Prevalence of natural infections of Xylella fastidiosa subsp. multiplex ST87 across plant species at Monte Argentario (Tuscany, Italy)

Mt. Argentario (Ma) is a 60 Km2 promontory located in the province of Grosseto, Italy, orographically isolated from the coast but connected to it by two thin dune cordons. The climate is typically Mediterranean (Csa according to Köppen and Geiger). The landscape, which contains a mix of vegetation and land-cover types, is dominated by typical Mediterranean sclerophyllous – evergreen forest (Natura 2000 habitat code: 9340) and by thermo-Mediterranean and pre-desert scrub (code: 5330). Scattered residential developments are also present. After the first finding of Xylella fastidiosa subsp. multiplex ST87 (ST87) at the end of 2018, the promontory was included in a demarcated area (DA) and a monitoring program was established in accordance with EU Regulations 789/2015 and 2020/1201. Although sampling efforts have been highly variable between years and across plant taxa, cumulative prevalence data (2019-2025) indicate that plant taxa differ in their susceptibility to natural infections by ST87, which is the only ST found within this DA to date. Twenty-nine taxa out of 175 that were monitored were found to be infected with a cumulative prevalence (N° infected/Total sampled) ranging from 0.21 (Polygala myrtifolia) to 0.001 (Myrtus spp.). However, several taxa with high cumulative prevalence, including P. dulcis, Polygala myrtifolia, Eleagnus angustifolia, and Lavandula spp. are not naturalized species and thus their presence on the territory is limited and mostly localized in private areas that are difficult to access. As a result, they tended to be under sampled during monitoring causing imprecise estimates and/or random fluctuations of prevalence as well as difficulty in correctly identifying infection trends. On the contrary, spontaneous and dominant host species such as Spartium junceum and Rhamnus alaternus, which form very large, widespread and easily accessible populations at Ma, have been more consistently sampled. Since correlation analyses allow excluding monotonic relationships between N and prevalence, they are very useful to study the trend of infections over time. S. junceum and R. alaternus have both shown a relatively high prevalence of infections in 2019–2020, a clear decline around 2022, and a partial rebound in subsequent years, which was particularly evident for S. junceum in 2025. The phenomenon will be thoroughly investigated during the 2026 monitoring campaign.