) Background , Current situation and management of the HLB and its vector in China

Huanglongbing(HLB), yellow shoot disease, is a destructive disease prevailing in the southern part of citrus producing area in China. The history of its occurrence has more than 100 years. “Yellow shoot” and ”mottling yellow leaf” are characteristic symptoms of HLB. Graft transmissibility and virus nature of HLB were experimentally confirmed in the mid 1950’. In 1970’, the pathogen was identified as rickettsia-like organism or a new type of mycoplasma-like organism. In 1990’, it was classified as Candidatus Liberibacter asiaticus. Recently, a phytoplasma related to Candidatus Phytoplasma asteri was discovered in some leaf samples collected in Guangdong Province. The results of heat sensitivity tests revealed that diseased budlings could be cured by the treatment for 50 minutes in 49B or 50B moist air, and the budwoods collected from diseased trees could be sterilized by immersing in 47B water for 10 minutes and repeat twice with the interval of 24 hours. The transmission of HLB by Asian citrus psyllid, Diaphorina citri Kuw. was experimentally confirmed in 1970’. Extensive surveys in different localities showed that the psyllid is not occurred in the localities wih high latitude or high altitude, and the northwards expansion of the distribution of psyllid is followed by HLB prevalence. HLB could be transmitted by 4-5 instar nymphs of the psyllid and by just emerged adults but not by 1-3 instar nymphs. In the graft transmission tests, as the bud or piece of stem was used as inoculum, the transmission rate was much higher than that as a piece of stem bark was used. As the budwoods were collected from the same diseased tree, and grafting conducted in different season of one year, HLB incidence of the progeny grafted in May-July was apparently lower than that grafted in other seasons. The paper on the exclusion of HLB by shoot-tip grafting was published in 1987. All of the varieties of genus Citrus and Fortunella are susceptible to HLB, whereas trifoliate orange is tolerant. Murraya paniculata and Clausena lansium have been identified to be susceptible to Ca. L. asiatics. As the graft transmissibility of HLB was confirmed in the mid 1950’, HLB was defined as a target of plant quarantine by the Department of Agriculture. Since late 1950’, mottling yellow leaf has been commonly used as main criterion for HLB diagnosis in the field, and since late 1990’, PCR has been used for exact diagnosis. Producing HLB free budlings has been recommended. In the past, mother block and nursery should be established in a separated area, 1.5-3 Km apart from existed citrus orchards. In recent years, the establishment of mother block and nursery in screen house was practiced in some localities. The budwoods were collected from healthy trees standing in the orchards with low HLB incidence, and treated by tetracycline immersing or intermittent heat treatment. Accompanying with other diseases control, shoot-tip grafting has been used in preparing HLB free mother plants. Strictly control psyllid in a large area by insecticides is the most important procedure in HLB control. In general, there are 1-2 sprays in the period after picking and before spring sprout, in spring growth, in summer growth, and in autumn growth, respectively. Removing diseased trees promptly is also an important procedure in HLB control. Usually, the diseased trees are removed after picking. The experience of Yangcun Citrus Farm with the acreage of about 2000 ha demonstrated that the annual HLB incidence could be kept less than 1% by strict control of psyllid and prompt removal of diseased tree. Since the early 2000’, much extension works for HLB control have been done by the Agricultural Department of Provincial Government in Zhejiang, Guangxi, Fujian (Yongchun) and Guizhou (Congjiang), and the quick spread of HLB in most areas has been effectively checked. Introduction: Huanglongbing (HLB), yellow shoot disease, is a destructive disease prevailing in the southern part of citrus producing area in China. According to old farmer’s memory, the history of its occurrence is about 120 years, and the record history goes back to 1919. The graft transmissibility and virus nature of HLB was confirmed in 1956, and it was determined as a quarantine target in 1957. Since 1960’, a group of isolated citrus nurseries have been established for propagating HLB free budings. After the late 1970’, the strict control of citrus psyllid has been emphasized in HLB control. In recent years, the HLB incidence was kept in low level in most citrus orchards, but it usually caused serious damage in others. 1. Distribution and importance In the Mainland of China, HLB and Asian citrus psyllid are distributed in Guangdong, Fujian, Guangxi, Yunnan, Southern Zhejiang, Southern Jiangxi, Southern Hunan, Southern Guizhou and in a limited area of Southern Sichuan. In 1980’, there was an apparent northward movement of HLB prevailing area occurred in Guangxi. Before 1980’, HLB prevalence had not been found in Northern Guangxi, but in 1980’, the distribution of citrus psyllid was found and followed by HLB epidemic. Since the early 2000’, the northward movements of HLB prevailing area have also occurred in Zhejiang, Hunan and Jiangxi Provinces. In the area where citrus had not been planted, the young citrus orchards usually have few trees shown HLB symptoms in 2-3 years after planting, if the psyllid was not controlled, the incidence of HLB would quickly increase, and the orchard would be destroyed in about 8 years after planting. If the new orchard established close to the orchards with high HLB incidence, it would be destroyed by HLB in 1-3 years after planting.