Bois noir epidemiology and management

BOIS NOIR EPIDEMIOLOGY Bois noir (BN) is the most important disease of the phytoplasma-associated grapevine yellows (GY) complex, causing important economic losses in all major wine-growing areas by reducing fruit quality and yields. BN has been associated with ‘Candidatus Phytoplasma solani’ (CaPsol, subgroup 16SrXIIA or stolbur group) (Quaglino et al., 2013). The main insect vector of CaPsol is the planthopper Hyalesthes obsoletus Signoret (Maixner, 1994), erratically transmitting CaPsol to grapevine and living preferentially on stinging nettle, bindweed and other herbaceous plants and shrubs present in the vineyard and surrounding areas (Mori et al., 2020). Molecular approaches along with transmission trials unveiled that other polyphagous insect species can play a role in vectoring CaPsol to grapevine, including Reptalus panzeri Löw, Aphrodes makarovi Zachvatkin and Dicranotropis remaniaca Guglielmino (Quaglino et al., 2019). In addition, Reptalus artemisiae (Becker), Macrosteles quadripunctulatus (Kirschbaum), Anaceratagallia ribauti (Ossiannilsson), and Pentastiridius leporinus (L.) were also identified as putative CaPsol insect vectors, still lacking defined epidemiological role. Moreover, many other weedy hosts were found infected by CaPsol, showing the existence of several reservoir plants in the different agroecosystems (Chuche et al., 2018). The multifaceted ecology of BN highlighted the great genetic diversity and adaptability of CaPsol phytoplasma. BOIS NOIR MANAGEMENT The complex epidemiology of BN, involving several insect vector species transmitting CaPsol from multiple plant hosts to grapevine, makes very difficult the disease containment in vineyards. Sustainable Bois noir management is based on grapevines and insect vectors strategies. Bois noir management strategies focused on grapevine The selection of healthy propagating material and the choice of grapevine varieties showing low susceptibility to the pathogen, can play a key role in the prevention of BN. To eliminate phytoplasma from infected mother plants, hot water treatments, cryotherapy, and tissue cultures were used, underlying the possible and promising applications of such methodologies on a large scale (Pierro et al., 2024). Nevertheless, after the introduction of phytoplasma-free planting materials in vineyards, the presence of insect vectors able to transfer phytoplasma into healthy plants still represents a concrete risk, thus the BN management strategy needs to be implemented by multiple management approaches. About curative strategies, the utilization of agronomical practices (partial uprooting and pulling of symptomatic plants, grafting of materials from recovered vines to symptomatic plants) and the treatments of the canopy of BN-affected grapevines with resistance inducers or biostimulants showed promising results in inducing recovery and increasing berry production in some grapevine cultivars. The application of these strategies should be extended to wider geographic areas and grapevine varieties to have further confirmation of their effectiveness, optimizing the formulations of resistance inducers, and clarifying the involved mechanism of actions. Bois noir management strategies focused on insect vectors Since H. obsoletus (and many other BN insect vectors) is a polyphagous species, whose life cycle is not restricted in the vineyard, but also involves mainly surrounding areas, hedges and forests, the use of insecticides cannot significantly reduce neither the vector population, nor BN incidence (Mori et al., 2008). For the control of H. obsoletus on herbaceous plant hosts, chemical weeding, soil tilling, and frequent cuts were proposed (Mori et al., 2016); however, it should be considered that weed management may produce a relocation of insects, including those transmitting CaPsol, from wild species to grapevine obtaining a fast and significant raising of BN incidence in vineyards. Alternatively, in organic vineyards, programmed cuts until late summer are highly needed to promote the development of perennial grasses, competitive with stinging nettle (Maixner et al., 2010). Novel and sustainable vector control strategies have been recently developed such as the application of the biocontrol agents (entomopathogenic fungi and nematodes) against H. obsoletus (Moussa et al., 2021) and the microbial resource management (endosymbionts) in insect vectors (Gonella et al., 2011). In some area the use of chaste tree in the vineyard surroundings as a trap plant for H. obsoletus to prevent BN spreading in the vineyard was suggested (Sharon et al., 2005). Considering that BN epidemiology involves multiple plant hosts and several insect vectors, the disease management should include an integrated approach based on the direct and indirect vectors control and the use of resistant grapevines.