Exploring the potential of the almond bacterial microbiota as biocontrol agents: an in vitro evaluation of antagonism against pathogens

Abstract: Almond (Prunus amygdalus) is a very important fruit tree species, as almonds are
the most consumed nut worldwide. Similarly to other agricultural resources, almond production may be reduced due to pests and diseases, the latter mostly including fungal and bacterial pathogens. In this study, we explored the potential of the almond bacterial microbiota to be used as biological control agents against diseases. A total of 1090 bacterial morphotypes were isolated from almond samples collected on two consecutive seasons, 2022 and 2023, from different locations, plant materials and cultivars. In vitro assays were conducted to evaluate the antagonistic activity of those bacterial isolates against the pathogenic fungi Colletotrichum chrysophilum, C. godetiae, Diaporthe amygdali, Diplodia seriata, Eutypa lata and Monilinia laxa, and the bacterial pathogen Xanthomonas arboricola pv pruni. The inhibition halos and relative bacterial and fungal growth were measured, whereas a global antagonism score was visually assigned for all pathogens. Our results revealed that 18 bacteria exhibited the highest antagonism against all six pathogenic fungi, whereas 16 isolates performed similarly on X. arboricola growth. Moreover, three bacteria showed full antagonism to all fungal and bacterial pathogens. These results suggest the efficacy of the tested bacteria as potential biocontrol agents and a promising environmentally sustainable alternative to chemical pesticides though more studies on survival and performance in field conditions are still required to confirm it.