Curent Situation of “Candidatus Liberibacter Asiaticus” in Guangdong, China, Where Citrus Huanglongbing Was First Described

Journal of Integrative Agriculture 2012, 11(3): 424-429 March 2012 RESEARCH ARTICLE

Curent Situation of “Candidatus Liberibacter asiaticus” in Guangdong, China, Where Citrus Huanglongbing Was First Described

DENG Xiao-ling1, GAO Yi-di1, CHEN Jian-chi2, PU Xue-lian1, KONG Wei-wen3 and LI Hua-ping1

1 Laboratory of Citrus Huanglongbing Research, South China Agricultural University, Guangzhou 510642, P.R.China 2 San Joaquin Valley Agricultural Sciences Center, USDA-ARS, Parlier, California 93648, USA 3 School of Horticulture and Plant Protection, Yangzhou University, Yangzhou 225009, P.R.China

Abstract

Citrus Huanglongbing (HLB, yellow shoot disease) was first observed in the coastal Chaoshan Plain of Guangdong Province, China, in the late 19th century based on descriptions of yellow shoot symptoms. “Candidatus Liberibacter asiaticus” has been considered as a putative pathogen associated with HLB since 1994. Information about the curent prevalence of this bacterium is important for HLB control in Guangdong and also provides useful reference for HLB study elsewhere. In 2007, we collected HLB symptomatic citrus samples from 16 cultivars in 12 prefecture cities, mostly in the north and west regions of Guangdong, where major citrus fruits are currently produced. Among the 359 samples collected, 241 (67.1%) were positive for “Ca. L. asiaticus”, distributed in 15 out of the 16 cultivars from all 12 cities, indicating the widespread prevalence of “Ca. L. asiaticus” in Guangdong Province. The detection rates varied from 16.7 to 100% depending on location and cultivar. Lower detection rates were found in newer citrus cultivation cities among the previously less popular but now promoted cultivars. In reviewing the citrus management and pest control practice, we believe that infected nursery stocks play a key role in the current spread of “Ca. L. asiaticus”.

Key words: citrus, Huanglongbing, Candidatus Liberibacter asiaticus

1956b). HLB was also referred to by different names, INTRODUCTION such as Jitouhuang (yellow chicken head, related to leaf yellowing), Lajiaoye (pepper leaf, describing the Citrus Huanglongbing (HLB), yellow shoot disease, was small and upright leaves), and Genfu (root rotting), as first described and studied in Guangdong, China summarized by Lin (1956b). (Reinking 1919; Chen 1943; Lin 1956b). HLB has been Studies on the etiology of HLB have been met with a serious problem in two historic citrus production areas, great challenges. A milestone was established by Dr. namely, the Chaoshan and Pearl River Delta plains (Fig. Lin K H (Lin 1956a) who established a hypothesis of 1) and remains endemic there (Liao et al. 2007; Tang viral etiology (infectious agent) involving insect vectors. et al. 2007). Typical HLB symptoms were initially de- Following the report of Jagoueix et al. (1994), scribed as one or more yellow shoots or tip-chlorosis “Canidatus Liberibacter asiaticus”, an alfa- from the green canopy, followed by leaf yellowing, proteobacterium, was later confirmed to be associated mottling, defoliation, and tree decline that reduced and with HLB in Guangdong (Deng and Tang 1996). A finally destroyed tree productivity (Chen 1943; Lin phytoplasma was detected in HLB samples (Chen et al.

Received 7 April, 2011 Accepted 4 July, 2011 Correspondence DENG Xiao-ling, Mobiel: 13925067776, Fax: +86-20-38297040, E-mail: [email protected]; LI Hua-ping, E-mail: [email protected]

2009). Yet, data about HLB-associated phytoplasma to reemerge in early 2000s (Liu 2003). Understanding and other possible agents are very limited. Nevertheless, the prevalence of “Ca. L. asiaticus” provides critical b