Evaluation of resistance of kiwifruit varieties (line) against bacterial canker disease and correlation analysis among evaluation indexes

Abstract:【Objective】In recent years, as the continuous increase of the global kiwifruit cultivation , the occurrance of the bacterial canker disease has been increased The disease has the characteristics of strong pathogenicity, fast transmission speed, and difficulty to eradicate, resulting in restriction of the development of kiwifruit industry. The disease is caused by Pseudomonas syringae pv. actinidiae (Psa). Up to now, the research on the disease mostly focuses on the biological characteristics of the pathogen, cultivation management technology and field control methods. Selection of varieties resistant to the dis- ease would be beneficial to the industry. The analysis of the resistance of different kiwifruit accessions to bacterial canker disease and the correlation between different evaluation indexes are of great significance to breeding new kiwifruit varieties resistant to the disease.【Methods】The kiwifruit varieties (lines) used in this study were all from the Kiwifruit Germplasm Collection of Zhengzhou Fruit Re- search Institute, Chinese Academy of Agricultural Sciences. Pseudomonas syringae pv. actinidiae (Psa), kiwifruit bacterial canker pathogen, was provided by the Institute of Horticulture, Zhejiang Academy of Agricultural Sciences. Firstly, specific primers PsaF1 and PsaR1 were used for PCR amplification to identify the correctness of the strain, and then the Psa was cultured in beef extract peptone medium (shaker temperature 22-24°C, oscillation frequency 200 r·min-1, time 24 h), and the microbial concen- tration of the Psa was diluted to 1×108 colony-forming units cfu·mL-1 prior to inoculation. The One-year old detached shoots, approximately 0.8 cm in diameter, were sterilized with 75% alcohol and then cut into 10 cm shoots, the ends of the shoots were dipped in candle wax to reduce dehydration. A wound of about 1-1.5 cm was made with a file from each end of the shoot and the Psa was added to the wound with a pipette. The inoculated shoots were placed in trays, which were placed in an artificial climate in- cubator at 22-24 °C and 80% relative humidity for 12 h day/night cycles. The ploidy of different kiwi- fruit varieties (lines). was detected by flow cytometry to analyze the relationship between disease resis- tance and ploidy. The test data were statistically analyzed using Excel and SPSS 13.0 software.【Results】6 days after the Psa inoculation of kiwifruit isolated shoots, the disease infection occurred in some vari- eties (lines). The inoculation area became reddish brown and exhibited milky white bacterial pus. 21 days after the Psa inoculation, the disease indexes of different varieties were calculated and classified according to the evaluation standard of kiwifruit bacterial canker disease resistance. The results indicat- ed that‘Hongyang’and‘2-72’exhibited high susceptibility;‘Hort16-A’‘6-65’‘1-74’‘14-57’and‘6-20’exhibited susceptibility;‘2-43’‘Jintao’‘Jinyan’‘Youxi-52’showed tolerance;‘Xuxiang’‘Hward’‘Bulunuo’‘Hongbaoshixing’‘SE1- 5’‘P1- 2’‘P4- 2’‘N4- 32’‘P6- 1’‘P3- 3’‘11- 17’ ‘N4-25’‘N14-9’‘Jinkui’and‘TML’showed resistance; and‘N12-16’‘N12-3’and‘Huate’showed high resistance. In addition, 21 days after the Psa inoculation, lesion length was measured witha vernier caliper. The average length of disease spots of high susceptible varieties, susceptible varieties, tolerant varieties, resistant varieties and high resistant varieties were 29.53, 15.63, 10.12, 2.86, 0.79 mm, respectively. The results showed that the higher the disease resistance of kiwifruits was, the small- er the length of lesion was; The SPSS correlation analysis showed that the disease indexes of 29 kiwi- fruit varieties (lines) were significantly positively correlated with the lesion lengths, and the correlation coefficient R was 0.957. In addition, the ploidy of 29 kiwifruit varieties (lines) was detected by flow cy- tometry. Among A. chinensis, high susceptible varieties‘Hongyang’,‘2-72’and susceptible varieties‘1-74’and‘Hort16A’were diploid, while tolerance varieties‘Jintao’and‘Jinyan’were tetraploid. The nine A. deliciosa kiwifruit materials were all hexaploid ploidy, among them three materials showed susceptibility, one showed tolerance, and the remaining 5 showed resistance, which accounted for 55.6%. The seven A. arguta kiwifruit materials were all tetraploid, and all showed resistantce. The sixA. eriantha kiwifruit materials were all diploid, and all showed resistance or high resistantce. There was a tendency that among A. chinensis, the biger the ploidy number was, the stronger the disease resistance degree was. However, the effect of interspecific ploidy on resistance was not significant.【Conclusion】Among the methods for the identifying the resistance of kiwifruit to the canker disease, kiwifruit detached shoot inoculation was a common and effective method. The mothod was simple to operate and easy to master, and the identification results were statistically reliable. The disease index was signifi- cantly and positively correlated with the lesion length. It seems that the tetraploidy of the kiwifruit accessions would be more resistant to the disease than the diploidy ones in A. chinensis.