2. An Overview of Fungal Diversity and Frequency

Investigations on the mycobiota of plants frequently reported new taxa or new species distribution, and several fungi are still undiscovered or undetected. Numerous higher plants have developed a variety of resistance mechanisms to prevent fungal infections. However, the presence of weakly pathogenic fungi in healthy plant tissues highlights the evolutionary continuum between latent pathogens and symptomless endophytes [15]. Generally, all plants have symbiotic interactions with fungal endophytes which can influence host performance in terms of disease resistance [154,155,156], stress tolerance [157], and biomass accumulation [158]. Fungal endophytes may also change according to plant tissues colonized [159], phenological growth stages, host genotypes [160], and geographical distribution areas [161].

Investigations on the mycobiota of plants frequently reported new taxa or new species distribution, and several fungi are still undiscovered or undetected. Numerous higher plants have developed a variety of resistance mechanisms to prevent fungal infections. However, the presence of weakly pathogenic fungi in healthy plant tissues highlights the evolutionary continuum between latent pathogens and symptomless endophytes ^[15]. Generally, all plants have symbiotic interactions with fungal endophytes which can influence host performance in terms of disease resistance ^[32][33][34], stress tolerance ^[35], and biomass accumulation ^[36]. Fungal endophytes may also change according to plant tissues colonized ^[37], phenological growth stages, host genotypes ^[38], and geographical distribution areas ^[39].

In this review, a total of 428 endophytic species belonging to 122 fungal genera have been found in association with 13 plant genera (

Table 1

). The greatest level of fungal diversity was reported in in association with 

Bauhinia

 with 43 fungal genera and 94 fungal species, and 

Cornus

 with 44 fungal genera and 78 fungal species. The degree of fungal recovery from 

Acacia

 (29 genera, 51 species), 

Albizia

 (14 genera, 27 species), 

Berberis

 (17 genera, 29 species), 

Caesalpinia

 (19 genera, 42 species), 

Cassia

 (15 genera, 19 species), 

Ligustrum

 (20 genera, 29 species), and 

Nerium (21 genera, 37 species) was nearly half in comparison to the abundance noted in the genera 

 (21 genera, 37 species) was nearly half in comparison to the abundance noted in the genera Bauhinia and Cornus. Nonetheless, the lowest diversity showed for Hamamelis (4 species/genera), Jasminus (7 species, 1 genera), Lonicera (3 species/genera), and Robinia (6 species/genera) was also due to the lack of published research about fungal endophytes in these plant genera.

Table 1. Endophytic fungi isolated from Acacia (AC), Albizia (AL)., Bauhinia

 and 

 (BA), Berberis (BE), Caesalpinia (CP), Cassia (CS), Cornus

. Nonetheless, the lowest diversity showed for 

 (CO), 

Hamamelis (4 species/genera), 

 (HA), Jasminus

 (7 species, 1 genera), 

 (JA), Ligustrum (LI), Lonicera

 (3 species/genera), and 

 (LO), Nerium (NE), Robinia

 (6 species/genera) was also due to the lack of published research about fungal endophytes in these plant genera.

 (RO). Columns report the number of isolated fungal species. The total number of records calculated per fungal genus is indicated as Tot. SF. The total number of records per plant genera is indicated as Tot. SP. Fungal genera are sorted by alphabetic order.

The literature evidenced that several fungal endophytes live in association with the investigated plants. The most representative genera in terms of abundance of isolated species were 

Aspergillus

 (40 spp.), 

Penicillium

 (30), 

Fusarium

 (29), 

Colletotrichum

 (27), 

Alternaria

 (14), and 

Cladosporium (14). These genera include ubiquitous and generalist fungi as well as several plant pathogens and saprobes [162,163,164].

(14). These genera include ubiquitous and generalist fungi as well as several plant pathogens and saprobes ^[40][41][42].

It is worth noting the relative homogeneity in distribution of fungi such as 

Colletotrichum

, 

Fusarium,

 and 

Alternaria

 among these plant genera. In fact, 

Colletotrichum

 was undetected only in 

Lonicera

 and 

Robinia

, 

Fusarium

 in 

Caesalpinia

, and 

Hamamelis, Jasminus,

 and 

Alternaria

 in 

Cassia

 and 

Lonicera.

 Although scarcely abundant, the fungal genus 

Phyllosticta

 was almost reported for all selected plants except for 

Albizia

, 

Jasminus

, 

Robinia,

 and 

Hamamelis.

 Other endophytic fungi were detected more occasionally. Future surveys may reveal the presence of additional fungal species also from less investigated plants, such as 

Robinia

, 

Jasminum,

 and 

Lonicera

.

The presence of pathogenic or saprotrophic fungi has already been discussed by several authors [165,166]. 

The presence of pathogenic or saprotrophic fungi has already been discussed by several authors ^[43][44]. 

Table 1

 shows that several of the listed fungi were apparently restricted to a single plant genus or at least exhibit some preference for a particular one. Some common and ubiquitous pathogens have been recovered in more than one plant host. This is the case of 

F. oxysporum

 (8 host plant species belonging to 7 different genera), 

A. alternata

, 

A. niger

, 

C. gloeosporioides

 (7 host plant species), 

N. oryzae

 (4 host plant species), 

B. dothidea

, 

C. globosum, C. acutatum

 (3 host plant species), 

A.

ochraceus

, 

A. pullulans

, and 

C. truncatum

 (3 host plant species).

3. 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 

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 ^[45]. 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. officinalis

, 

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, 

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 ^[46]. 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 

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 ^[47][48]. 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. officinalis

, 

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, 

species may cause severe fruit rot mainly occurring in pre- and post-harvest ^[49]. Moreover, 

C. truncatum, the causal agent of anthracnose disease affecting several leguminous crops [171], has been collected from 2 plant genera, namely 

the causal agent of anthracnose disease affecting several leguminous crops ^[49], 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, 

, is the causal agent of seed and seedling blight in several crops, such as rice, millet, sugarcane, and rice, and of maize leaf spot ^[50]. Besides, 

B. dothidea

 reported in association with 

A. karroo

, 

Cornus

 sp

.,

 and 

C. officinalis

 may cause cankers, dieback, fruit rot, and blue stain in woody plants, including 

Acacia, Eucalyptus, Vitis

, and 

Pistachio [12]. Concerning the species 

^[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 (

) in the USA ^[51]. 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, 

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 ^[52]. 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 

, is the causal agent of blossom blight in strawberries ^[53]. 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, 

), 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 ^[54]. 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, 

spp., maize, and cotton ^[55]. 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, 

), is the causal agent of dieback, root rot, and blights for a wide range of plant hosts, mainly located in tropical and subtropical regions ^[56]. 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].

, is the causal agent of the dogwood anthracnose, a devastating disease that was firstly documented in the USA and then introduced into Europe ^[57].

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 [11,23,24,25]. 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 [182,183,184,185,186,187,188,189,190], 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.