[Screening, identification and biocontrol effect of antagonistic actinomycetes against the pathogen of Cytospora sp. for apple tree].

The pathogen of Cytospora sp. was considered as the target fungus to evaluate the bio-control potential of antagonistic actinomycetes against Cytospora sp. in the present study, which was isolated from the rhizosphere soil of apple tree. Moreover, the antagonistic effect of actinomycetes against Cytospora sp. growth was determined by the method of confrontation and growth rate, and the screened antagonistic strain was identified by the method of morphology and molecular biology. Also, the inhibitory activity of strain ZZ-9 against the conidium germination and mycelia growth of Cytospora sp. was determined in vivo, and its bio-control effect on Cytospora sp. growth was determined in detached apple twigs. The results showed that fifteen strains of actinomycetes had the inhibitory effect on Cytospora sp. growth, among all the isolated strains, which accounted for 18.8% and especially the inhibitory rate of eight strains was more than 50%. In addition, among all the screened strains, the strain ZZ-9 presented the highest inhibitory rate of 96.4%, which was significantly higher than those of the other isolated strains. The strain ZZ-9 was initially identified as Streptomyces rochei according to cultural characteristics, physiological and biochemical properties and 16S rDNA analysis. The nucleotide sequences in GenBank accession number was registered as KT986228. Different dilution of ZZ-9 fermentation had significant inhibitory effect on Cytospora sp. conidium germination and mycelia growth. The inhibitory rates of the 50 times fermentation on both conidium germination and mycelia growth were more than 80%. The inhibited mycelia's color was deepened, the mycelia branches were increased, and the ends of hyphae were swelled and deformed, even the protoplasm was concentrated and released. The bio-control effect of the ZZ-9 stock solution on Cytospora sp. growth was more than 75% in detached apple twigs. Thus, our results indicated that the strain ZZ-9 could be used for controlling apple tree Valsa canker in vivo and vitro.