Potential of the entomopathogenic fungus Metarhizium robertsii to control radicicole grape phylloxera Daktulosphaira vitifoliae

Abstract: In most viticulture countries, the grape phylloxera (GP) Daktulosphaira vitifoliae (Fitch) (Hemiptera: Phylloxeridae) is controlled by planting grafted plants on rootstocks resistant to this pest. In the search for alternative protective methods, the effectiveness of Metarhizium robertsii in protecting non-grafted Vitis vinifera grapevines against radicicole GP was assessed in vitro. In the first approach, GP eggs from a single clonal lineage were infected with M. robertsii strain EF3.5(2) obtained from a French vineyard. In bioassays, three application methods (egg spraying, root spraying, and both) were assessed using fresh woody root sections of V. vinifera cv. Cabernet Sauvignon. M. robertsii pathogenic effect translated to a significant reduction of 90.0 % by the egg spraying, 70.0 % by the root spraying, and 86.1 % by spraying both in the survival probability of GP larvae and adults compared to controls, and 92.0 %, 89.0 % and 93.0 % in the reduction of successful development to the adult stage after 28 days post-treatment, respectively. In the second approach, the impact of M. robertsii’s association with non-grafted V. vinifera roots on radicicole GP establishment and symptom induction was assessed in a pot experiment. After 76 days post-GP inoculation, a remarkable 91 % reduction in damage was observed in plants co-inoculated with M. robertsii and GP, compared to those infected solely with GP. Furthermore, 100 % of M. robertsii-inoculated plants and 92.0 % of GP-M. robertsii co-inoculated plants were colonized by the fungus at the rhizosphere level. Notably, grapevine growth remained unaffected during the trial by neither M. robertsii inoculation nor GP infestation. In conclusion, these findings highlight the potential of M. robertsii for durable association and GP biocontrol in non-grafted grapevines. Future investigations are warranted under field conditions to validate and optimize the larger-scale practical application of this biocontrol strategy.