The entomopathogenic fungus Metarhizium robertsii and its endophytic potential in grapevine to regulate radicicole grape phylloxera populations

Abstract

Grape phylloxera (GP) (Daktulosphaira vitifoliae Fitch, 1855) devastated European vineyards in the 19th century by attacking Vitis vinifera roots. Today, grafting onto resistant American rootstocks remains its primary control strategy, but GP continues to threaten non-grafted vineyards, prompting the search for alternative control solutions. The entomopathogenic fungus Metarhizium robertsii parasitizes insects and can colonize plant roots internally as an endophyte. This study investigated the potential of M. robertsii to associate with non-grafted V. vinifera roots while controlling root-feeding GP populations. Four native M. robertsii strains from Argentina, Australia, France, and Germany were evaluated for their potential to associate with non-grafted grapevines in the rhizosphere and in the root-endosphere in greenhouse experiments. Rhizosphere colonization at 68–70 days post-inoculation ranged from 60 % (Argentinian strain) to 100 % (German strain), while root-endosphere colonization reached up to 33 % by the German strain. No significant strain colonization rate differences were found, and no adverse effects on grapevine growth or leaf pigment content were observed. Root section bioassay on V. vinifera ligneous roots sprayed with fungal suspension of the French strain EF3.5(2) showed reduced GP individual survival (70-90 %) and GP adult development (89-93 %), depending on the fungal spraying method. The number of nodosities induced by GP on inoculated non-grafted grapevine with M. robertsii strain EF3.5(2) was reduced by 91 %. Moreover, qPCR analysis showed induction of phylloxera-responsive defense genes in leaves 24h after grapevine inoculation with the same strain. These results suggest M. robertsii could offer a sustainable biological control option for GP in non-grafted vineyards. Further field trials are needed to evaluate its persistence and efficacy under field conditions.