these strategies would of course be applied jointly.
There are a number of problems in breeding improved varieties of citrus. Of the four main groups of citrus (sweet orange, mandarin, grapefruit and lemon), only one, mandarin, is a true species. The other three are interspecific hybrids which cannot be improved by conventional breeding techniques. Even in the case of mandarin, one of the main mandarin varieties, Satsuma, is derived by mutation rather than by selective breeding, making further improvement very difficult. It is important to obtain more systematic genetic information about existing varieties, and also about the traits which breeders wish to manipulate.
Another problem in breeding citrus varieties is the long period the trees take to reach maturity, and the large size of citrus trees. Both of these factors mean that the breeding of new cultivars requires a large area of land for an extended period of time, adding to the cost. Other biological factors which limit the development of new cultivars are sexual incompatibilities and sterility, and widespread nucellar embryony.
In Japan, a single variety, Satsuma mandarin, represents 75% of citrus production. There have been efforts to diversify into a wider range of varieties, including some new varieties, but these have not been very successful. Several new cultivars have been developed in recent decades in Japan which have superior fruit quality, but rather low productivity. They tend to need fairly warm climatic conditions. Some citrus breeders suggest that future breeding programs in Japan should emphasize, not only fruit quality and yield, but also the ability of cultivars to adapt to a wide climatic range.
The conservation of germplasm is vital, including traditional varieties which are now being rapidly replaced by a relatively limited number of improved ones. Japanese citrus breeders are now producing interspecific hybrids of
Microcitrus. They cannot be used directly for fruit, because of their poor flavor, but show promise as rootstock. Also of great importance are related wild species, many of which at present have little or no direct economic value. Some species are coming close to extinction with the destruction of their natural habitats. It is of vital importance to conserve this germplasm. The importance of genetic mapping was emphasized, and also of clarifying the phylogenetic relationships and the range of genetic diversity in citrus and its relatives.
The exchange of genetic materials between countries is another important aspect. Apart from disease and quarantine, one major problem limiting the international movement of citrus germplasm is the patenting of biological resources under WTO. However, it was also agreed that all national citrus breeding programs would benefit from free access to a wide range of genetic materials.
Biotechnology is likely to be very important in breeding new citrus varieties. Genetic engineering and other developments in molecular biology offer the possibility of expanding the number of gene combinations, including transgenic plants which have improved disease resistance and other valuable traits. Tissue culture is also an important technique:
in vitro plant regeneration may generate valuable mutations, as well as new somatic hybrids for rootstock. Because somatic hybrids are tetraploid, this technique is also potentially useful in developing scion cultivars, especially seedless triploids. Genetic transformation should make it possible to modify only a single selected trait of an otherwise successful cultivar, without changing any of its other traits.
The high cost of farm labor, often the most expensive production input, makes it important to reduce the cost of labor-intensive practices such as pruning and fruit thinning. Chemical fruit thinning is being tested for Satsuma mandarin in Cheju island, Korea, where the trees tend to bear very heavy crops on alternate years. Foliar sprays of chemicals such as thichlopyr acid and ethychlozate can give practical fruit thinning if applied at the correct time. However, the effect is not completely standard in different orchards or in different years.
There are also various methods of improving fruit quality. If the roots suffer from water stress during fruit ripening, the fruit tends to have a higher sugar content.
Root confinement also gives a higher fruit sugar content, although yields are reduced. Cultivating citrus trees in enclosed boxes is based on the same principle. The problem with the method is its high cost, including the high labor cost for irrigating the boxes individually. Training branches along trellises gives better exposure of fruit to light and sun, but again at a relatively high labor cost.
Repeated applications of chemical pesticides tend to destroy populations of natural enemies. Improved systems of IPM are being developed that conserve important natural enemies. One example is a successful program of biological control of the red wax scale in Cheju island, Korea, using an introduced parasitoid from Japan. Cheju is in a good situation to control insects pests by biological methods, since it is an island, relatively isolated from other land masses, with strong prevailing winds which can quickly disperse introduced natural enemies.
The fact that so many thousands of metric tons of citrus are grown in Asian countries demonstrates the profitability and success of the citrus industry. However, in many countries production has been falling in recent years, largely because of competition from imports, including imported orange juice. Pressure from imported citrus fruit is likely to hit the region's citrus farmers even harder under WTO. There is also competition from other fruits which come onto the market at the same time as citrus, and even competition from processed snack foods, which young people tend to prefer to fresh fruit.
Future citrus output is likely to be governed, not by the production capacity of farmers, but by the strength of market demand and by price levels. Efforts to reduce the labor requirements of citrus production, by integrated technology and improved cultural practices, will be of major importance. Improved disease control, thus giving orchards in the tropics a longer life span, will also play a vital role in reducing costs and making the region's citrus farmers more competitive.
Research, and extension of the results to farmers, have done a great deal to raise both the yields and quality of citrus grown in the region over the past few decades. The outlook is promising, particularly in terms of our ability to produce improved varieties with traits such as superior resistance to pests and diseases, good eating and storage qualities, and high yields. However, the conservation of citrus germplasm, especially in the Southeast Asian region that is the center of origin of citrus, must be a priority if such varieties are to become a reality.
Location: Cheju Citrus Research Institute, Korea
Date: October 22-25
No. Participating Countries: 7 (Japan, Korea, Malaysia, Philippines, Republic of China, Thailand, USA)
No. Papers: 15
No. Participants: 23 plus 50 observers
Co-sponsor: Cheju Citrus Research Institute, Rural Development Administration
1. Worldwide citrus production and innovative technology for citrus cultivar improvement
- Fred G. Gmitter, Jr.
2. An overview of citrus production in Japan: new trends in technology
- Isao Iwagaki
3. Citrus production in Korea
- Dai-Joon Kang
4. Citrus production in Taiwan
- Ming-Hsiung Lu
5. Citrus production in Thailand
- Ravie Sethpakdee
6. Horticultural characteristics and utilization of Cheju local citrus species
- Han-Yong Kim
7. The citrus industry in the Philippines, its problems and prospects
- Alfredo V. Palo
8. Overcoming the problems of citrus production in Malaysia- Weng-Wah Ko
9. Strategy in reducing production and marketing costs of citrus in Taiwan
- Tzu-Bin Huang
10. Case study of tangerine growing in the central plain of Thailand
- Ampaiwan Paradornuwat
11. Chemical fruit thinning for Satsuma mandarin- Doo-Khil Moon
12. Citrus breeding in Japan: new citrus cultivars and their genetic background
- Hirohisa Nesumi
13. New developments in citrus breeding
- Nobumasa Nito
14. Current status and future direction of pest management for citrus insect pests in Korea
- Hong-Sun Kim
15. Significance of sucrose synthetic enzymes in the improvement of citrus cultivars
Figure 1 Citrus Trees Growing in a Greenhouse, Cheju Island, Korea. This Is a Common Practice to Extend the Harvest Season.
Figure 2 Citrus Orchard in Taiwan. Grassy Strips Provide Access Paths and Drainage, without Soil Erosion.
Figure 3 Citrus Orchard in the Central Plain of Thailand. the Water Table on This Plain Is High, and Trees Are Often Tended by Workers in Small Boats.