Verticillium wilt of Ailanthus altissima in Italy caused by V. dahliae: new outbreaks from Tuscany

Abstract

Verticillium spp., including V. nonalfalfae and V. dahliae, are known vascular wilt pathogens of the invasive Ailanthus altissima (tree-of-heaven) in the United States and in Europe. Herein we provide evidence of the presence of a previously unreported wilt disease of A. altissima in Tuscany (Central Italy). Several isolates were collected from two locations and identified as V. dahliae, based on microscopical features of conidiophores, conidia and microsclerotia. Genomic DNA was extracted from the mycelium, the ITS region was amplified and the sequence was deposited in GenBank as VdGL16 (accession no. MK474459). BLASTn analysis showed 100% similarity with V. dahliae. To confirm pathogenicity of VdGL16, inoculations of Ailanthus seedlings were performed with the root dipping technique whereas mature trees were steminoculated. All inoculated seedlings exhibited wilt symptoms after 20 days, while mature Ailanthus trees showed wilting and dieback after six months. The pathogen was easily re-isolated from seedlings and re-identified as V. dahliae, thus satisfying Koch's postulates. Results from intraspecific resistance screening of nine seed sources from across Italy revealed that Ailanthus provenances from all the six sampled regions were susceptible to V. dahliae. Stem inoculated adult plants exhibited abundant production of epicormic sprouts along the stem within six months, and most of these sprouts wilted following initial dieback of the main stem; furthermore, sprouting from the crown was intense. Petioles and rachises tissues of leaves fallen from infected trees were a good source for re-isolation of the pathogen; we proved that such petioles and rachises can effectively transfer the fungus to healthy Ailanthus seedlings via root infections. Host-specificity of the V. dahliae isolate VdGL16 was also determined on 40 non-target species/varieties/cultivars. The isolate caused disease in herbaceous species belonging to five botanical families: Asteraceae, Lamiaceae, Leguminoseae, Linaceae and Solanaceae. Given the difficulties in countering Ailanthus invasion with mechanical and chemical methods, the biological control using Verticillium may provide an efficient, low cost and sustainable control of this invasive species.

Control ofAilanthus altissimausing cut stump and basal bark herbicide applications in an eighteenth‐century fortress

“…As an alternative to herbicides, a promising more sustainable control technique for this invasive species could be the use of biological control agents, such Verticillium nonalfalfae , a highly efficient fungus that has been demonstrated to be able to control A . altissima (Harris et al ., 2013; Kasson et al ., 2014; Maschek and Halmschlager, 2017; Pisuttu et al ., 2020). This method could be a valid alternative in particular in environmental fragile areas and in areas frequented by people where herbicide applications are restricted or prohibited.…”

Physiochemical responses of Ailanthus altissima under the challenge of Verticillium dahliae: elucidating the decline of one of the world’s worst invasive alien plant species

“…In accordance with the above outcomes, our research group recently provided evidence of the presence of a previously unreported wilt disease of Ailanthus in Tuscany (Central Italy), then attributed to V. dahliae. The isolated strain (i.e., VdGL16) was proven to be virulent towards both Ailanthus seedlings from several Italian provenances and adult trees, not infectious in trees or shrubs of economic or landscape interest, whereas only a few herbaceous species of horticultural and forage concern resulted susceptible among the 40 non-target species/varieties/ cultivars tested (Pisuttu et al 2020a). This study also allowed to develop on optimal inoculation method for adult trees, both in terms of infection efficacy and environmental safety (i.e., to avoid accidental spread of conidial suspension).…”

“…However, also chemical herbicides have been shown to be generally ineffective against the re-sprouting ability of Ailanthus. Moreover, their use is expensive and laborious, not to mention their negative impacts on non-target vegetation (being them non-selective) and ecosystems, which is the main reason of the increasing restrictions in the use of such chemical products (Pisuttu et al 2020a). So, for instance, glyphosate, the most frequently used herbicide both worldwide and in the EU which has been used for several decades is currently approved in the EU until 15 December 2022 (https:// ec.…”

“…The virulence and the spread of these fungal species seem strictly related to the climatic conditions: infections due to both Verticillium species have been found in countries characterized by temperate climate such as Pennsylvania, Virginia, and Ohio in the USA (Schall and Davis 2009a;Snyder et al 2013;Rebbeck et al 2013), as well as in Austria in Europe (Maschek and Halmschlager 2017), where V. nonalfalfae is more aggressive (Brooks et al 2020) and best adapted than V. dahliae. Conversely, infections caused only by V. dahliae have been reported in Hungary (Izsépi et al 2018) and in different Mediterranean regions (i.e., Greece, Italy, and Spain;Skarmoutsos and Skarmoutsou 1998, Longa et al 2019, Pisuttu et al 2020a, Moragrega et al 2021 where V. nonalfalfae has been rarely reported also on other hosts, because its optimum growth temperature is around 22 °C and is infrequently present outside the 10-27 °C range (Maschek and Halmschlager 2017). These reports thus suggest to focus on the role of V. dahliae (able to survive up to 30 °C; Pegg and Brady 2002), currently the only natural endemic enemy reported as able to limit Ailanthus invasion in the warmer environments.…”

Occurrence of Aculus mosoniensis (Ripka, 2014) (Acari; Prostigmata; Eriophyoidea) on tree of heaven (Ailanthus altissima Mill.) is expanding across Europe. First record in France confirmed by Barcoding

“…The early synonymy between Aculops tailhangensis and Aculus mosoniensis suggested by de Lillo et al ( 2017) based on morphological characters was supported by a high similarity between their internal transcriber spacer 1 sequences (Cristofaro & de Lillo, 2019;Marini et al, 2021). In Europe, Aculus mosoniensis, first reported in Hungary (Ripka & Érsek, 2014) and recently in Italy (de Lillo et al, 2017), and the vascular wilt pathogen, Verticillium dahlia, isolated in Italy (Pisuttu et al, 2020), are the only two potential European biological control agents of tree of heaven. Aculus mosoniensis forms dense populations mainly at the under surface of the leaflets of the young compound leaves, causing leaf edges to curl upwards and turn yellowish in color.…”