**3. Other Endophytic Fungi and Their Interactions with Pests and Pathogens of** *Citrus*

Besides the above reports, essentially dedicated to pathogenic species/genera upon the aim to assess the epidemiological impact of latent endophytic stages, additional data have been recorded on the overall species assemblage in a few contexts (Table 1). A study carried out on *C. limon* in Cameroon [36] pointed out that yellowing of leaves affects foliar endophytic communities, and that interactions among endophytes may be a factor driving the yellowing process. In fact, yellow leaves presented a less varied species assortment dominated by *C. gloeosporioides* in the absence of species belonging to the Mycosphaerellaceae, otherwise common in healthy leaves. In vitro observations in dual cultures showed that the latter were inhibited and overgrown by *C. gloeosporioides*, even if capable to revert this inhibitory effect when pre-inoculated, which was interpreted as deriving from production of fungitoxic metabolites. This study also demonstrated a low occurrence of species in the Xylariaceae, which are usually quite widespread as tree endophytes [67,68].

The endophytic occurrence of a few yeast species was documented in an investigation carried out on *C. sinensis* in Brazil [53]. By means of scanning electron microscopy, it was observed that these microorganisms are mostly localized around stomata and in xylem vessels. Isolates of the species *Rhodotorula mucilaginosa*, *Meyerozyma* (*Pichia*) *guilliermondii*, and *Cryptococcus flavescens* were inoculated in healthy plants, and re-isolated, without causing any kind of disease symptoms. Quite interestingly, the authors noted that *M. guilliermondii* primarily occurred in plants colonized by *Xylella fastidiosa*, the causal agent of citrus variegated chlorosis (CVC), and that the bacterium could thrive on a supernatant separated from cultures of a strain of this species. This finding represents an indication that the presence of the yeast could stimulate the pathogen and could be responsible for more severe disease symptoms. More recently, strains of *M. guilliermondii* have been recovered, along with strains of *Hanseniaspora opuntiae* and *Pichia kluyveri*, from tangerine peel in China. However, it is questionable if this record can actually concern endophytic occurrence considering that authors refer that fruits were purchased on the market rather than being directly collected in the field [59].

Indeed, interactions between endophytic bacteria and fungi are complex, and the assortment of strains which can be recovered is largely influenced by the antagonistic interactions as mediated by the production of antibiotics. In this respect, strains of *P. citricarpa* isolated from *Citrus* spp. in Brazil were found to possess inhibitory properties toward several endophytic *Bacillus* spp. from the same source, while a stimulatory effect was assessed towards the gram-negative *Pantoea agglomerans*, which can be taken as an indication of the opportunity to investigate possible interference with the development of *X. fastidiosa* [56].

Antagonistic properties by an isolate of *Muscodor* sp. from *C. sinensis* were reported against *P. citrocarpa* as deriving from the production of volatile organic compounds (VOCs) [61]. Actually, such properties are known for endophytic isolates of *Muscodor* and other genera of xylariaceous fungi, such as *Hypoxylon* (=*Nodulisporium*) and *Xylaria*, reported from many plant species [69] and also occurring in citrus plants [35,36,49,52].

Endophytic strains belonging to two species of*Diaporthe*,*D. terebinthifolii* and the already-mentioned*D. endophytica*, displayed inhibitory properties against *P. citrocarpa* in vitro and on detached fruits. Moreover, their transformants expressing the fluorescent protein DsRed proved to be able to actively colonize citrus seedlings, and to remain viable in the plant tissues for one year at least. These evidences support their

possible use in the biocontrol of this pathogen [70]. Antifungal properties have also been reported for a strain of another fungus belonging to the Diaporthales (*Lasmenia* sp.), which was recovered from *C. medica* var. *sarcodactylis* [52].

Rather than just concerning agents of cryptogamic diseases, protective effects by endophytic fungi may pertain several kinds of pests [71,72]. Actually, data available in the literature concerning citrus plants are limited but encourage further assessments. For instance, a ustilaginomycetous yeast endophytic in grapefruit (*Citrus paradisi*), *Meira geulakonigae*, was found to be able to reduce populations of the citrus rust mite (*Phyllocoptruta oleivora*) [60]. More recently, two strains of *Beauveria bassiana* were inoculated in seedlings of *C. limon* through foliar sprays and proved to be able to colonize the plants endophytically. Besides increasing plant growth, they caused 10%–15% mortality on adults of the Asian citrus psyllid (*Diaphorina citri*), and the females feeding on the treated plants laid significantly fewer eggs [55]. It is not unlikely that more evidence in this respect can be gathered from targeted investigations concerning naturally occurring endophytes, considering that protective effects have been documented for endophytic strains of *F. oxysporum* against aphids [73] and nematodes [74].

As a general ecological trait, endophytic fungi seem to be absent in seeds of citrus species [65]. This is to be taken as an indication that these microorganisms are not adapted to a vertical spread, and most likely colonize citrus plants coming from the surrounding environment.
