Study on the main overwintering sites and survival ability of the pathogen of kiwifruit bacterial canker

【Objective】Kiwifruit bacterial canker, caused by Pseudomonas syringae pv. actinidiae (Psa), poses a severe threat to the healthy development of the kiwifruit industry in Shaanxi Province. Identifying the main overwintering sites of this pathogen and understanding its survival ability in different environments are of great theoretical and practical significance for precisely reducing the initial infection sources in the following spring and effectively controlling the bacterial canker of kiwifruit. 【Methods】To determine the main overwintering sites of Psa, as well as its survival ability and pathogenic activity, this study systematically collected samples of bark, canes, fruit stalks, soil, and exudate during winter. A specific quantitative real-time polymerase chain reaction (qPCR) technique was employed to detect the Psa-carrying rate in these samples, aiming to accurately understand the distribution of the pathogen in different locations. Through artificial inoculation experiments on detached branches, orchard soil, and water, and by regularly detecting the viable bacterial counts, the survival time of Psa in these different environments was quantitatively analyzed. The leaf-disk vacuum infiltration method was used to test the pathogenicity of Psa strains isolated and purified from winter bark, dead canes, soil, and Nongda Jinmi canes that had been inoculated for 109 days. This was to clarify whether the overwintering pathogen still maintained its pathogenic activity and to evaluate its potential to cause an epidemic in the following spring.【Results】The detection results showed that the bark of diseased vines had the highest Psa detection rate, reaching 75.0%. The detection rate of dead canes on the ground was 37.8%, which was significantly higher than the 10% detection rate of canes piled in ditches and fruit stalks of diseased trees. The Psa detection rate in the surface soil of diseased orchards was 25.0%. Among the exudates, the rust-colored waxy samples had the highest Psa detection rate, up to 80.0%, while the detection rates of reddish- brown and milky-white samples were 46.2% and 36.8% respectively. Studies on survival ability indicated that Psa had the longest survival period on detached canes, up to 109 days. In sterilized soil, it could survive for 55-65 days; in non-sterilized soil, the survival time was shortened to 43-55 days; and in soil containing residues, it was further reduced to 35-43 days. Psa had relatively strong survival ability in water environments, surviving for 63 days in tap water and 77 days in irrigation water. When the pathogenicity of the isolated Psa strains was tested, the average lesion areas of Psa strains BA12, BR5, S16, and M228-GFPuv1, which were isolated from winter bark, dead branches, soil, and Nongda Jinmi branches inoculated for 109 days respectively, on detached kiwifruit leaf disks, were 34.31, 25.45, 25.71, and 39.80 mm2 in sequence, all showing obvious pathogenicity.【Conclusion】 This study identified that the main overwintering sites of Psa in the kiwifruit- producing areas of Shaanxi Province are the vine bark, fallen canes, and surface soil, which together form the“three major pathogen reservoirs”of Psa. Specifically, the bark of diseased vines provides a stable overwintering environment for the pathogen; the dead canes widely distributed on the orchard ground are important carriers of the pathogen on the surface; and the surface soil of diseased orchards is also a key site for the pathogen to overwinter. There are obvious differences in the survival ability of Psa in different environments. Detached canes can serve as long-term storage sites for the pathogen, where Psa can survive for up to 109 days, making them an important source of initial infection in spring. The soil environment has a significant impact on the survival of Psa, and the soil microbial community inhibits the survival of the pathogen. Water is a potential transmission route for the pathogen. Psa can survive for 63 days in tap water and 77 days in irrigation water. During the exudation period, rainwater scouring or irrigation can cause the spread of the pathogen-carrying bleeding sap, which is a high-risk link for pathogen transmission. The Psa strains isolated from winter bark, dead canes, soil, and Nongda Jinmi canes inoculated for 109 days all showed significant pathogenicity, indicating that the overwintering pathogen still has the potential to cause an epidemic in spring. Based on the above research, a comprehensive prevention and control plan of“cane cleaning-soil purification-water control”is proposed. In terms of cane cleaning, fallen canes should be the focus of winter- spring orchard cleaning. Timely removal can reduce the amount of overwintering pathogens and the initial infection sources in spring. For soil purification, deep plowing and soil improvement of the surface soil should be carried out to improve the soil structure and microbial community, inhibiting the survival and reproduction of the pathogen. At the same time, the “two-before and two-after”medication strategy should be implemented to reduce the pathogen amount in vines and soil. Regarding water control, rain shelters should be set up during the bleeding sap period to reduce the spread of bleeding sap. Regular detection and disinfection of irrigation water and water storage containers should be carried out to block the secondary transmission route through water. This plan aims to precisely reduce the overwintering pathogen sources from the source, effectively block the epidemic of canker in the following spring, and provide a scientific and effective strategy for the sustainable control of kiwifruit canker.