Objective To isolate bacterium strains that have strongly inhibitory effects against Lasiodiplodia theobromae, the causal agent of mulberry root rot, and test its field control efficiency.

Method Bacterium strains were isolated from rhizosphere soil of healthy mulberry by plate dilution method.Strains with remarkable antagonist activities against target pathogen were screened by dual culture and spore-germination techniques. The antagonistic bacteria were identified based on morphological, physiological and biochemical characteristics as well as their gyrA and gyrB gene sequences. Field plot experiments were set up to determine the biocontrol effects of antagonistic bacteria under natural conditions.

Result Eight bacterium strains with inhibitory effects against L. theobromae were isolated, and YZ14-3 was the most effective among them. The inhibition rate of YZ14-3 and its culture extracts were 73.3% and 55.6%, respectively. The mycelia of L. theobromae in the inhibition zone were enlarged and malformed; the conidia could not germinate and disintegrated due to cell wall degradation in the culture extracts of YZ14-3. YZ14-3 was identified as Bacillus amyloliquefaciens using its morphological, physiological and biochemical characteristics as well as the phylogenetic tree based on the BLAST results with gyrA and gyrB gene sequences. Field tests showed that the biocontrol efficiency of YZ14-3 was 67.94%.

Conclusion The B. amyloliquefaciens strain YZ14-3 has remarkable inhibitory effect against L. theobromae, and therefore it can be used for biocontrol of diseases caused by L. theobromae.