Establishment of a Virus-Free Propagation System in Hunan Province, China Xiao Qiming, Wu Liyou, Liu Xueduan, Pan Chuanzhi, and Liao Zhenkun

ABSTRACT. Hunan Province is a major citrus-producing province in China with a total planted area of 213,000 ha. Several graft-transmitted diseases occur here, namely huanglongbing (greening), exocortis, tristeza and tatter leaf. The Hunan Virus-Free Propagation System was established in the early 1980s jointly by the China Department of Agriculture and the Hunan Department of Agriculture. Fifty-six cultivars selected from different areas of Hunan and some imported varieties have undergone thermotherapy, shoot-tip grafting and indexing. From these, 30 pathogen-free cultivars are maintained in a primary source mother block which has produced 170,000 containerized plants for secondary source blocks and 1.5 million plants for commercial plantings.

Hunan Province is one of the sequential polyacrylamide gel elec- major citrus-producing provinces in trophoresis have shown that Miya- China with a total planted area of gawa Wase and Owari satsumas are 213,000 ha in 1994. Important com- infected with viroid mercial cultivars include satsuma (CEVd) (5). mandarin, mandarin, navel Tatter leaf. Between 1987 and and other sweet oranges, 1992 trees in different commercial and pummelo. Since the early 1980s, orchards were indexed for citrus tat- new orchards have been established ter leaf virus (CTLV) using Rusk cit- with high quality varieties of navel range. The following varieties were , Bingtang orange, Valencia, found to be infected: Bingtang, Mang Ponkan mandarin and fragrant , , fragrant pummelo. Unfortunately, graft- pummelo and Ponkan mandarin. transmissible diseases were spread Since trifoliate rootstock is common, because of the lack of a control sys- typical symptoms of CTLV infection tem. For example, Bingtang orange were present, namely leaf yellowing mother trees were infected with both and leaf dropping, budunion crease exocortis and tatter leaf (6). The and the associated easy stem break- Hunan Virus-free Propagation Sys- ing (8). tem was, therefore, established Huanglongbing (HLB). HLB jointly by the China Department of was found in the southern province Agriculture (CDA) and the Hunan in 1982 (3). Following this, a survey Department of Agriculture (HDA). was conducted noting field symp- toms with confirmation by graft DISEASES IN HUNAN inoculation to Ponkan indicator seedlings and electron microscopy Many new varieties were intro- (EM). The HLB liberobacter was duced into Hunan in the 1970s, and detected in a few orchards in Chen, it is believed this resulted in the Yizhang and Rucheng Counties. The spread of diseases. The following are infected trees were destroyed, and known to occur: no new cases have been reported in Exocortis. the last nine years. rootstock is widely used in Hunan. Tristeza. By indexing using Field surveys together with biologi- Mexican indicator plants, citrus cal indexing using and tristeza virus (CTV) was shown to be 298 Thirteenth IOCV Conference, 1996-Surueys and Certification widespread, but it has not been a employed. The grafted plants major problem due to the use of were then indexed following the CTV-tolerant rootstocks. directions of Roistacher (5): Satsuma wilt. This new dis- a. HLB using Ponkan seedlings ease, not yet shown to be graft trans- and EM missible, is a serious problem in a b. CEVd using Etrog citron Ari- few orchards causing leaf curling, zona 861-S1 and electrophore- yellowing and leaf dropping (7). sis, c. CTV using Mexican lime, and HUNAN VIRUS-FREE d. CTLV using Rusk . PROPAGATION SYSTEM Thirty disease-free selections have been obtained and are now The first virus-free mother tree maintained in the primary orchard was established in Qiangy- mother tree foundation and ang County in 1982, but was aban- germplasm repository. The doned in 1984 because of citrus selections are eight satsumas, canker infection (8). In 1986 the eight navels, six other sweet CDA and the HDA selected the orange, four , two Zhaxi reservoir area in Anhua and two Bingtang county as a site for a disease-free oranges. propagation farm. It is under the 3. Establishment of primary direction of the Hunan Cash Crop mother block. The provincial Bureau (HCCB) in cooperation with mother block is a 7-ha site in the the Plant Protection Department of Zhaxi reservoir area in Anhua the Hunan Agriculture University County. It is surrounded by (HAU), the Hunan Plant Protection water on three sides, and the and Industrial Station (HPPIS) and area has warm temperatures, the Agricultural Bureau of Anhua fertile soils and is well watered. County. The system operation is All trees are inspected for true- shown in Fig. 1. The steps in the sys- ness to type and for absence of tem are as follows: disease symptoms before regis- 1. Selection of varieties. A list of tration for propagation pur- required citrus varieties was poses. One tree from each prepared by the HCCB, and the selection is kept in a germplasm HAU and people of the propaga- repository screenhouse under tion farm collected the budwood. strict sterilization and quaran- So far, 56 true-to-type selections tine regulations. The trees in the have been made, namely 14 foundation block are evaluated sweet orange, nine navel orange, for their horticultural qualities 15 satsuma, three Ponkan, three annually, and are re-indexed for summer orange, four tangerine graft-transmissible diseases and eight other types. every 2 to 3 years. The provincial 2. Elimination of pathogens. This mother block is the source of has been conducted by the HAU, budwood, rootstock seed and and was designed to eliminate seedlings for the secondary the four major pathogens mother blocks. recorded in Hunan. A combina- 4. Establishment of secondary tion of thermotherapy as mother blocks. The first second- described by Calavan et al. (1) ary mother block is a 2-ha site at followed by the shoot tip grafting the Anhua Citrus Propagation method of Navarro et al. (4), and Farm. In total there will be 10 including de Lange's (2) method such blocks and nurseries in dif- of grafting the emerging shoots ferent parts of Hunan, six of onto established seedlings was which were established by the Thirteenth ZOCV Conference, 1996-Surveys and Certification 299

Grafted plants Idaylnight) for 6 to 8 wk

Indexing for CEV, CTLV, CTV, HLB and other diseases

HLB and other diseases rDisease-free STG plants

Evaluation of horticultural Provincial primary characteristics and disease-free mother tree germplasm repository or reindexing of diseases 1foundation screenhouse

supply center foundation

Restricted for District level, 2to3yr 1 7;cted for 2 to 1 rRegistered nurseries

rCommercial orchards Fig. 1. Flowchart of the Hunan disease-free citrus propagation system.

end of 1994. These are located at budlings annually. The same Changning in southern Hunan, record keeping and quarantine Longhui in the center and Jing- regulations used for the pri- zhou in the west. Each second- mary blocks are used in these ary block has a total area of 3.5 blocks. Prior to the release of ha, 2 ha of which is for the nurs- budlings, they are inspected for ery and the remaining 1.5 ha as disease symptoms in order to be the budwood block. Each is certified. The provincial bud- designed to produce 0.5 million wood center and the secondary 300 Thirteenth IOCV Conference, 1996-Surveys and Certification

blocks can only supply budwood produced 0.3 million budlings and for three years and are then 1.2 million buds annually since renewed. Some secondary foun- 1992. Thus far, 1.5 million budlings dation trees are pruned and and 0.17 million containerized maintained as varietal evalua- plants have been provided to the sec- tion trees. ondary mother foundation and com- The provincial budwood center mercial orchards. and the disease-free nursery has

LITERATURE CITED 1. Calavan, E. C., C. N. Roistacher, and E. M. Nauer 1972. Thermotherapy of citrus for inactivation of certain viruses. Plant Dis. Rep. 56: 976-978. 2. de Lange, J. H. 1978. Shoot tip grafting-a modified procedure. Citrus Subtrop. J. 539: 13-15. 3. Huang Lihua, Xiao Qiming, and Mao Xiaolin 1984. Diagnosis of citrus huanglongbin in Hunan Province. Hunan Agr. Sci. 13(5): 37-38 4. Navarro, L., C. N. Roistacher, and T. Murashige 1977. Improvement of shoot tip grafting in vitro for virus-free citrus. J. Amer. Soc. Hort. Sci. 100: 471-479. 5. Roistacher, C. N. 1991. Graft Transmissible diseases of citrus. Handbook for detection and diagnosis. Publ. Div., FAO, Rome. 286 pp. 6. Shu Guangping 1985. A survey of CEV in Hunan Province. Hunan Agr. Sci. 14(4): 27-29. 7. Shu Guangping and Wu De Xi 1996. Satsuma green wilt, a new citrus disease, p. 412. (Abstr). In: Proc.13th Conf. IOCV., IOCV, Riverside 8. Wu Liyou, Xiao Qiming, and Liu Xueduan 1993. Regeneration of virus-free citrus and detection of citrus viruses in Hunan Prov- ince. J. Hunan Agr. Coll. 19(3): 31-37