**4. The Most Common Plant Pathogens**

The most frequent endophytes detected from the investigated plants are cosmopolitan and ubiquitous pathogens that may cause severe yield losses. In detail, *F. oxysporum* is responsible for the wilt of vascular tissues on numerous crops that may result in plant death, even if several strains have proved to be non-pathogenic [167]. It has been isolated from 8 different plant species belonging to 7 genera, namely *A. hindsii*, *A. julibrissin*, *B. malabarica*, *B. phoenicea*, *B. aristata, C. o*ffi*cinalis*, *L. lucidum,* and *N. oleander.* The fungus *A. alternata* may infect over 380 host plant species causing leaf spots, rots, and blights. It includes opportunistic forms in developing field crops as well as saprophytic strains that may cause harvest and post-harvest spoilage of harvested products. One of the major concerns represented by its infection is related to the production of mycotoxins that may be introduced in the food chain [168]. In this review, *A. alternata* has been found in association with 3 genera, in 7 plant species (*B. malabarica*, *B. racemosa*, *B. poiretii*, *B. aristata*, *Cornus* sp., *L. lucidum*, and *C. pulcherrima*). The saprophytic pathogen *A. niger* is responsible for the spoilage of a wide range of fruit, vegetable, and food products. It is also the causal agent of the black rot of onion bulbs, the kernel rot of maize, and the black mold rot of cherry [169,170]. It has been found within plant tissues of *A. arabica*, *A. lebbeck*, *B. fortificata*, *B. malabarica*, *B. racemosa*, *C. pulcherrima,* and *N. oleander* (7 plant species or 4 genera). Furthermore, three different species of *Colletotrichum* have been isolated from reviewed plants. *C. gloeosporioides* has been isolated from 7 plant species (3 genera), namely *A. hindsii*, *B. racemosa*,

*B. aristata*, *C. echinata*, *C. o*ffi*cinalis*, *C. stolonifera*, and *L. lucidum,* whereas *C. acutatum* has been found in *Cornus* spp*., Hamamelis* sp., and *H. virginiana* (3 species; 2 genera). Both *Colletotrichum* species may cause severe fruit rot mainly occurring in pre- and post-harvest [171]. Moreover, *C. truncatum,* the causal agent of anthracnose disease affecting several leguminous crops [171], has been collected from 2 plant genera, namely *A. hindsii* and *J. sambac.* Furthermore, *C. lunata*, was isolated from the tissues of 4 plant species (2 genera), including *B. malabarica, B. racemosa, B. phoenicea,* and *C. sappan*, is the causal agent of seed and seedling blight in several crops, such as rice, millet, sugarcane, and rice, and of maize leaf spot [172]. Besides, *B. dothidea* reported in association with *A. karroo*, *Cornus* sp*.,* and *C. o*ffi*cinalis* may cause cankers, dieback, fruit rot, and blue stain in woody plants, including *Acacia, Eucalyptus, Vitis*, and *Pistachio* [12]. Concerning the species *F. lateritium,* it has been extensively investigated as the causal agent of chlorotic leaf distortion on sweet potato (*Ipomoea batatas*) in the USA [173]. This fungus has been isolated from three different plant species and genera (*B. aristata, C. controversa,* and *L. lucidum*). Moreover, the common soil-borne fungus *G. candidum*, found in association with *B. vahlii, C. sappan,* and *L. lucidum,* is the causal agent of sour-rot of tomatoes and citrus fruits, and it is also one of the most economically important post-harvest diseases of citrus [174]. Also, *C. cladosporioides,* detected in *B. racemosa, C. echinata,* and *C. stolonifera*, is the causal agent of blossom blight in strawberries [175]. Other pathogenic fungi associated with these selected plants are less widespread and some of them are subjected to containment measures in some countries. This is the case of *N. parvum, N. oryzae, L. theobromae,* and *D. destructiva.* In particular, *N. parvum*, isolated as an endophyte in three *Acacia* species (*A. heterophylla, A. karroo,* and *A. koa*), is one of the most aggressive causal agent of Botryosphaeria dieback on the grapevine and it is known as an aggressive polyphagous pathogen attacking more than 100 plant hosts [176]. Also, *N. oryzae,* reported from *H. mollis, B. phoenicea, B. racemosa,* and *B. fortificata,* may reduce plant growth and seed quality of rice plants as well as *Brassica* spp., maize, and cotton [177]. Moreover, *L. theobromae,* found in association with six different plant species (*A. karroo, A. koa, B. racemosa, C. echinata, L. lucidum,* and *N. oleander*), is the causal agent of dieback, root rot, and blights for a wide range of plant hosts, mainly located in tropical and subtropical regions [178]. Finally, *D. desctructiva*, recovered from three different species of *Cornus*, is the causal agent of the dogwood anthracnose, a devastating disease that was firstly documented in the USA and then introduced into Europe [179].

Generally, closely related organisms, including pathogenic fungi as well as those non-pathogenic, may share similar ecological niches and may potentially interact among themselves. Their co-occurrence could be due to phylogenetic evolution or some unclear biological benefits gained [180,181]. The effects of this interaction may lead to a definition of spaces for development and survival. Nevertheless, it is widely known that non-indigenous species represent one of the greatest threats to native biodiversity [11,23–25]. In fact, a fungal invasion into a new ecosystem may change the native endophytic community structure, leading to the extinction of host-specialized fungi [182]. This antagonistic phenomenon is regulated by the production of antifungal compounds, mycoparasitism, or competition for space and resources [180], as well as a synergy of these interactions [181]. Biological invasions may set in motion a long-lasting cascade of effects on the plant host and associated species in unpredictable ways. Generally, the ecological importance of native species prior to the invasion may not be quantified because of the lack of information on fungal communities, especially for non-pathogenic fungal species. As a consequence of global trade and climatic or environmental changes, studies about the impact of new organisms on the ecosystem represent innovative challenges worldwide. In view of these considerations, even if fungal pathogens found in association with investigated plants are widely distributed in the EU [182–190], the risk posed by the introduction of potentially noxious species may be very high. Thus, our results suggest the importance of monitoring imported material to avoid the introduction of such alien species.

#### **5. Emerging and Potential Threats Due to Commercial Trade**

Several species reported in this review are Quarantine Pests (QP) or Regulated as Non-Quarantine Pests (RNQP), as defined by containment measures within the importing country [191]. Among the fungal pathogens found in *Cornus* species, *Elsinoe fawcettii* is listed as a QP in the EU, Tunisia, and Israel. This fungus is the causal agent of Citrus scab and it is one of the most important pathogens in many areas of citrus production [192]. *E. fawcettii* is common in South America and its presence has been detected in other areas such as Central and South Africa, India and South-Eastern Asia, and Australia [192].

Furthermore, the following pathogens are RNQP in the EU: *F. verticilloides* (isolated from *B. malabarica* and *A. lebbeck*), *C. acutatum* (isolated from *Cornus* spp., *H. virginiana*, and *Hamamelis* sp.), *S. sclerotiorum* (isolated from *C. stolonifera*), and *V. dahliae* (isolated from *Cornus* sp.). Outside the EU, the following species are listed as QP: *L. theobromae* and *P. palmivora* (in Morocco), *A. nidulans*, *A. macrospora, C. kahawae*, *C. citrullina*, *C. herbarum*, *C. pallescens*, *A. brassicicola*, *F. semitectum*, *F. verticillioides*, *N. oryzae*, and *P. longissima* (in Mexico), *P. graminis* (Canada and USA), *Diaporthe tersa* (in Israel), *C*. *acutatum* (in Tunisia and Israel), and *C. gloeosporioides* and *P. capitalensis* (in Egypt) [192].

Organisms that move across continents may or may not become dangerous depending on several factors, and unexpected consequences may occur [193,194]. The current knowledge about the fungal community associated with ornamental plants and their interaction with the environment is fragmentary. Fungi species generally well known as pathogens, are not necessarily pathogenic when isolated as endophytes [6–8]. Genetic mutation can occur in virulent pathogens, transforming the original pathogen into a nonpathogenic strain [9]. Likewise, even though some endophytes are mutualistic, this does not imply that they will not have negative impacts if introduced in a new ecosystem [6,9]. Alien pathogens can often encounter more susceptible host plants and different microbial and abiotic environments without their own 'natural enemies'. The so-called 'risk pathway' defined by international protocols tend to assume that the pathogen will attack a plant host taxonomically similar to that of the susceptibile plant species in its native countries. However, an invasive pathogen may spread to new target hosts, when introduced in a new ecosystem, and novel pathogen combinations can occur [11]. The disease outcomes of these combinations may be extremely complex and the invasive pathogen populations can reach explosive distribution levels that are usually difficult to eradicate once established [23–25]. Beyond the damage which may occur on the host plant species and local microbial communities, biological invasions may affect entire ecosystems and the connected ecosystem processes and services, such as soil fertility, fire control, hydrology, and recreation and tourism amenities [23–25]. In response to expanding global trade, several EU regulations [27–29] and international protocols [195,196] are aimed at regulating over-dissemination and accidental introduction of plant diseases. However, despite existing laws and efforts to prevent the introduction of potential pathogens at ports of entry, many of them will evade detection and establish alien populations [197,198]. Many pathogenic fungi may be undetected, transported in the form of inocula as endophytes, propagules, mycelium, or spores of vegetative material. In addition, large import volumes often permit the inspection of only a small proportion of the introduced plants. According to the precautionary principle, all imported plant species should be considered as a potential threat (vectors of fungi), therefore the presence and establishment may not depend on the number of arrivals. As a consequence, even a reduced amount of infected plants, which can easily escape phytosanitary inspections, may cause the introduction and the spread of diseases with devastating outcomes [199]. The development of tools, such as new molecular diagnosistics [200] and volatile compounds detection devices [201], that allow the rapid and on-site identification of potentially invasive species and the screening of large volumes of plants, clearly appears to be essential [202]. Despite increasing trade, targeted investment in biosecurity may be effective to reduce pathogen introduction and limit the establishment of alien microorganisms. Thus, we highlight the importance of surveillance due to the potential risk of accidental introductions in the absence of effective biosecurity measures.
