Pathogen identification of kiwifruit bacterial canker and screening of indoor control agents

【Objective】 This study aimed to identify the pathogen of kiwifruit bacterial canker in Heping County， Guangdong Province， screen effective agents to control the disease， thereby providing a theoretical basis for the diagnosis and control of kiwifruit bacterial canker.【Method】 Diseased branches of kiwifruit bacterial canker were collected from Heping County， Guangdong Province. Pathogenic bacteria were isolated using the spread plate method， and their pathogenicity was verified according to Koch’s postulates. Based on Gram staining， morphological characteristics observation， and sequence analysis of 16S rRNA and carA genes， the taxonomic status of the pathogen was clarified. Effects of diffe-rent temperatures on pathogen growth were analyzed to clarify the difference between the pathogen and Pseudomonas syringae pv. actinidiae， a common pathogen of kiwifruit bacterial canker. Using the inhibition zone method and methyl thiazolyl tetrazolium （MTT） assay， indoor toxicity of seven tested agents （1.5% tetramycin AS， 85% thiadiazole copper TC， 31.5% zhongshengmycin TK， 20% cuaminosulfate acetate AS， 98% oxine-copper TC， 83.8% kasugamycin TC， and 98% streptomycin sulfate TC） was evaluated.【Result】 One bacterial strain， designated as GD， was isolated from the branches infected by kiwifruit bacterial canker. The strain GD was identified as the pathogen of kiwifruit bacterial canker based on Koch’s postulates， whose colonies were off-white to light yellow， with a smooth and humid surface and slight raised appearance. The strain GD was identified as a Gram-negative bacterium by Gram staining； microscopic observation revealed that the cells were short rod-shaped， with blunt and rounded ends， and they were relatively uniform in width， with their sizes of （0.5-1.0） μm×（2.0-3.0） μm. According to analysis of 16S rRNA and carA gene sequences， the strain was finally identified as Pectobacterium carotovorum subsp. actinidiae， which was different from the common kiwifruit bacterial canker pathogen Pseudomonas syringae pv. actinidiae. The optimal growth temperature of Pectobacterium carotovorum subsp. actinidiae was 35 ℃， and it still maintained certain viability in a high-temperature environment of 40 ℃； in contrast， the optimal growth temperature of Pseudomonas syringae pv. actinidiae was 25 ℃， and its growth was inhi-bited when the temperature exceeded 35 ℃. The results of indoor toxicity tests showed that 1.5% tetramycin AS demonstrated the strongest inhibitory effect against both Pectobacterium carotovorum subsp. actinidiae and Pseudomonas syringae pv. actinidiae. At the concentrations of 6.25， 3.125， and 1.5625 mg/mL， the diameters of inhibition zone against the two pathogens were significantly higher than those of the conventional control agent 98% streptomycin sulfate TC （P<0.05）. The half maximal inhibitory concentration （IC₅₀） values of 1.5% tetramycin AS against strain Pectobacterium carotovorum subsp. actinidiae and Pseudomonas syringae pv. actinidiae were 24.28 and 6.29 μg/mL， respectively.【Conclusion】 The kiwifruit bacterial canker pathogen in Heping County， Guangdong Province is Pectobacterium carotovorum subsp. actinidiae， whose optimal growth temperature is 35 °C， and it is more tolerant to high temperature than the common kiwifruit bacterial canker pathogen Pseudomonas syringae pv. actinidiae. 1.5% tetramycin AS shows excellent inhibitory effects on both Pectobacterium carotovorum subsp. actinidiae and Pseudomonas syringae pv. actinidiae， there by it can be taken as a candidate agent for controlling kiwifruit bacterial canker.