The Need for Virus-Free Nursery Plants
In the past, citrus plants were propagated by budding or grafting. There were no indexing practices for virulent pathogens such as viruses or viroids. By the 1970s, most trees growing in Taiwan and other parts of Asia were infected with some detrimental virus or pathogen. "Likubin", a combination of tristeza virus and greening disease, is the most damaging citrus disease in Taiwan, with symptoms similar to those of greening disease in other countries. To reduce disease infestation, growers in Taiwan moved their citrus growing areas from the warm lowlands, where soil was fertile and cultivation was easier, to the colder highlands. In this way, they avoided infestation of their trees by psyllids (Diaphorina citri), the vector insects of "likubin". In Taiwan, exocortis virus infests citrus trees island-wide, lowering their productivity. However, growing citrus trees in the highlands and using ordinary nursery plants meant higher production costs, and higher transport costs when shipping fruit to the market.
Program for Virus-Free Nursery Plants
Taiwan's virus-free citrus budwood program was launched in 1976. The program included:
- Scion groves with protective measures to prevent secondary infection with pathogens by insect vectors. These citrus scion groves provide virus-free budwood for citrus nurseries.
- The establishment of citrus nurseries, both government and private, to provide virus-free citrus plants to citrus growers ( Fig. 1(0)).
How to Produce Virus-Free Citrus Plants
Budwood is cut from one of the registered scion trees in a scion grove. The rootstock is grown from seed. Rootstock varieties include a bitter pummelo (Citrus grandis), "Sunki" tangerine (Citrus sunki), Rangpur lime (Citrus limonia) and hybrids such as Carrizo citrange and Swingle citrumelo.
Most of the viruses which infect citrus tree cannot be transmitted through seed. Grafting or budding virus-free budwood onto rootstock grown from seed produces almost 100% virus-free plants. The young plants are kept in a greenhouse or screenhouse until they are large enough for sale to growers ( Fig. 2(0)).
Results and Advantages of the Program
Taiwan's virus-free citrus budwood program has been in operation for 33 years. During that time, the area planted in citrus has fallen by 26%, but the total production of citrus fruits has fallen by only 19%. Their value is slightly higher than in 1976. This is because of the higher productivity and longer life-span of citrus orchards where virus-free nursery plants are used.
Index of Images
Figure 1 Greenhouse Used As Nursery for Virus-Free Plants
Figure 2 Healthy Young Virus-Free Citrus Plants
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