A PRELIMINARY STUDY ON THE ESTABLISHMENT OF DISEASE-FREE MOTHER TREES AND NURSERY TREES OF CITRUS IN FUJIAN PROVINCE

The integrated disease management of practicing severe quarantine, cultivating disease-free nursery tree,eradicating diseased plants and killing vectors is the effective measure to control citrus yellow shoot disease and other virus diseases. Planting disease-free nursery tree is the key of control measures, because it can prevent the citrus tree in new areas from yellow shoot disease and reduce or eradicate infection centre in the new citrus orchards of the old diseased areas.From spring of 1978, we began the research work on establishing disease-free mother trees and nursery trees in a well isolated place in Min-Hou county of Fujian province. Parent source trees were selected from various citrus areas in Fujian. Most parent source trees were chosen for true-to-type,good fruit production and very healthy in appearance. The budsticks taken from each parent source tree were treated by hot-moist air, antibiotic and tip-stem micrograft for an attempt to eradicate yellow shoot pathogen and other virus carried. The treated bud was grafted on the rootstock grown from seed which was disinfected by hot water treatment for propagating mother tree candidates. The mother tree candidates were indexed by three ways of electron microscope exam i nation,biological test and serological test for detection of virus and virus-like pathogen, such as yellow shoot procaryote, tristeza virus and exocortis virosis which have been known in China. Buds were taken from these disease-free mother trees in the following year for propagating disease-free nursery trees by grafting them on rootstocks growing from hot water treated seeds(see Fig.1). By the spring of 1983, 11 of 19 parent source trees, namely sweet orange CV. Snow, sweet orange CV. Tai 1, sweet orange CV. Shenghui, sweet orange CV. Liu, sweet orange CV. Shu llou, mandarin CV. Bei-keng 2,mandarin CV. Bei-Lan, mandarin CV. Tai 1, Ponkan CV. Tin-Ma, ponkan CV. Nan-Jen and Tankan, were chosen and used as disease-free mother trees, and planted in the mother tree foundation in Min-Hou county. When the inspections were made cut on the occurrence of diseases after 2-3 years, none of 3897 mother trees showed yellow shoot disease and other abnormities. In 1984, cooperated with Fujian Agricultural Bureau, we set up other three disease-free mother tree foundations in Chang-Tai county, Yong-Chun county and San-Min county. During this period, 20,000 nursery trees were propagated and sold to various villages and national citrus farms. Those orchards planted with such nursery trees are free from yellow shoot disease,but some of them planted with other nursery trees together were soon infected by Canker disease. The results show that it is possibility to establish mother tree foundation free of yellow shoot disease and other virus diseases above mentioned for propagating disease-free nursery trees in the well isolated localities by methods.