Rhizospheric Bacteria: Promising Candidates for Biocontrol of Apple Trunk Pathogens

Abstract

Biocontrol provides a compelling alternative to fungicide applications for plant disease management. In the present study, bacteria from the rhizosphere of different fruit trees in Morocco were tested for their potential to inhibit causal agents of trunk diseases in apple trees, including Lasiodiplodia theobromae, Diaporthe eres, Neopestalotiopsis rosae and Diplodia seriata. These pathogens pose a considerable threat to worldwide apple production. Fifteen rhizobacterial isolates demonstrated notable antifungal activity against the tested fungal pathogens in vitro. Sequencing analysis classified these isolates into three bacterial genera: Bacillus, Stenotrophomonas and Alcaligenes. In vitro experiments demonstrated that Bacillus species were the most effective in inhibiting the mycelial growth of the aforementioned pathogens. For instance, B. subtilis PH31Z8 was highly effective against D. seriata (94.22% growth inhibition after 3 days), B. amyloliquofaciens PH34Z5 showed strong antagonistic activity against L. theobromae (94.12% inhibition) and D. eres (93.10%), whereas B. tequilensis AH31Z6 demonstrated notable efficacy against N. rosae (74.66%). Most of these bacterial strains secreted hydrolytic enzymes that can degrade fungal cell walls. In plant growth promotion assays with Brassica napus seedlings, the selected bacteria, particularly strains PH1Z8 and PM6Z12, enhanced plant growth compared with the negative controls. Experiments under glasshouse conditions revealed limited effectiveness of the antagonistic bacteria in reducing infections on apple plants, except for D. seriata treated with B. amyloliquefaciens PH34Z5, B. subtilis PH31Z8 and B. siamensis PC4Z9, which showed notable results. This study provides essential groundwork for advancing research on the biological control of apple canker diseases.