Protective and curative effect of an experimental biocomplex against Flavescence dorée phytoplasma infection and transmission by Scaphoideus titanus under laboratory conditions

Abstract

Flavescence dorée (FD) is one of the most devastating diseases threatening grapevine cultivation in Europe. The causal agents are phytoplasmas, which reside in phloem vessels of infected plants and in the body of leafhopper vectors, the most important being Scaphoideus titanus. Current control mainly relies on vector management, which is often insufficient to prevent from disease outbreaks. Suitable methods to interfere with phytoplasmas in planta are constantly sought to support insecticide sprays. We tested a biocomplex containing trace elements and soluble sugars, with potential systemic spread and antimicrobial activity, with the aim to verify its capability to penetrate plant phloem and to interfere with phytoplasma survival in vector-inoculated plants, using two FD isolates (FD-C and FD-D). The penetration into the plant phloem, as well as its acquisition by phloem-feeding vectors, were tested using an iridium-marked biocomplex. Phytoplasma infection was detected by real Time PCR and compared in plants exposed to biocomplex treatment before inoculation (short-time and mid-time preventive application), and to treatment after inoculation (curative application). We also assessed if the biocomplex interferes with the vector competence of S. titanus, by conducting phytoplasma transmission trials with sequential inoculation steps, where untreated and biocomplex-treated grapevine plantlets were sequentially offered to infectious leafhoppers. Foliar treatment with the biocomplex supported the penetration of phloem tissues, and the product was successfully acquired by the experimental phytoplasma vector Euscelidius variegatus after feeding on the phloem of treated plants. When the biocomplex was applied on grapevines, it induced a significant reduction of phytoplasma infection regardless of FD isolate, both as a preventive and curative treatment, indicating an antibacterial activity in the plant. Moreover, sequential inoculation experiments showed a decrease of S. titanus inoculation rates after vector exposure to a biocomplex-treated plant, suggesting a reduction of transmission efficiency that was higher using the FD-C rather than FD-D isolate. Our results indicate the tested biocomplex as a promising tool for the creation of anti-phytoplasma formulations to support vector management in FD containment.