Effect of ultrasonic and heat treatment on killing ulcerative pathogens in branches and seedling growth in kiwifruit

Abstract: 【Objective】Bacterial canker of kiwifruit caused by Pseudomonas seudomonas syringae pv.actinidiae (Psa) is currently the major threat to its commercial production worldwide, which belongs to a typical low-temperature disease and mainly infects the arms, lateral branches and leaves of kiwifruit.The main transmission routes of P. syringae pv. actinidiae (Psa) who always has colonized in the phloem and xylem ducts are boughs and seedlings. As traditional chemical preventions are difficult to kill P.syringae pv. actinidiae (Psa) parasitizing in branches, nursery stock treatment is frequently more effective. Thus, the effects of ultrasonic + heat treatment on killing P. syringae pv. actinidiae (Psa) and seedling growth are evaluated, respectively, which can provide a theoretical basis for bacterial canker controlling by reducing the spread of P. syringae pv. actinidiae (Psa) in kiwifruit branches and seedlings.【Methods】Kiwifruit branches with bacterial canker were selected as raw materials, and the sterilization rates of ultrasonic + heat treatment were determinated through the colony counting method. Single factor experiment was carried out at different treatment temperatures (6 gradients every 5 ℃ from 25 to50 ℃) , treatment times (7 gradients every 10 min from 0 to 60 min) , and ultrasonic frequencies (6 gradients every 20 kHz from 0 to 100 kHz) . Based on the optimum treatment temperature, ultrasonic frequency and treatment time were obtained from the single factor test, and then Box-Benhnken center experiment (Taking the best value point of each single factor as the center and from the upper and lower regions taking one level value as the design level, 17 test points were designed with three factors and three levels, among which the zero point test was repeated 5 times to estimate the test error) , response surface and the Design – Expert 8.0.5 b statistical software regression analysis were used to obtain optimum sterilization process after ultrasonic + heat treatment. Finally, kiwifruit seedlings treated by the optimum process were planted in field to observe their survival rate, leaf area, seedling height, leaf number and growth situation of near-ground diameter.【Results】The best single factor condition was treating temperature at 45 ℃, ultrasonic frequency at 80 kHz and treating time for 30 min, respectively. Further response surface analysis showed that the sterilization rate was up to 97.75% after the optimal sterilization process was applied (treating temperature at 45 ℃, ultrasonic frequency at 80 kHz and treatment time for 30 min) . The results of regression analysis briefly showed that the sterilization rate under optimal conditions was 97.88%, which showed no significant difference from the results of response surface optimization (α = 0.01) . The results of model variance analysis showed that the regression model was very significant (p = 0.000 1) , and the lack of fit (p = 0.083 3 > 0.01) showed no significant difference, which meant that the fitting degree of the model had the greatest influence. And following model variance analysis showed that both the primary terms ( A (p < 0.000 1) and C (p = 0.000 3) and secondary terms (A2 (p < 0.000 1) and C2 (p = 0.000 7) ) had a significant difference in the model. And the A and A2 were very significant, which indicated that temperature was the most important factor in the sterilization process. Besides, the interaction showed that the order of interaction strength was AC >BC > AB, which meant the interaction between treatment temperature and time had the greatest influence on the sterilization rate. Furthermore, the effect of each single factor on sterilization rate hardly showed a simple linear relationship. The order of the efficacy is A (treatment temperature) > B (treatment time) > C (ultrasonic frequency) . In the end, the results of transplanting experiment of the treated kiwifruit plants showed that the survival rate of the treated group was 100%, and the leaf area, seedling height, number of leaves and near-ground diameter had no significant difference between the control and treatment (α = 0.05) .【Conclusion】The results remarkably showed that the optimal sterilization process of ultrasonic and heat treatment (treating temperature at 45 ℃, ultrasonic frequency at 80 kHz and treating time for 30 min) can not only kill P. syringae pv. actinidiae (Psa) living in kiwifruit shoots and seedlings effectively, but also have no negative effect on its survival rate and growth. And this method was clean, efficient, convenient and quick, comparing with the current chemical and biological prevention. What's more, this treatment provided a new direction in the prevention and treatment of P. syringae pv. actinidiae (Psa) . Therefore, this study has a potential application for the P. syringae pv. actinidiae (Psa) controlling in the infected branches of kiwifruit plantings in the field.