Potential of bioinsecticidal Bacillus thuringiensis inoculum to suppress gray mold in tomato based on induced systemic resistance

Abstract The potential of the active ingredient of a commercial bioinsecticide, XenTari ® ( Bacillus thuringiensis [ BT ] serovar aizawai strain ABTS‐1857), to suppress gray mold in tomato plants was elucidated. First, a suspension of the active ingredient of XenTari ® and a liquid culture of the bacterial strain as BT inocula were sprayed onto detached leaves or drenched into pots of tomato seedlings, and then, propagules of the gray mold fungus, Botrytis cinerea, were inoculated onto the leaves. The gray mold disease was significantly suppressed when rhizospheres were drenched with either inoculum, but not when inocula were sprayed onto detached leaves of seedlings. Both BT inocula were verified not to directly inhibit the mycelial growth of B. cinerea based on in vitro culture plate assays. Additionally, real‐time RT‐PCR analysis verified that the active ingredient increased the expression levels of defence‐related genes, such as PR‐1 ( P6 ) and P4 , in the leaves of tomato seedlings. These results suggest that the active ingredient has the potential to suppress gray mold disease in tomato, not through direct antagonistic interactions with B. cinerea , but rather through systemic activation of the plant defence system by increased expression of several defence‐related genes.