Characteristics and Biocontrol Activity of Soil Bacterial Isolates Against the Emerging Pathogen Fusarium solani in Thai Durian (Durio zibethinus)

Fusarium solani is an emerging pathogen responsible for Fusarium-related diseases in durian trees in Thailand. Several chemical fungicides and biocontrol agents are ineffective in controlling these diseases, which affects durian trees and reduces yields. This study aimed to identify soil-derived bacteria with biocontrol activity against F. solani that surpasses traditional biocontrol bacteria. The characteristics and biocontrol efficacy of effective isolates were analyzed. Four isolates from 107 bacterial isolates were identified as effective biocontrol agents against F. solani. Isolate S301 exhibited the highest inhibition at 74.31%, exceeding that of the traditional biocontrol bacterium Bacillus subtilis. These isolates antagonized F. solani by producing siderophores, fungal cell wall lytic enzymes, and hydrogen cyanide, and by promoting plant growth. Molecular and phylogenetic analyses identified the four isolates as members of the Bacillus genus, specifically B. safensis, B. thuringiensis, B. subtilis, and B. cereus. The application of B. safensis strain S101 and B. subtilis strain S301 showed potential to reduce fungal disease symptoms on Monthong durian leaves. These findings are the first to demonstrate the potential of B. safensis and B. subtilis as promising bacterial biocontrol agents for managing F. solani-related diseases in durian trees in Thailand.