Selection of Beneficial Microorganisms as Potential Biological Control Agents Against Verticillium Wilt of Olive, and Their Validation in the Field

The control of Verticillium wilt, caused by the soilborne fungi Verticillium dahliae, is one of the major challenges of current olive growing. The lack of effective chemical products against this disease, among other factors inherent in the biology of the pathogen, makes its control difficult, with the search for integrated management alternatives being necessary. Thus, the objective of this study was to evaluate the efficacy of 25 beneficial microorganisms (biological control agents; BCAs), 20 fungi and 5 bacteria isolated from different plants, on mycelial growth of V. dahliae (isolates V-180 and V-323) and on the microsclerotia viability of the pathogen in two naturally infested soils. Likewise, the efficacy of the BCAs on the progression of Verticillium wilt in olive plants of cv. Picual was evaluated under controlled conditions. Finally, the effect of a selection of the most effective BCAs was validated under field conditions along two experimental years. Among the 25 BCAs evaluated, Fusarium proliferatum PV-353 and PV-339, and F. oxysporum PV-1030 inhibited V. dahliae mycelial growth by more than 50%, regardless of the isolate of the pathogen. However, only F. proliferatum PV-353 and F. oxysporum PV-1030 significantly reduced microsclerotia viability by more than 50% in naturally infested soils. Aureobasidium pullulans PV-380, F. proliferatum PV-339 and PV-353, Trichoderma asperellum PV-346, and Bacillus velezensis PV-700 significantly reduced the severity and the progression of the disease in planta. Field trials confirmed A. pullulans PV-1033, B. velezensis PV-700, and T. citrinoviride PV-1041 as the most effective BCAs. Natural products were included in the field as reference compounds (e.g. willow and seaweed extracts), and their efficacy against the disease was also confirmed. This study highlights the difficulty of identifying consistently effective BCAs, and their variability depending on the V. dahliae isolates.