Although many citrus species originated in tropical Asia, most modern commercial varieties are not best suited to the humid tropics. Throughout tropical Asia, citrus production is severely limited by diseases such as tristeza virus, greening and phytophthera. There is also considerable damage from insect pests, including the insect vectors which spread many of these diseases. Citrus growers in Asia need improved varieties which are adapted to local climatic conditions and have good disease and pest resistance. However, collections of citrus germplasm in Asia are limited, particularly in tropical countries.
The pomelo, or shaddock, has the largest fruit of any citrus species. The fruit, yellowish-green in color and either round or pear-shaped, is a popular fruit in Asia. Because it is thought to resemble the moon, it is a favorite fruit among Chinese people at Mid-autumn Moon Festival. It is also relatively easy to grow, and is more tolerant of diseases and pests than most other kinds of citrus. There is great potential for improved pomelo production, especially for varieties with high-quality fruit. The aim of this survey was to identify and evaluate available pomelo genetic resources, including indigenous cultivars, which could be used in quality and production improvement programs.
Vietnam was chosen for the survey because of its abundant citrus genetic resources, and because the Vietnamese government is strongly promoting fruit production. Since Vietnam covers a very wide range of climatic zones, from cool temperate highland areas in the north to lowland tropical areas in the south, there are a wide range of fruit crops, and many indigenous varieties. In 1995, there were 320 thousand hectares of fruit orchards in Vietnam. Over the next three years, the Vietnamese Government has the goal of expanding the area planted in fruit to one million hectares.
Eighty percent of the population in Vietnam live in the countryside and depend on farming for their livelihood. Fruit production is seen as a promising way of raising farm incomes and the standard of living in rural areas. Increasing the area planted in fruit trees will need a plentiful supply of disease-free planting materials for farmers.
Citrus trees grow slowly and take several years to come into production, while virus disease and greening may have minor or hidden symptoms for some years. Citrus development programs in several Asian countries have already been badly damaged by the use of seedlings which were thought to be healthy, but were in fact infected with disease. The Vietnamese government is anxious that this will not happen to Vietnam's commercial pomelo production, which is still in its very early stages.
The Long Dinh Fruit Research Center of Vietnam's Ministry of Agriculture, Forestry and Fisheries has identified 26 areas of fruit production in Vietnam. Pomelo is grown in five of these.
The survey results indicated that the most promising varieties in the northern region are Dao, Kha, Linh, Suu, My and Dien. In the Central region, two promising varieties, Thanh Tra and Dao, are grown in back gardens rather than being used for commercial production. Seven promising cultivars were found in the south: Thantra, De Xanl, Nam Roi, Duong Da Lang, Dung La Vam, Da Xanh, and Long Da Lang.
Samples of these promising varieties have been collected, and are now being indexed for virus and other diseases. Material which is infected with virus will be cleaned by heat treatment and other laboratory treatments. Once the material has been certified free of virus and other pathogens, it will be used in a program carried out in cooperation with the Department of Plant Pathology and Entomology of National Taiwan University to produce certified disease-free mother trees. These in turn will be sent to Vietnam for use as pomelo nursery stock and as planting materials for farmers.
Figure 1 Pommelo for Sale in Roadside Stall
Figure 2 â€œWen-Tanâ€ an Improved Pommelo from Taiwan