Bacillus velezensis CE 100 controls anthracnose disease in walnut trees (Juglans regia L.) by inhibiting Colletotrichum gloeosporioides and eliciting induced systemic resistance.

Biological control of plant diseases is recognized as an effective and environmental friendly alternative to chemical fungicides. We demonstrated the dual biocontrol strategy of Bacillus velezensis CE 100 through the hydrolytic activity of chitinase and β-1,3-glucanase and the elicitation of induced systemic resistance (ISR) against Colletotrichum gloeosporioides that causes anthracnose disease in walnut trees. B. velezensis CE 100 produced a maximum of 62.1 units mL^-1 (132.9 units mL^-1) chitinase and 5.2 units mL^-1 (9.4 units mL^-1) β-1,3-glucanase enzymes in the broth culture (crude enzyme fraction), and inhibited spore germination and mycelial growth of C. gloeosporioides by 81.6% and 22.6%, respectively, at 100 µl mL^-1 of crude enzyme fraction. The inoculation of B. velezensis CE 100 induced the production of pathogenesis-related (PR) chitinase in walnuts by 2.1-fold, and to a lesser extent PR β-1,3-glucanase, and reduced anthracnose disease severity by 3.0-fold compared to the control group. The bacterium produced a maximum of 11.4 µg mL^-1 indole-3-acetic acid (IAA) and improved the chlorophyll content, shoot length, and root collar diameter of walnut trees compared to the fungicide treatment and control groups. B. velezensis CE 100 demonstrated the prospect of controlling walnut anthracnose by direct antagonism and ISR against C. gloeosporioides, while simultaneously enhancing walnut growth through IAA production.