Investigation And Disease-management Development Of Citrus Huanglongbing In Guangdong Province

Citrus huanglongbing(HLB)has been a serious deisease to citrus not only in the Guangdong province in China,but to the global citrus industry around the world.In recent years,trees etiolation is continuously reported in large areas of citrus growing regions,about a third of which is in Guangdong province's 240000 hectares of citrus orchard.To characterize the occurance of different types of HLB,we analysed more than 2000 randomly collected citrus leaf samples in the field,including 16 citrus varieties at 18 cities in the Guangdong province from August 10,2013 to January 14,2015,as well as more than 800 citrus leaf samples in nurseries from September 4,2013 to December 2014,by real-time PCR.Our results showed that detection rate was high on plants in the field(corresponding to 1506 positive samples),from which the detection rate reached 71.2%.Among the analysis of 16 citrus varieties,we found that some samples had detection rate lower than 50%,including Bergamot,Sweet pomelos,Huazhou red pomelo,and Pomelo,etc.In contrast,detection rate of some samples were 100%,including Wenzhou mandarin oranges,Fujian orange,Murcott orange,and so on.Moreover,low but sematic detection rate was found in nursery plants.In total,there were 51 positive samples detected,corresponding to a detection rate of 6.1%.Among them,the positive detection rate of the Hongjiang orange and Sugar orange were about 6.5%,while the Tribute orange and Fructus aurantii were not detected.According to the survey results,we focused on four citrus orchards in the Guangdong Province.We randomly selected 100 trees for chlorophyll content and HLB bacterial copy numbers measurement in each orchard.At the same time,we built a quantitative relationship between the 16S rDNA quantitative Ct value and bacterial titers(copy numbers).Based on the chlorophyll content and 16S rDNA quantitative Ct parameters,we were able to establish a disease classification criteria.Research results from the four orchards showed that 49%citrus plants with yellowing phenotype were diagnosied as HLB positive,14%citrus plants with yellowing phenotype were diagnosied as HLB negative;20%citrus plants appeared no yellowing phenotype were diagnosied as positive,while 17%citrus plants appeared no yellowing phenotype were diagnosied as negative.Therefore,scientific prevention and protection is very necessary for HLB control.To explore efficient prevention and control approaches of citrus HLB,we foliar sprayed plant growth regulators on two-years-old plants,and injected chemicals in eight-year-old adult plants.The results showed that foliar regulator-spraying could maintain pathogen growth rate at low rate:application of IAA and NAA composite regulators had pathogen growth rate at 26.08%and 32.34%,respectively;Injection of three kinds of chemicals for the prevention and controling of citrus HLB had a relatively better effection.Comparison of different types of injection results showed that the Sugar orange plants injected with Ampicillin had a relative control effect of 86.98%,while the Tribute orange plants injected with Ciprofloxacin had a relative control effect of 87.24%.By sampling multiple citrus tress from diverse orchards,we made a comprehensive survey of HLB development in the Guangdong province.We also explored citrus tree restoration as a management for trees infected with HLB.Our results will be helpful to develop an efficient HLB control system.