Kale (Brassica oleracea L. var. acephala) rhizosphere bacteria suppress Pythium aphanidermatum-induced damping-off of cabbage, produce biofilm and antimicrobial volatile compounds

In this study, rhizosphere and endophytic bacteria of kale (Brassica oleracea L. var. acephala) cultivars ‘Winterbor’ and ‘Black Magic’ were isolated and their efficacy to suppress damping-off of cabbage caused by Pythium aphanidermatum was evaluated. On nutrient agar medium, 10 bacterial isolates with different morphological characteristics were isolated from kale roots and rhizosphere soil. In an in vitro dual culture assay, two rhizosphere bacterial isolates (KRB1 and KRB2) obtained from ‘Winterbor’ exhibited high levels of inhibitory activity against the mycelial growth of P. aphanidermatum. Based on 16S rDNA sequence analysis, these bacterial isolates were identified as Pseudomonas alcaligenes (KRB1) and Rheinheimera mangrovi (KRB2). Soil application of P. alcaligenes KRB1 and R. mangrovi KRB2 reduced the damping-off incidence in cabbage by 42.4% and 45.5%, respectively compared to the infected control under controlled pot culture conditions. Both bacterial strains demonstrated the ability to form biofilm; however, P. alcaligenes KRB1 produced five times more biofilm than R. mangrovi KRB2. These bacterial strains produced volatile compounds that suppressed P. aphanidermatum growth in an in vitro assay. Gas chromatography-mass spectrometry analysis of volatile compounds revealed that P. alcaligenes KRB1 and R. mangrovi KRB2 predominantly produced L-alanine ethylamide, (S)-, accounting for 35.9% and 29.5% of the peak area, respectively. The capability to form biofilms and release anti-oomycete volatile compounds likely contributes to the biocontrol effectiveness of these bacterial strains against P. aphanidermatum.