Antagonistic microorganisms against Verticillium wilt in yellow-fleshed kiwifruit and their effect on Bruno rootstock physiology

Verticillium wilt, caused by Verticillium nonalfalfae MLST2, is an emerging disease affecting yellow-fleshed kiwifruit in Chile, and effective management strategies are currently unavailable. This study evaluated the biocontrol potential of antagonistic microorganisms against V. nonalfalfae MLST2, their interaction with different kiwifruit rootstocks, and their effects on plant physiological performance. Six Pseudomonas protegens and six Bacillus velezensis isolates were screened in vitro, and the most effective strains were subsequently evaluated with Trichoderma harzianum RGM3510 under controlled conditions. B. velezensis BcB1 and P. protegens C025 showed the highest in vitro inhibition of mycelial growth (46.2 and 40.2%, respectively). However, disease suppression in planta depended strongly on the rootstock genotype. Among the evaluated rootstocks, Z5Z6 consistently exhibited the lowest wilt severity, whereas P. protegens C025 significantly reduced disease development compared with the inoculated control. In contrast, B. velezensis BcB1 failed to suppress disease and, in some rootstocks, increased wilt severity. Although cumulative disease severity in Bruno plants was not significantly reduced by the biological treatments, P. protegens C025 produced disease levels comparable to the non-inoculated control. Moreover, treatments containing P. protegens improved stomatal conductance and photochemical reflectance index, indicating lower physiological stress associated with pathogen infection. These results demonstrate that the success of biological control against V. nonalfalfae MLST2 is determined by the interaction between antagonistic microorganisms and rootstock genotype, highlighting P. protegens C025 in combination with the Z5Z6 rootstock as a promising strategy for the integrated management of Verticillium wilt in kiwifruit.