Effects Of Citrus Fruit Quality By Citrus Huanglongbing And Preliminary Screening Of Agents Against Huanglongbing In The Field

Citrus Huanglongbing(HLB) is a destructive disease affecting the citrus production and seriously threat to the development of citrus industry. Several reports has revealed the quality of citrus fruit could be affected by HLB. However, only individual citrus variety and few evaluation indicators have been used and analysed in previous studies. In this study, to evaluate the quality of citrus fruit affected by HLB, a total of six citrus cultivars collected in Guangdong and Jiangxi were analyzed, including Hongjiangcheng(Citrus sinensis Osbeck CV. Hongjiangcheng), Newhall(Citrus sinensis Osbeck CV. Newhall), Gonggan(Citrus reticulata Blanco var. Gonggan), Tankan(Citrus reticulate Blanco CV. Tankan), Shatangju(Citrus reticulate Blanco CV. Shatangju) and Kinokuni(Citrus reticulate Blanco CV. Kinokuni). The evaluation indexes included fruit appearance quality, internal quality, coloring quality and sensory evaluation. Quantitative evaluation of fruit quality of different varieties affected by HLB had been provided the theoretical basis for the scientific assessment of HLB-infected fruits from different citrus varieties and the foundation for the analysis of disease mechanism.So far, no effective agent had been reported to prevent and control of HLB. Several studies had been repoeted field trials to screen effective agents for controlling HLB. However, due to the technological limitation, effect evaluation was only based on the changes of visible symptoms, which lacked the objective evaluation index. In this study, six kinds of agents had been applied in the field, including tetracycline hydrochloride, thiodiazole-copper, zhongshengmycin, tetramycin, ningnanmycin and resist(plant activator). The 8-years HLB-infected citrus trees, located in Qingyuan of Guangdong province, were conducted by two treatment methods, the trunk injection and the foliar spray with root irrigation. Effect of different agents and treatment methods were evaluated by the development of symptoms and the titers of “Ca. L. asiaticus” were also evaluated by Taq Man Real-time PCR. Objective effect evaluation of 6 varieties agents to HLB can provide the scientific basis for comprehensive prevention and HLB control. The main results were shown as follows:HLB have significant impact on citrus fruit appearance quality. Compared with the healthy fruit, the fruit size and fruit weight of HLB-infected fruit significantly decreased. After affected by HLB, the fruit shape had been changed. Moreover, the fruit shape index and firmness had been increased(P<0.05). However, the seed number and seed rate has no significant differences between HLB-infected and healthy fruits(P>0.05). The peel rate were significantly increased on HLB-infected fruits of Hongjiangcheng, Newhall, Gonggan and Tankan significantly, The residue rate of HLB-infected fruits from Hongjiangcheng, Newhall and Shatangju were also increased and the peel thickness of HLB-infected fruits were only increased on Newhall and, reduced on Hongjiangcheng and Gonggan.By measuring the internal quality index of healthy fruits and HLB-infected fruits, compared with the healthy fruit, the total soluble solid content, vitamin C content and TSS/TA ratio of the HLB-infected fruit were significantly decreased(P<0.05). The titratable acid content of HLB-infected citrus from Gonggan, Tankan, Shatangju and Kinokuni significantly increased, but significantly decrease on Newhall. No obvious different had been identified between HLB-infected citrus and healthy fruit of Hongjiangcheng. Except the Shatangju and Kinokuni, the edible rate of all HLB-infected fruit were reduced. The fruit juice percentage were significantly decreased(P<0.05), except the Gonggan and Shatangju. These results showed that the inner quality of citrus fruit had been significantly influenced by HLB.The measuring of the fruit coloring quality between healthy fruits and HLB-infected fruits revealed that the peel brightness and color intensity of HLB-infected fruit significantly decreased(P<0.01) However, the color hue angles of peel significantly increased(P<0.01) on the HLB-affected fruit relative to the healthy fruit. The pulp brightness of HLB-infected fruit obviously increased on Hongjiangcheng, Newhall, Gonggan and Tankan, but decreased on Shatangju and Kinokuni. Except the Gonggan and Shatangju, the pulp color intensity of HLB-infected fruits significantly decreased(P<0.05). The color hue angles of pulp only significantly increased on HLB-infected fruit from Shatangju.The sensory evaluation of HLB-infected and healthy fruits from all six varieties had revealed that the sweetness, citrus flavor of HLB-infected fruits were obviously dropped that caused the obvious off-flavor. The tartness of HLB-affected fruits were increased on Hongjiangcheng, Tankan and Kinokuni, while it was obviously decreased on Gonggan. Besides the Gonggan, the richness of HLB-infected fruits from other varieties were obviously dropped. Except the Gonggan and Shatangju, the melting quality of HLB-infected fruits from other citrus varieties were much worse than healthy fruits.The result of field trials showed that the trunk injection of tetracycline hydrochloride 1g/L can significantly reduce the “Ca. L. asiaticus” titer(P<0.05). Compared with the controls, the symptoms of the HLB-infected trees didn't get worse, which showed a certain control efficacy of HLB. However, the leaf spraying and root irrigation with tetracycline hydrochloride didn't show the obvious decrease of “Ca. L. asiaticus” titer in HLB-infected trees(P >0.05). No obvious effect had been identified on HLB control by other agents.