Assessment of entomopathogenic fungi and their extracts against a soil-dwelling pest and soil-borne pathogens of olive

Abstract

The olive fruit fly Bactrocera oleae (Gmelin) and the olive pathogens Verticillium dahliae, Phytophthora megasperma and Phytophthora inundata are major worldwide limiting factors for olive tree production. Entomopathogenic fungi, such as Metarhizium spp. and Beauveria spp., are suitable for olive fly control by either targeting adults or pupariating larvae and puparia in the soil beneath the tree canopy. Such soil treatments could have an additional impact on other communities of microorganisms present in the soil, such as soil-borne olive pathogens. The present work explores the possible dual biocontrol of both olive pests and diseases with entomopathogenic fungi. Twelve indigenous isolates of Beauveria bassiana, Beauveria pseudobassiana, Beauveria varroae and two isolates of Metarhizium brunneum from olive crops (soil and phylloplane) were evaluated in soil treatments against medfly (Ceratitis capitata Wied.) pre-imaginals. There were significant differences among the isolates in the total percentage of non-viable puparia and the total the puparia and adults showing fungal outgrowth, with percentages ranging from 27.5% to 82.5%, which highlights the potential of soil treatments with entomopathogenic fungi not only to reduce the target pre-imaginals but also the adults emerging from them, therefore contributing to the overall reduction of the next adult generation. The strains EAMa 01/58-Su of M. brunneum and EABb 09/16-Su of B. bassiana were evaluated for antifungal activity against two strains of V. dahliae (V117, V004), one of P. megasperma (PO20), and one of P. inundata (PO47) with one strain of Trichoderma atroviride used as a reference in the antagonism assays. In the dual plate culture assays, T. atroviride caused a 64–79% inhibition of Phytophthora spp. and V. dahliae mycelial growth. Meanwhile, the mycelial growth inhibition rates of M. brunneum and B. bassiana were 42–62% for Phytophthora and 40–57% for V. dahliae, with both entomopathogens causing inhibition halos, supporting the mechanism of antibiosis. Such a mechanism is also supported by the inhibitory activity of the crude extracts of these two M. brunneum and B. bassiana strains against these olive pathogens. Also, after the dialysis of the crude extracts against water in a molecular porous membrane with a cut-off of 3500Da, the antifungal activity was mostly retained (100% inhibition rates) in the dialyzed fractions, indicating that the active compounds were secondary metabolites. Likewise, these dialyzed fractions inhibited the germination of conidia (96–100%) and microesclerotia (58–96%) of V. dahliae and propagules of P. megasperma (38–100%). These results indicate that entomopathogenic fungi have the potential for dual biocontrol of olive soil pests and diseases.