Efficacy of biofungicides based on Bacillus against apple scab (Venturia inaequalis) and their influence on rhizosphere fungal communities

Fungal phytopathogens cause crop diseases that pose a serious threat to food security worldwide. The most common and effective method to control crop diseases is the use of chemical fungicides . However, chemical fungicides can negatively impact the environment, human health, and the abundance and diversity of beneficial soil microorganisms . Environmentally friendly methods based on biological control agents (BCAs) have been proven to be selective and effective against phytopathogens, making them a viable ecological alternative to chemical fungicides. This study aimed to investigate the efficacy of BCAs based on Bacillus spp. against apple scab ( Venturia inaequalis ) and to assess their effect on fungal communities in the rhizosphere of apple trees compared to chemical fungicides. We evaluated Bacillus velezensis BZR 336g (3.0, 4.0 and 5.0 l ha −1 ) and Fitosporin-M, L, based on the Bacillus subtilis strain 26D (2.0 l ha −1 ), as biofungicides, and chemical treatments using Zato, WDG (trifloxystrobin 500 g l −1 ) and Score, EC (difenoconazole 250 g l −1 ). We assessed disease incidence on apple leaves and fruits, fruit yield, as well as the abundance and diversity of rhizosphere fungal communities using plating, RT-PCR, and DNA metabarcoding. Biofungicides applied to leaves and fruits demonstrated up to 80 % efficacy against V. inaequalis . Treatment with trifloxystrobin and difenoconazole resulted in a sharp decrease in fungal diversity in the rhizosphere, along with an increase in the absolute and relative abundances of fungal phytopathogens, primarily Fusarium , Rhizoctonia , and Verticillium . Since the total number of rhizosphere fungi remained stable, we can conclude that the development of phytopathogens in the rhizosphere of apple trees under chemical fungicides was due to the depletion of ecological niches previously occupied by saprotrophic and plant symbiotic fungi. Therefore, the long-term use of chemical fungicides may eventually lead to incidences of root rot and other plant diseases caused by soil-borne phytopathogens developed in the rhizosphere. The application of BCAs decreases the incidence of diseases in plants without affecting the diversity of rhizosphere soil fungi and can be used to reduce reliance on chemical fungicides.