Screening and possible mechanism of strains against black spot of Korla pear

Abstract:

[Background] Alternaria alternate-induced black spot, among the major diseases of Korla pear during storage, lowers the quality of the fruit, thus causing huge economic loss. [Objective] To screen strains with strong inhibitory effect on A. alternata, explore the underlying mechanism, and thus to lay a theoretical basis for postharvest preservation of Korla pear.[Methods] The rhizosphere soil samples of healthy Korla pear plants were collected and bacteria were isolated and purified by serial dilution and plating. The antagonistic strains were screened by plate confrontation assay and cylinder plate method. The preliminary identification of antagonistic bacteria was based on morphological characteristics and 16S rRNA gene sequence analysis. The antifungal spectrum of antagonistic bacteria was determined by plate confrontation assay. The extracellular enzyme species of antagonistic bacteria were detected by specific plate. The enzyme activity was quantified with 3,5-dinitrosalicylic acid colorimetric method and Folin-phenol method. Spore germination test and scanning electron microscopy (SEM) were employed to detect the antagonistic effect of the bacteria on A. alternaria. [Results] JE53 and JE56 with strong activity against A. alternaria were screened out from 152 isolates, with inhibition zone diameters of 22.49 mm and 22.40 mm, respectively. According to phylogeny analysis, they were identified as Paenibacillus polymyxa (JE53, GenBank accession No. OL966428) and Bacillus subtilis (JE56, GenBank accession No. OL966429), separately. The results of antifungal spectrum showed that JE53 and JE56 had different degree of inhibition on 9 plant pathogens such as Rhizoctonia solani. They produced four kinds of extracellular enzymes. The supernatant of the fermentation broth could significantly suppress the spore germination of A. alternaria, with inhibition rates of 70.30% and 88.87%, respectively. SEM showed that the suppressed hyphae were rough and distorted.[Conclusion] JE53 and JE56 inhibit A. alternaria, which have the potential of further development and application.