Progress in study of new Citrus viruses and viroids diseases in China

Abstract：Citrus is one of the most popular fruit crops of the world from the equator to 41 degrees of latitude north and south. As a perennial plant, citrus is high value crop which is infected by a big number of viruses and viroids in the field due to vegetative propagation and grafting. Some of these pathogens cause severe crop losses and often reduce the productive life of the orchards. The most important cause for the worldwide spread and accumulation of viruses and viroids has been the international trading of infected nursery plant material. Subsequently, vector transmission might occasionally play a role in the spreading of these agents at the local level. In China, citrus was infected by more than 20 virus and viroids diseases, including Citrus tristeza, Citrus tatter leaf, Citrus satsuma dwarf, and Citrus exocortis etc, which severely affect its production as well as the quality. During recent years, there has been significant increase in the citrus production in China, and the citrus production of China stayed stably ca 33 million tonnes, which is of economical importance to farmers in the southern part. With the development of new detecting techniques, especially the next-generation sequencing, three new citrus virus and three viroids diseases were identified in China. These diseases are becoming more harmful to citrus production. Progress in studies on the new citrus virus and viroids diseases was summarized. The results obtained in recent years are as following: (1) Citrus yellow vein clearing virus (CYVCV) , a member of genus Mandarivirus, family Alphaflexiviridae, is the causal agent of yellow vein clearing disease. CYVCV was first observed in Ruili, Yunnan province of China in 2009. Now this virus is widely distributed in China. CYVCV infects most citrus species, varieties and hybrids, and lemon (Citrus limon) is most sensitive to CYVCV. CYVCV is transmitted through vegetative propagation of infected buds, scion or rootstocks, and by mechanical inoculations ofsap extracts onto herbaceous indicator hosts. CYVCV can also be transmitted by Aphis craccivora and A.spiraecola from lemon to bean (Phaseolus vulgaris) , and from bean to bean. No other transmission ways are known. Comparison of the whole genome sequences of CYVCV isolates indicated that there is a very low level of sequence heterogeneity among CYVCV isolates of different geographic origins and hosts. (2) Citrus chlorotic dwarf-associated virus (CCDa V) , a recently described member of the family Geminiviridae, is considered the causal agent of citrus chlorotic dwarf disease (CCDD) . CCDD is considered to be the most serious disease of citrus in Turkish, only sweet orange showed some tolerance to the disease. Until recently, CCDD had not been reported to be present in any area outside of Turkey. However, in 2009, the typical symptoms caused by CCDD were first observed at one Eureka lemon yard in Ruili, Yunnan province, and now CCDa V was still restricted in few orchards in China. CCDa V is transmitted through vegetative propagation of infected buds, scion or rootstocks, and by stem-slash inoculation of sap extracts onto sensitive citrus species, such as C. macrophylla, rough lemon (C. jambhiri) and sour orange (C. aurantium) . CCDa V could also be transmitted by the Japanese bayberry whitefly (Parabemisia myricae) effectively. (3) Citrus leaf blotch virus (CLBV) , a member of the family Flexiviridae, has a 9 kb single-stranded, positive-sense genomic RNA encapsidated by a 41 ku coat protein. CLBV dispersal occurs primarily by propagation of infected buds, but seed transmission at low rates has been detected in at least three citrus species or hybrids. CLBV was first detected and characterized in a Nagami kumquat (Fortunella margarita) in Spanish, showing bud union crease on Troyer citrange (C. Sinensis × Poncirus.trifoliate) . In recent years, CLBV was fond in Prunus avium、Actinidia chinensis and Citrus in China. (4) Three new viroids were detected, including Citrus bark cracking viroid (CBCVd) , Citrus viroid V (CVd-V) , and Citrus viroid V (CVd-VI) . Three new viroids species have been identified in citrus and belong in two genera: Cocadviroid (CBCVd) , and Apscaviroid (CVd-V; CVd-VI) , all belonging to the family Pospiviroidae, and they are unencapsidated, small, circular, covalently closed, single-stranded RNAs of 246 to 401 nucleotides (nt) . Three new viroids infect most citrus species, varieties, hybrid of genera citrus. They are transmitted through vegetative propagation of infected buds, scion or rootstocks, and by knife-cut inoculation of sap extracts onto sensitive citrus species. 42 samples of 33 cultivars with such viroid symptoms as stunting, bark scaling, and cracking on P. trifoliata rootstock, were collected from citrus orchards. CBCVd was detected in cv.Meishan No. 9 (C. sinensis) from Sichuan, cvs. Akemi (C. reticulata) and Nishirokaori (C. reticulata) from Zhejiang, while CVd-V was detected in cvs. Nishirokaori, Haruka (C. tamuranua) , and Kiyomi (C. unshiu ×C. sinensis) from Zhejiang, Hunan and Chongqing, respectively. CVd-VI was detected by RT-PCR in nine samples (20.9%) collected from four different growth regions (Hunan, Sichuan, Chongqing and Zhejiang) .Selection of resistant cultivars is the most effective way of combating virus and virods diseases of citrus, but so far after many years of research the success is rather limited. As a result, control strategies are mainly focused on the production and use of virus-free plant propagative material and control the vectors.