**4. Biological and Ecological Traits**

As introduced above, endophytes are basically defined by their ability to spread in host tissues without inducing disease symptoms. However, the contraposition with pathogens is not so obvious, considering that many pathogens have a latent stage in their life cycle during which they are characteristically asymptomatic. The duration of this stage is very variable, and the pathogenic shift often depends on changes in the host susceptibility induced by several kinds of stress, which may reduce their tolerance or trigger a more aggressive behavior by the latent pathogen. For instance, plant stress is presented as a fundamental factor stimulating the pathogenic behavior of *L. theobromae* on dogwoods (*Cornus florida*), also considering the occasional failure of artificial inoculations during pathogenicity trials [129]. Genetic factors also actively influence the lifestyle shift of plant-associated fungi, as documented in a dedicated study disclosing repeated conversions during the evolutionary history of several species [130].

Members of the Botryosphaeriaceae are renowned as latent pathogens with a wide host range and geographical distribution [50,131]. Confirming this general feature, *L. theobromae* exerts such an ecological adaptability, particularly in tropical and subtropical regions [4,5]. However, the recent increasing trend in temperature may result in a major range expansion, placing more known and unknown hosts at risk.

Until recently, the incidence of latent pathogens has been underestimated, particularly in the trade of forest and horticulture plants and products; indeed, endophytes have been long disregarded in quarantine measures [131], which has enabled fungi to spread in plant germplasm circulating around the world [2,132]. With specific reference to *L. theobromae*, it has been conjectured that this fungus might have spread from Mexico to other subtropical countries through the trade of pine seeds [8]. Another hypothesis based on a phylogeographic approach considers the possible spread to South America from Africa to have repeatedly occurred as a consequence of human activities [28]. The availability of molecular techniques for the routine screening of plant material has increased the awareness that this risk has to be monitored [133]. In fact, besides considering pathogenic fungi of crops with an undefined latent stage [134,135], the European Food Safety Agency (EFSA) has recently started to consider the potential presence of disease agents occurring as endophytes in traded ornamental plants [136]. This concern is further supported by data gathered in this review, also considering that several hosts belong to widespread tree genera in boreal forests (e.g., *Pinus*, *Populus*, *Quercus*, *Taxus* and *Zelkova*). On the other hand, the accumulation of data on the occurrence of endophytes also provides

an indication that some plants could be exempt. As an example, a recent review on the endophytic fungi of olive tree (*Olea europaea*), gathering all the available data concerning this important crop, has disclosed that, to date, there are no citations concerning *L. theobromae*, not only in the Mediterranean area but also in several tropical and subtropical countries where the plant has been introduced [3].

Many fungi reported for their endophytic occurrence are better known as plant pathogens. This is to be interpreted not only considering a more or less enduring latent stage within the disease cycle, as introduced above, but also with reference to a variable capacity by plant species to host certain fungal pathogens without showing symptoms of infection. Besides the more established concepts considering an improvement of host fitness in terms of growth promotion and protection against pests and pathogens, in the case of renowned disease agents, it has been conjectured that the capacity of a plant to host and promote their horizontal spread in the biocoenosis reflects a competitive advantage against other susceptible species [137]. This concept is quite appropriate for *L. theobromae*, which has such a high number of hosts as both a pathogen and an endophyte.

The problematic discernment of the real relationships with the host plant particularly emerged in our overview of the endophytic occurrence of *L. theobromae*. Indeed, defining this fungus as an endophyte in crops where it is known to cause disease (at least 46 plant species listed in Table 1, underlined) arouses a certain perplexity and raises the question of how to consider observations in the wild. The subject of plant pathology basically consists of diseases affecting crops or forest plants, and thorough assessments concerning fungal diseases of non-crop species are infrequent. In the absence of previous records and symptom descriptions, how can we be sure that a fungus isolated from "asymptomatic" tissues of a plant growing in whatever natural context is not exerting pathogenicity? It is worth observing that most of the plant species listed in Table 1 are not crops, and that for the majority of them, there is just a single finding, which is not at all sufficient for making a decision in this respect. Moreover, rather than being focused on the moment and circumstances of isolation, the issue should be considered with reference to the entire life cycle of the host plant: in this respect, how to consider reports of endophytic occurrence in centuries-old trees such as baobabs? [15,50].

Besides baobabs, there are more plants where it has been clearly demonstrated that the presumed endophytic occurrence is rather considered to refer to isolations carried out during the latent stage of the disease cycle. This is the case of cashew (*Anacardium occidentale*), where *L. theobromae* was recovered from healthy tissues at a distance of up to 80 cm from cankers caused by the same, and it was found to transmit through apparently healthy propagation material [92]. In other cases, the issue may be considered to have a "topographical" connotation, basically when the fungus exerts its pathogenic aptitude in some plant parts only. In fact, endophytic asymptomatic colonization of mango (*Mangifera indica*) shoots and branches has been shown to be prodromal to postharvest fruit rot [93,138]. In the case of *Aquilaria* spp. used for the production of agarwood, designating *L. theobromae* as an endophyte seems inappropriate too; in fact, resin formation is promoted as a reaction to an infection process which rather qualifies the fungus as a pathogen [139]. Likewise, internal infections by *L. theobromae* are reported to cause blue stain of wood after felling in *Pinus elliottii* [8], as well as in *Terminalia* spp. [87] and rubberwood (*Hevea brasiliensis*) [140]. It is worth considering that in similar cases observed on neem (*Azadirachta indica*) [141] and *Ficus insipida* [142], the occurrence of the fungus is merely referred to as a pathogenic association.
