Resistance evaluation and utilization of different kiwifruit cultivars to Pseudomonas syringae pv. actinidiae

Abstract:【Objective】Kiwifruit is one of the most popular commercial fruit crops around the world be- cause of its high vitamin C content and good taste. However, kiwifruit bacterial cancer caused by Pseu- domonas syringae pv. actinidiae (Psa) has been widely reported as the biggest limited factor of kiwi- fruit production. In particular, with the large-scale planting of susceptible cultivars, this disease has displayed a rapid spreading trend in recent years. Utilization of resistant kiwifruit cultivars has always been recognized as the most cost-effective and environment-friendly strategy for disease control, but there is still a lack of knowledge about the disease resistance of different cultivars in kiwifruit produc- tion. This study aimed to screen out Psa-resistant kiwifruit cultivars through in vitro and natural induc- tion method and to establish the evaluation criterion for local kiwifruit industry, which will be helpful for effective disease control and resistance breeding in the future.【Methods】For indoor resistance iden- tification, total 12 kiwifruit cultivars were selected for screening through the wounding method of de-3 tached branches when the susceptible cultivars Hongyang and Hayward were used as the controls. The bacterial suspension of Psa was prepared to use sterile water to obtain a concentration of 1×108 cfu·mL-1, and kiwifruit cultivars were inoculated with 10 ul bacterial suspension on the annual healthy branches. After 15 days of inoculation, the epidermis of the branches near the inoculation site was cut off, and the size of the lesions was measured. The cultivar resistance was classified as: high susceptibility (HS), the length of the lesion and the degree of browning are greater than those of Hongyang; disease resistance (R), the length of the lesion is less than that of Hayward; medium susceptible (MS), the lesion length is between Hongyang and Hayward, but closer to that of Hongyang; medium resistance (MR), the lesion length is between Hongyang and Hayward, but closer to that of Hayward. For field trails, 23 kiwifruit cultivars were identified by natural induction methods in the field. I n the canker disease-endemic or- chard, the branches of these cultivars were grafted onto the rootstocks in the spring of 2011, and the dis- ease characteristics and disease index were monitored and recorded from 2012 to 2019, when the dis- ease index was used as the standard to rank the resistance of each cultivar. In addition, the canker dis- ease rates of different cultivars grown in a large scale in Sichuan province were investigated to compare their differences in disease resistance under large-scale planting conditions. Based on the resistant culti- vars screened (such as Cuixiang, Cuiyu, Xuxiang, and Hayward) in this study, the reasonable utilization of these resistant kiwifruit cultivars was conducted instead of those susceptible cultivars in the disease- serious orchards, and its application effect on prevention was investigated, and finally the control of bac- terial cancer was evaluated through calculating the disease incidence and disease level on 3-year regrafting plants during the peak period of canker disease.【Results】The results showed there were significant differences in disease resistance among different cultivars. After 15 days of inoculation in vitro, the symptoms of susceptible materials were characterized as brown lesion at the inoculation site, extending from the wound to both ends in a long strip, followed by a large extension and pus formation at the wound with disease development (e.g., Hongyang, Hort16A, and Jinyan). The symptom of disease-resis- tant materials displayed a relatively smaller browning area at the inoculation site, with lighter color and only spreading around the wound, which were typically observed in the cultivars such as Hayward, Longshan, and Guihai 4. Based on the resistance grades to cancer disease, the tested cultivars were distinct in different resistance grades, among which, the high resistance accounted for 2 when medium re- sistance and medium susceptibility were 2 and 6 cultivars, respectively. Among the 12 cultivars tested, none of the highly susceptible cultivars displayed almost with close symptoms as Hongyang, while all highly resistant cultivars showed a little lesion. For field resistance evaluation of canker disease, the numbers of cultivars with high resistance, resistance, medium resistance, medium susceptibility, suscep- tibility and high susceptibility were 3, 2, 6, 5, 4 and 3 through naturally induced inoculation in the field, respectively. Among them, the plants of Hongyang, Hort16A and Jinyan cultivars had almost dead symptoms, when those cultivars including Jinkui, Huate and Kuilu did not show obvious disease symp- toms from 2011 to 2019. Furthermore, the cultivars Hongyang, Hort16A and Jinyan belonging to Actin- idia chinensis were highly susceptible in almost all kiwifruit producing areas, and the disease incidence of these three cultivars reached up to 100%, eventually leading to the destruction of the entire orchard. The disease incidence of Guichang also increased by 45.25%, and showed weaker resistance than that of Cuixiang and Xuxiang. The cultivars with high resistance to the canker disease included Hayward, Cuiyu and Kuilu, whose plants had less than 1% individuals over five years old suffered from the can- ker disease. When high-sensitivity cultivars like Hongyang and Hort16A were replaced by Cuixiang, Cuiyu, Xuxiang, Hayward and other disease-resistant cultivars in the diseased orchards, the disease inci- dence had a significant decrease, and the relative control effect of canker disease was more than 85% as compared to the control orchards with almost 100% of disease incidence.【Conclusion】Through a com- prehensive evaluation of the resistance among different kiwifruit cultivars, it is clear that the current main kiwifruit cultivars have differences in resistance to canker disease. Actinidia arguta and A. eriantha had the highest resistance in all cultivars, followed by A. deliciosa, while A. chinensis is of the low- est resistance to canker disease. The combination method of the indoor inoculation of detached branches and naturally induced inoculation is practicable for resistance identification of kiwifruit cultivars. The results show that screening and utilization of disease-resistant cultivars is an important way to avoid the harm of kiwifruit bacterial canker disease.