Identification of Monilinia yunnanensis Causing Brown Rot in Korla Fragrant Pear and Evaluation of Bacillus siamensis PL55 as a Biocontrol Agent

To identify the primary pathogen responsible for brown rot in mature Korla fragrant pears and to screen for effective biocontrol bacterial strains, a research team collected 30 samples of brown rot-infected fruits and 84 samples from healthy trees (including branches, leaves, and fruits) from three Korla pear cultivation areas in Korla, Aksu, and Kashgar between September and October 2024. Investigation into the severity of the disease revealed the local field incidence rate of brown rot ranged from 17% to 31%. From the 30 disease samples, 23 morphologically identical fungal strains were isolated, with 13 strains isolated from Korla City (56.5%), 6 from Aksu City (26.1%), and 4 from Tumushuk (17.4%), all detected in infected fruits. Pathogenicity tests were conducted using both inoculation by wounding with mycelium and spray inoculation experiments to study the pathogens’ effect on brown rot in Korla fragrant pears. From the 84 samples of healthy trees, 55 bacterial strains were isolated, and the antagonistic bacteria’s inhibitory effect on the isolated pathogens was determined using the dual-culture method. The pathogen was identified by morphological characteristics and phylogenetic analysis based on multi-locus sequencing (ITS-TUB2-Lcc2). Antagonistic bacterial strains were identified through morphological observation and 16S rDNA sequencing. The results demonstrated that the pathogen isolated from the diseased Korla fragrant pear tissues was identified as Monilinia yunnanensis. Among the biocontrol antagonistic bacteria isolated from the branches, bark, and leaves of healthy Korla fragrant pear trees, Bacillus siamensis exhibited significant inhibitory effects against the pathogen, with an in vitro inhibition rate of (88.18 ± 3.43)%.