**1. Introduction**

*Fusarium* species have been commonly reported in the majority of bioclimatic regions and ecosystems, where they occur as endophytes, latent plant pathogens, or soil saprobes, thus showing a considerable ecological plasticity [1–3]. Some *Fusarium* species may cause severe plant diseases, and contaminate crop productions with mycotoxins, which are secondary metabolites (SMs) of major concern to food and feed safety worldwide [4–7]. This aptitude may not only involve pre- and post-harvest plant pathogens, but also strains which develop endophytically without causing disease symptoms [8,9]. On the other hand, the release of bioactive SMs in plant tissues by endophytic strains may induce defensive responses against pests and pathogens [10], with positive implications on plant growth [11].

The genus *Fusarium* is also well known for a controversial taxonomic history, where species descriptions were basically founded on key morphological characters [3]. In the last decades several studies considered data on SM production as a possible support in *Fusarium* taxonomy [12–14]. However, such a sound approach has been impaired by the finding that synapomorphy, i.e., the occurrence of certain common characters in distantly related organisms, has notably affected the classification of *Fusarium* strains in the past [15]. More recently, the advances in the DNA-sequencing technology allowed the identification of *Fusarium* spp. based on multi-gene genealogies [16–19], thus improving the phylogenetic accuracy and the taxonomic resolution [20–22]. Nevertheless, the ongoing deposition of DNA sequences in database resulting from the manifold surveys of natural populations of *Fusarium*, together with the characterization of novel species, may result in incorrect matches and, subsequently, in misleading identifications [23].

In this work, we report a case study describing a *Fusarium* strain (A1021B) that was recovered as an endophyte of common spindle (*Euonymus europaeus*) at the Astroni Natural Reserve near Naples, Italy. The fungus was provisionally defined to belong to the *Fusarium incarnatum-equiseti* species complex (FIESC) [24]. Fusarinol, a derivative of fusaric acid (FA), was the main extrolite purified from cultures grown in Czapek-Dox broth (CDB) [25]. Afterwards, we found FA to be the major SM produced by A1021B in potato dextrose broth (PDB), thus confirming that FA production in vitro is influenced by the culture medium composition [26–28]. Previous studies reported that FA production is strain-dependent even in species known as common producers, and it can be stimulated in some reluctant *Fusarium* strains by co-cultivation with other fungi [29]. Factors regulating gene expression are fundamental in explaining variation in SM production. Gene clusters for FA synthesis have been detected in many *Fusarium* spp. [15,30], and the deletion of specific genes has been reported to affect the production of FA and related compounds [31,32].

Our results appeared in contrast with the mycotoxin profile commonly associated with FIESC members. In fact, previous investigations failed to detect the production of FA in species/strains ascribed to this species complex [26,33–35], which are mainly known as trichothecene producers [35,36]. FA was listed among the mycotoxins of *F. equiseti* in a couple of recent papers [15,28], but no specific references were provided supporting this inference. Therefore, further investigations were undertaken concerning both the authentic taxonomic identity of strain A1021B and its biochemical and molecular characterization.
