Entomopathogenic nematodes and fungi to control Hyalesthes obsoletus (Hemiptera: Auchenorrhyncha: Cixiidae)

Hyalesthes obsoletus Signoret (Hemiptera: Auchenorrhyncha: Cixiidae) is a univoltine, polyphagous planthopper that completes its life cycle, including the subterranean nymph cryptic stage, on herbaceous weeds. In vineyards, it can transmit ‘Candidatus Phytoplasma solani’, an obligate parasitic bacterium associated with bois noir (BN) disease of grapevine, from its host plants to grapevine when occasionally feeding on the latter. The main disease management strategies are based on vector(s) control. Insecticide treatments on grapevine canopy are completely inefficient on H. obsoletus, due to its life cycle. Consequently, control of this planthopper focuses on the nymphs living on the roots of their host plants. Such practices, based on herbicide application and/or weed management, can reduce vector density in the vineyard but can impact the environment or may not be applicable, highlighting the necessity for alternative strategies. In this study, the efficacy of entomopathogenic nematodes (EPNs; Steinernema carpocapsae, S. feltiae, Heterorhabditis bacteriophora) and fungi (EPFs; Beauveria bassiana, Metarhizium anisopliae, Isaria fumosorosea, Lecanicillium muscarium) against H. obsoletus nymphs (EPNs) and adults (EPNs and EPFs) was assessed under laboratory and greenhouse conditions. The majority of examined EPNs and EPFs were able to kill H. obsoletus exhibiting a range of effectiveness. S. carpocapsae (among EPNs) and I. fumosorosea (among EPFs) were found to be the most effective biocontrol agents in all trials carried out. Advantages and limitations of such promising biocontrol agents were discussed. Ecological competency and conditions that can impede or enhance the EPNs and EPFs performance should be investigated to optimize their performance under field conditions.

Hyalesthes obsoletus is an active vector of Wood blackening in the Republic of Moldova

Hyalesthes obsoletus (Signoret) is an important vector of Wood Blackening in the Republic of Moldova. To identify the causes of the epidemiology of this disease in vineyards, the period of flight and the preference of the host plant H. obsoletus in the field were studied. Monitoring of vineyards showed that this species was found on the Field Bindweed (Convolvulus arvensis L.), and during the period of mass flight, the leafhopper was observed on Xanthium strumarium and Gorets bindweed (Fallopia convolvulus L.). The captured leafhoppers were diagnosed with the presence of Bois Noir stolbur phytoplasm (STOL). The adults of H. obsoletus were also caught from other herbaceous plants: Povoy fence (Calystegia sepium L.), Garden quinoa (Ariplex hortensis L.), Sow thistle (Sonchus oleraceus L.), Medicinal dandelion (Taraxacum officinale L.), as well as c Stinging nettle (Urtica dioica L.). Changes in climatic conditions in the region, causing premature drying of the grass cover, forces cicadas - vectors to switch to actively growing plants during this period, including grapes, which contributes to the widespread of the disease.

Relationships between Hyalesthes obsoletus, Its Herbaceous Hosts and Bois Noir Epidemiology in Northern Italian Vineyards

Hyalesthes obsoletus is the vector of “Candidatus Phytoplasma (Ca. P.) solani,” the causal agent of grapevine yellows Bois noir (BN). The relationships among the planthopper, its main herbaceous hosts as phytoplasma reservoirs (Convolvolus arvensis and Urtica dioica) and BN spreading were studied in northern Italy. In two areas the relationship between host plants and the phenology and survival of planthopper adults was investigated in potted plants and in field conditions. Moreover, H. obsoletus ecology, newly symptomatic grapevine occurrence and “Ca. P. solani” tuf-types’ presence were studied in two vineyards (2014–2019). An earlier occurrence of H. obsoletus adults on C. arvensis than U. dioica and better adult survival of the originating host were observed. When U. dioica was prevalent, the vector occurred almost exclusively along the ditch outside the vineyard. Hyalesthes obsoletus amount varied widely from year to year and nymphal mortality due to late frosts was supposed. In one vineyard, the amount of newly symptomatic grapevines was significantly correlated with vector abundance in the previous year. The “Ca. P. solani” tuf-type was influenced by vector population levels on the two hosts. Since the abundance of H. obsoletus populations on the two hosts influences BN epidemiology and dynamics and the “Ca. P. solani” tuf-type, this must be considered in BN control strategies.

Proposal of A New Bois Noir Epidemiological Pattern Related to ‘Candidatus Phytoplasma Solani’ Strains Characterized by A Possible Moderate Virulence in Tuscany

Bois noir (BN), associated with ‘Candidatus Phytoplasma solani’ (CaPsol), is the most widespread disease of the grapevine yellows complex worldwide. In this work, BN epidemiology was investigated in a case study vineyard where an unusual CaPsol strain, previously detected only in other host plants, was found to be prevalent in grapevine. Experimental activities included: symptom observation; sampling of symptomatic vines, Auchenorrhyncha specimens, and weeds; molecular detection and typing of CaPsol strains; statistical analyses for determining possible relationships between CaPsol relative concentration, strain type, and symptom severity. Among insects, Reptalus quinquecostatus was the most abundant and was found to be highly infected by CaPsol, while Hyalesthes obsoletus, the main CaPsol vector, was not caught. Moreover, R. quinquecostatus harbored CaPsol strains carrying uniquely the stamp sequence variant St10, also identified as prevalent in vines and in the majority of weeds, and all the secY variants identified in the vineyard. Statistical analyses revealed that CaPsol strains carrying the St10 variant are not associated with severe symptoms, suggesting their possible moderate virulence. Based on such evidence, a new BN epidemiological pattern related to these CaPsol strains and involving grapevine, R. quinquecostatus, and/or weeds is proposed. Furthermore, the possible presence of other players (vectors and weeds) involved in CaPsol transmission to grapevines was highlighted.

Lavender Decline in France Is Associated with Chronic Infection by Lavender-Specific Strains of “Candidatus Phytoplasma solani”

ABSTRACT Lavender decline compromises French lavender production, and preliminary data have suggested the involvement of “Candidatus Phytoplasma solani” in the etiology of the disease. In order to evaluate the epidemiological role of “Ca. Phytoplasma solani,” a 3-year survey was conducted in southeastern France. “Ca. Phytoplasma solani” was detected in 19 to 56% of the declining plants, depending on seasons and cultivars, and its prevalence was correlated with symptom severity. Autumn was more favorable than spring for phytoplasma detection, and “Ca. Phytoplasma solani” incidence was higher in Lavandula angustifolia than in Lavandula intermedia hybrids. Detection of the phytoplasma fluctuated over months, supporting the chronicity of infection. Three “Ca. Phytoplasma solani” secY genotypes, S17, S16, and S14, were the most prevalent in lavender fields and were also detected in nurseries, whereas strains detected in surrounding bindweed and wild carrots were mostly of the S1 and S4 genotypes. This suggests that lavender is the main pathogen reservoir of the epidemic. Adults and nymphs of the planthopper vector Hyalesthes obsoletus were commonly captured in lavender fields and were shown to harbor mainly the prevalent phytoplasma genotypes detected in lavenders. The “Ca. Phytoplasma solani” genotype S17 was transmitted to Catharanthus roseus periwinkle by naturally infected H. obsoletus. Finally, the inventory of the bacterial community of declining lavenders that tested negative for “Ca. Phytoplasma solani” by 16S rRNA deep sequencing ruled out the involvement of other phloem-limited bacterial pathogens. IMPORTANCE The etiology and main pathways for the spread of lavender decline, an infectious disease affecting French lavender production since the 1960s, have remained unclear, hampering the development of efficient control strategies. An extensive survey of lavender fields led to the conclusion that “Candidatus Phytoplasma solani” was chronically infecting declining lavenders and was associated with large infectious populations of Hyalesthes obsoletus planthoppers living on the crop itself. Lavender appeared to be the main reservoir host for lavender-specific phytoplasma strains, an unusual feature for this phytoplasma, which usually propagates from reservoir weeds to various economically important crops. These results point out the necessity to protect young lavender fields from the initial phytoplasma inoculum coming from surrounding lavender fields or from infected nurseries and to promote agricultural practices that reduce the development of H. obsoletus vector populations.