Transmission of Verticillium nonalfalfae via root contact from inoculated Ailanthus altissima in close- to- nature conditions

Ailanthus altissima is an invasive alien species in Europe. Biological control of this tree species by Verticillium nonalfalfae is a potential alternative control approach. This study investigates host specificity, pathogenicity and transmission of V . nonalfalfae to neighbouring plants with root contact in mini- ecosystems. V . nonalfalfae led to dieback of all inoculated Ailanthus trees. Furthermore, V . nonalfalfae was transmitted to neighbouring Ailanthus trees, causing wilt and dieback, and, in one case, to Quercus petraea , which did not display any disease symptoms. Lastly, V . nonalfalfae could not be detected in the soil, which suggests transmission via root contact.

It transmits via intraspecific root grafts within Ailanthus stands . The use of V. nonalfalfae for biocontrol of Ailanthus has been tested in several studies (Brooks et al., 2020;Maschek & Halmschlager, 2018), although the fungus poses a potential danger for agricultural crops (Kasson et al., 2015). The pathogenicity of the fungus has also been tested on various non-target tree species co-occurring with Ailanthus (Kasson et al., 2015;Maschek & Halmschlager, 2018). This study goes one step further by testing host specificity and transmission of V. nonalfalfae from inoculated plants to neighbouring tree species by root contact. It focused on whether (i) Inoculation with V. nonalfalfae kills young Ailanthus', (ii) Inoculation of young Ailanthus' causes symptoms (weakening/wilting) on or leads to death of neighbouring trees via root contact, and (iii) V. nonalfalfae is transmitted through soil, using a mini-ecosystem approach in a greenhouse.

S H O R T C O M M U N I C A T I O N
Transmission of Verticillium nonalfalfae via root contact from inoculated Ailanthus altissima in close-to-nature conditions intermingled with those of the central Ailanthus (Figure 1 A and B).
Mini-ecosystems were placed in a greenhouse and watered sufficiently twice a week during the whole vegetation period.
Inoculation of Ailantex® (0.2 ml) took place 4 weeks after planting and followed the procedure of Maschek and Halmschlager (2018). For controls, the central Ailanthus solely received a cut with the same tool used for inoculation to reproduce wounding.
Disease progression was monitored in categories and intervals of ratings of disease severity were adapted to the observed symptoms in order to adequately record any changes during the phases of rapid disease development. In total, observations covered a time span of 313 days post inoculation (dpi). Plants were sprayed against aphids using a pyrethroid 39 dpi following manufacturer's instruc-

| RE SULTS AND D ISCUSS I ON
The first symptoms on inoculated Ailanthus trees (yellowing of leaves) appeared 14 dpi, followed by wilting and leaves with necrosis (24 dpi) and death after 31-53 dpi. When the central Ailanthus' were harvested 55 dpi, only one had started to re-sprout already.
Later, five of the six trees re-sprouted. Finally, 152 dpi, even the re-sprouted central Ailanthus' had died. Therefore, V. nonalfalfae led to the complete dieback of all inoculated Ailanthus', even if they started to re-sprout (Assumption i; Wilcoxon rank-sum test, p < .001; Figure 1c,d). These findings are in accordance with studies conducted in Europe and in the United States (Brooks et al., 2020;Kasson et al., 2015;Maschek & Halmschlager, 2018 Figure 1e). Neither any other neighbouring tree species nor control trees displayed any disease symptoms.
Verticillium nonalfalfae was found in all tree organs examined of inoculated central Ailanthus' (Table 1). For all but one inoculated Ailanthus V. nonalfalfae was detected in at least 2 out of 4 tree or-

ACK N OWLED G EM ENTS
We wish to thank Erhard Halmschlager and Oliver Maschek for providing the control agent and the latter for performing the inoculations (V. nonalfalfae), Thomas N. Sieber for inputs concerning the statistical analyses and Flavia Mäder for assisting with the establishment of mini-ecosystems.

PE E R R E V I E W
The peer review history for this article is available at https://publo ns.com/publo n/10.1111/efp.12720.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request. TA B L E 1 Presence of Verticillium nonalfalfae in the different tree organs such as roots, stem, shoots and leaves, from the different tree species with corresponding treatment (+ = central tree inoculated with V. nonalfalfae; -= noninoculated neighbouring tree in the same mini-ecosystem)