Effectiveness of Endophytic and Rhizospheric Bacteria from <i>Moringa</i> spp. in Controlling <i>Pythium aphanidermatum</i> Damping-Off of Cabbage

In this study, endophytic and rhizospheric bacteria were isolated from <i>Moringa olifera</i> and <i>M. perigreina</i> from Oman, and their in vitro antagonistic activity against <i>Pythium aphanidermatum</i> was tested using a dual culture assay. The promising strains were tested further for their compatibility and potential for plant growth promotion, biofilm formation, antifungal volatile organic compound (VOC) production, and the biological control of <i>P. aphanidermatum</i> damping-off of cabbage (<i>Brassica oleracea</i> L.) under greenhouse conditions. A total of 12 endophytic and 27 rhizospheric bacteria were isolated from <i>Moringa</i> spp. Among them, <i>Bacillus pumilus</i> MPE1 showed the maximum antagonistic activity against <i>P. aphanidermatum</i> in the dual culture assay, followed by <i>Paenibacillus glucanolyticus</i> MPE3 and <i>Pseudomonas indica</i> MOR3 and MOR8. These bacterial isolates induced abundant morphological abnormalities in the hyphae of <i>P. aphanidermatum</i>, as observed via scanning electron microscopy. The in vitro cross-streak assay showed that these bacterial isolates were compatible among themselves, except for <i>P. indica</i> MOR8 × <i>P. glucanolyticus</i> MPE3. These antagonists released VOCs that restricted the growth of <i>P. aphanidermatum</i> in an in vitro assay. These antagonistic bacteria released 2,4-dimethylheptane and 4-methyloctane as the predominant volatile compounds. Of the four antagonistic bacterial strains, <i>P. indica</i> MOR8 was capable of forming biofilm, which is considered a trait that improves the efficacy of rhizosphere inoculants. The results of the greenhouse experiments showed that the soil treatment with <i>B. pumilus</i> MPE1 showed the highest reduction (59%) in the incidence of <i>P. aphanidermatum</i> damping-off in cabbage, evidencing its potential as a biological control agent for the management of this disease. Further research is needed to characterize the antifungal traits and activities of <i>B. pumilus</i> MPE1 and to assert its potential use against other soil-borne plant pathogens.