There is no concise and unambiguous definition for grasslands (Gibson, 2009). The definition of a grassland could be based on the absence of specific vegetation features (Milner and Hughes, 1969) when using a physiognomic or structural approach (Bazzaz and Parrish, 1982; Sims, 1988). A more promising definition, however, was given by Risser (1988), who indicated that grasslands are “types of vegetation that are subject to periodic drought, that have a canopy dominated by grass and grass-like species, and that grow where there are fewer than 10 to 15 trees per hectare”. Grasslands are distributed throughout the world’s land area except on the continent of Antarctica (Gibson, 2009). Based on the “The Pilot Analysis of Global Ecosystems (PAGE)” classification, grasslands cover 52,544,000 km² or 40.5% of the world’s land mass (excluding urban areas according to night time lights) (White et al., 2000; Kassam, 2002). According to the PAGE classification, grasslands cover more land area than the other major vegetation cover types (White et al., 2000). For example, forests cover 28.97 × 10⁶ km² and agriculture covers 36.23 × 10⁶ km² (White et al., 2000). Furthermore, grasslands are the second largest land type inhabited by humans (nearly 800 × 10⁶ people), second only to agricultural land, which holds 2.8 × 10⁹ people according to 1995 estimates (White et al., 2000; Gibson, 2009). Grasslands also occupy comparatively large areas of many major watersheds in the world (Gibson, 2009).

The effect of species diversity on the productivity of a community has been explained by two mechanisms. They are: (i) the sampling effects, which state that the probability of finding key-trait species in a community is reduced due to lower species richness (Aarssen, 1997; Huston, 1997; Tilman et al., 1997) and (ii) the niche complementarity hypothesis, which states that less diverse communities with competing species utilize resources incompletely (Naeem et al., 1994; Tilman et al., 1996; Hector et al., 1999). Furthermore, the loss of interactive competition and loss of species diversity have been shown to increase mutualism, predation, herbivory, and infectious disease transmission (Bond, 1993; McNaughton, 1994; Chapin et al., 1997; Chapin et al., 2000). Mitchell et al. (2002) tested the long-standing hypothesis (Elton, 1958; van der Plank, 1963) that the lower diversity of plant species increases the severity of diseases, focusing mainly on specific pathogens. Several studies have previously suggested that this hypothesis applies only to a small number of plant species and genotypes (Garrett and Mundt, 1999; Zhu et al., 2000). The relaxed interspecific competition due to the decreased plant species richness has been shown to increase the abundance of one or more species existing in a local community, which typically also increases the abundance of one or more host species for specialist pathogens (Aarssen, 1997; Huston, 1997; Tilman et al., 1997). The basic mechanism of the diversity-disease hypothesis is that a decreasing number of plant species allows for an increased local abundance of other singular species, which then facilitates the spread of diseases specific to that species within the community (Burdon and Chilvers, 1976; Knops et al., 1999; Chapin et al., 2000). The extent of the differences among different species is correlated with the ecological effects of species diversity (Tilman and Lehman, 2014); hence, the susceptibility of different species for a particular disease must vary. Thus, the local mechanisms of the diversity-disease hypothesis vary highly, as the host abundance depends on numerous biotic and abiotic factors in communities (Mitchell et al., 2002). Apart from host abundance, many other factors such as microclimate and the competitive ability of host plants also influence the disease level in ecosystems with decreasing diversity (Boudreau and Mundt, 1992; Boudreau and Mundt, 1994; Zhu et al., 2000; Boudreau and Mundt, 2018).

Most studies examining the diversity-disease hypothesis have been focused on agriculture or silviculture (Mitchell et al., 2002). However, studies by Kranz (1990) revealed that natural, communities with higher diversity did not necessarily have low disease levels, whereby the diversity of forest and pasture communities was higher while in meadow and agricultural field communities it was lower (Kranz, 1990). As such, species diversity is related to species composition, microclimates, and many other factors (Kranz, 1990; Mitchell et al., 2002). Agronomic intercropping, with increased species diversity, subsequently decreases the prevalence of diseases (Mitchell et al., 2002), especially fungal diseases. Experimental studies on the diversity-disease hypothesis with regards to intercropping are fewer (Boudreau and Mundt, 2018), but intercropping can change the microclimate and the competitive nature of the crops and all these factors together can either increase or decrease disease severity (Boudreau and Mundt, 1992; Boudreau and Mundt, 1994; Zhu et al., 2000; Boudreau and Mundt, 2018).

It has been shown that cultivating a variety mixture or multiline with multiple genotypes of one crop reduced the severity of airborne fungal diseases when compared with its intercrop cultivation (Wolfe, 1985; Finckh and Wolfe, 1997; Garrett and Mundt, 1999; Zhu et al., 2000; Boudreau and Mundt, 2018). In agricultural systems, the main mechanism of reducing the spread of the disease is through reducing the host abundance (Burdon and Chilvers, 1977; Burdon and Chilvers, 1982; Chin and Wolfe, 1984; Wolfe, 1985; Alexander et al., 1986; Burdon, 1987; Garrett and Mundt, 1999; Zhu et al., 2000; Boudreau and Mundt, 2018). We can overlook highly diverse natural grassland ecosystems through the modern knowledge of agricultural multiline even though natural grasslands are more complex than the agricultural multiline.

Disease susceptibility and the dominant species present are two aggregated characteristics of a community that can influence the spread and severity of a disease (Mitchell et al., 2002). Mitchell et al. (2002) tested several hypotheses relating to disease levels in a community and found that (1) the loss of less susceptible species in a community increased the disease levels within the community more than the loss of highly susceptible species, (2) the loss of less susceptible species from a plant community increased the disease levels in that community, and (3) the loss of the dominant species increased the prevalence of species-specific pathogens more than the loss of rare species, when all other conditions were equal. However, in certain instances, the most abundant species can be more susceptible to disease, and in such cases the above hypotheses are not valid and the effect of losing the dominant, highly susceptible species is countervailing (Arneberg et al., 1998).

Grasslands are highly diverse ecosystems with many interactions. Plant communities in grasslands are comprised mainly of diverse species of grasses, including genotypes of the same species, and various dicots. However, the major part of the community is represented by grasses. Population structure of grasslands is crucial for the productivity as well as the resource utilization of grasslands. However, pathogens in grasslands play a crucial role in the productivity of these communities. Herein we investigate how fungal pathogens presence affect the dynamics of grasslands. Population dynamics of fungal pathogens are highly influenced by the diversity and the population structure of grassland is in question. Furthermore, the dynamics of specialist pathogens and generalist pathogens are contrasting. Increased host diversity of grasslands increases generalist pathogens while reducing specialist pathogens. However, this may be highly vary based on the many biotic and abiotic factors in grasslands. Hence, interactions between pathogens and their host in natural grasslands are complex.

This review brings together a vast amount of information on fungal pathogens in grasslands and then discusses their role within and effects on grassland ecosystems. This also describes the pathogenic fungal communities in grasslands and discusses well-studied pathogenic fungal species reported on host grasses.

Pathogenic fungi significantly affect the population biology of grasses and their contribution to plant communities by affecting the physiology and chemical composition of those grasses (Gibson, 2009). This in turn affects the population ecology of grassland communities and ecosystems, especially when they affect the dominant species of those communities (Burdon et al., 2006). However, the pathogenic effects of fungi cannot be studied by considering only the fungi themselves; they manifest their effects via complex species interactions (Ampt et al., 2019).

Ecologists have observed that two main aspects drive the interactions between plant species and their natural enemies in natural ecosystems—host-specificity and density-dependency (Janzen, 1970; Connell, 1971; Bever et al., 2015). According to the “pathogen hypothesis”, the negative effects from a pathogen on a host must either be species-specific affecting certain species only, or a generalist which can have negative effects on multiple host species (Hersh et al., 2012; Ampt et al., 2019). Mommer et al. (2018) studied monocultures of four grass species and four forbs found in natural grasslands. In this study, the Internal transcribed spacer (ITS) based Next-Generation Sequencing (NGS) analysis of soil-borne fungal species showed a clear difference in the host specificities of fungal communities among grasses and forbs. In addition, host specificity has been apparent in plant-soil feedback studies, where the same species or the functional group, has been grown and the plant growth was reduced (Petermann et al., 2008; Mangan et al., 2010; Hendriks et al., 2013; Cortois et al., 2016). It is important to use NGS complemented analysis in conjunction with traditional pathology knowledge to gain a proper knowledge of the interactions between soil-borne fungal pathogens and their plant hosts (Mommer et al., 2018). An insight into the host specificity of soil-borne fungal pathogens in natural grasslands has been provided by Klironomos (2002). Klironomos (2002) isolated fungi from roots of several rare plants found in Canadian meadows, which were later identified as species of Cylindrocarpon, Fusarium, and Verticillium. Although identification of these fungi was only to the level of the genus, in the ecological context host specificity was clearly demonstrated upon inoculation to the field as there was a reduction in plant growth as a result of fungi isolated from conspecific roots but not from heterospecific roots (Klironomos, 2002). Consequently, this demonstrated that those fungi had only a negative impact on their preferred host (Gibson, 2009).

Biodiversity experiments in natural grasslands have also demonstrated pathogen dilution in foliar (Knops et al., 1999; Mitchell et al., 2002; Rottstock et al., 2014) and soil-borne fungi (Mommer et al., 2018). Rottstock et al. (2014) showed that foliar pathogen incidence and severity were reduced with increased species diversity. However, Ampt et al. (2019) suggested that the results might be due to a sampling error. Another study examined the effect of plant density on soil-borne disease dynamics in relation to seedling mortality in agricultural systems (Otten et al., 2004; Otten et al., 2005; Hiddink, 2008). During experiments on Wageningen grassland biodiversity (Van Ruijven et al., 2003; Van Ruijven and Berendse, 2005; Van Ruijven and Berendse, 2009; Cong et al., 2014), nearly 50% of the variation in fungal community composition could be explained by the density of the below ground host components. Hence, host density has a major impact on the fungal community. Moreover, above ground fungal pathogens play a major role in biomass of the grasslands (Cappelli et al., 2020). The recent studies by Cappelli et al. (2020) showed that the infection incidence is highly affected by trade-off between plant growths and defines with the consideration of diverse grasslands and generalist pathogens as the generalist pathogens can spread out easily within a highly diverse grassland.

Although host specificity and host density both play major roles in the disease dynamics within a community, heterospecific neighbors also have an impact (Otten et al., 2005; Eppinga et al., 2006). Increased heterospecific neighbor plants, or the host diversity, reduce the host density of the specialist pathogens while increasing the density of generalist pathogens (Petermann et al., 2008). This evidence was observed in agricultural intercropping, where the presence of maize reduced Phytophthora blight severity and spread in pepper plants (Yang et al., 2014). The heterospecific neighbor effect can be explained in two ways by considering the below ground disease dynamics (Ampt et al., 2019). First, there are direct neighbor effects occurring through plant traits and root exudates. Second, there are indirect neighbor effects through the root microbiome (Ampt et al., 2019). Heterospecific neighbors can either act as barriers or vectors for diseases (Otten et al., 2005; Ampt et al., 2019). Host plants surrounded by non-host plants are less likely to become infected, and hosts that are surrounded by the hetero-specific hosts or symptomless hosts are susceptible to becoming infected (Malcolm et al., 2013). Furthermore, phylogenetically distant hosts are less likely to infect other hosts, and thereby plant communities with phylogenetically diverse hosts are less likely to spread disease within themselves (Gilbert and Webb, 2007; Haas et al., 2011; Wehner et al., 2014; Gilbert and Parker, 2016). The root system of different species may also either enhance or reduce disease transmission by physical or chemical means (Newsham et al., 1995). Intercropping of maize and pepper reduced the spread of Phytophthora blight due to the higher degree of root intermingling (Yang et al., 2014). Similarly, the dense and tightly intermingled roots of natural grasslands reduce the spread of disease (Kesanakurti et al., 2011; Ravenek et al., 2014; Frank et al., 2015). Moreover, complex below ground chemical composition and chemical communication has an impact on disease dispersal as well, but these mechanisms are not yet well explained (Ampt et al., 2019). Indirect neighbor effects via the root microbiome have been shown in natural grasslands inhabited by species of Streptomyces (Bakker et al., 2013; LeBlanc et al., 2015).

In natural grasslands, hosts, fungi, and pathogens are highly correlated. Below-ground host components shape total fungal composition while above-ground pathogens shape the biomass of grasslands. Moreover, generalist pathogens are highly dispersed in grasslands characterized with rich diversity. Hence, in grasslands, disease incidence highly relies on the plant growth, diversity of grasslands, and generalist pathogens in grasslands. Host specificity, host density, and host diversity play huge roles in the dynamics of grasslands. Heterospecific neighbors disturb the spread of specialist pathogens in grasslands. However, the presence of heterospecific neighbors increases the spread of generalist pathogens. However, heterospecific hosts can be either barriers or vectors if the host is symptomless. The close phylogenetic relatedness of grasses can increase disease spread among grasses with shared traits. Heterospecific neighbors can affect the spread of diseases through plant traits or root microbiomes. However, the tightly intermingled roots of heterospecific neighbors reduce specialist pathogens in grasslands.

The majority of studies on grassland pathogens have been focused on above ground systems and monocultures for a few primary reasons (Ampt et al., 2019). The air-borne dispersal and pest-based dispersal of above ground pathogens attributes to fast spread of the pathogen and can be detected much more easily, especially under epidemics (Mitchell et al., 2002; Rottstock et al., 2014). Particularly in monoculture systems, the diseased areas can be visually seen as gaps in cultivation drawing immediate attention. Furthermore, much of the research attention in this area has been focused on monocultures, as they are often a part of economically important agricultural systems (Ampt et al., 2019).

Plant species richness has been shown to increase plant productivity, and soil-borne fungal pathogens play an important role in plant productivity (Maron et al., 2011; Schnitzer et al., 2011; Cardinale et al., 2012; Mommer et al., 2018). Experiments on how biodiversity loss affects ecosystem functioning in experimental grasslands has shown that monocultures perform less well than polycultures in measures of plant productivity (Tilman et al., 1997; Hector et al., 1999; Tilman et al., 2001; Van Ruijven and Berendse, 2005). However, the mechanism responsible for this scenario is not clear (Cardinale et al., 2012). The most promising strategy for explaining the difference in plant productivity has been by focusing on plant-plant interactions, resource partitioning, and facilitation between plants (Mueller et al., 2013; Ravenek et al., 2014; Wright et al., 2017; Jesch et al., 2018). Furthermore, interactions between the plants and soil biota are important contributor for positive biodiversity effects. The main hypothesis regarding the effect of soil-borne pathogens is that their negative impact is stronger in plant monocultures and weaker in mixed-plant ecosystems (Maron et al., 2011; Schnitzer et al., 2011; de Kroon et al., 2012; Mommer et al., 2018). Ecologists have investigated the “pathogen hypothesis” using a black box method, such as comparing plant growth on soil with and without the soil biota (Kos et al., 2013; Mccarthy-Neumann and Nez, 2013; Hendriks et al., 2015; Cortois et al., 2016; Wubs and Bezemer, 2018). Schnitzer et al. (2011) grew plant communities in both field and sterile soil. The result showed that on sterilized soil, the positive effects of plant species richness on plant productivity disappeared. Sterilization enhanced the productivity at lower biodiversity but did not affect biodiversity of soil with high plant species richness. Again, Schnitzer et al. (2011) re-inoculated the sterilized soil using a soil wash containing soil-borne fungi and observed that the productivity for the lower diversity group was reduced. This emphasized the importance of soil-borne fungi on biodiversity patterns. Maron et al. (2011) experimented on the relationship between plant diversity and above-ground plant biomass in fungicide treated and non-treated soils. Application of systemic fungicides thiophanate ethyl and mefenoxam adversely affected this relationship indicating the positive effect exerted by soil fungi on plant diversity and productivity (Maron et al., 2011). The experiments of Maron et al. (2011) and Schnitzer et al. (2011) demonstrated the importance of host specific microorganisms in determining the diversity-productivity relationship. Hence it can be stated that a deep understanding of the roles played by soil-borne fungi in grasslands is essential in defining the exact roles of grassland associated fungi.

The most studied soil-borne pathogens of grasses are the ones that have been known to cause severe diseases for many hosts, especially for agriculturally important crops (Ampt et al., 2019). For example, Fusarium oxysporum causes Fusarium wilt on nearly 100 monocot and dicot hosts (Michielse and Rep, 2009). Soil-borne fungal diseases have been reported from commercially grown medicinal herbs and turf grass species found in natural grasslands (Smiley et al., 2005). Furthermore, soil-borne fungal diseases have also been identified in commercial monocultures of dicots found in grasslands (Gaetán et al., 2004). However, this knowledge has not been found to provide very much insight into fungal-plant interactions of highly diverse grassland ecosystems (Ampt et al., 2019).

Natural grasslands are diverse ecosystems housing a variety of monocot and dicot plants (Herben et al., 1993; Van Der and Sykes, 1993). Hence, grassland ecosystems are highly static yet show a wide variety of spatiotemporal dynamics (Herben et al., 1993; Van Der and Sykes, 1993). Soil-borne pathogens play a major role in these spatiotemporal dynamics (Olff et al., 2000). Soil-borne fungal diseases in natural grasslands are rarely observed, either because the disease incidence is low in higher diversity communities or other species replace the poorly performing species before they have a significant effect (Gilbert, 2002; Burdon et al., 2006; Alexander, 2010). Soil-borne pathogens in grasslands are highly diverse due to the overall high species diversity and the genotype diversity for a given species (Termorshuizen, 2014; Dassen et al., 2017; Yang et al., 2017; Bach et al., 2018). Although plant root systems in grasslands are colonized by a wide variety of fungi, only a few have been isolated and had their pathogenicity thoroughly examined through Koch’s postulates (Vandenkoornhuyse et al., 2002). Mills and Bever (1998) isolated five species of Pythium from two types of perennial grass, Danthonia spicata and Panicum sphaerocarpon, from a 50-year-old grassland. They reintroduced four species of Pythium, and it reduced the biomass of D. spicata and P. sphaerocarpon, while Anthoxanthum odoratum (Poaceae) and Plantago lanceolata (Plantaginaceae) were not adversely affected by the four reintroduced species of Pythium. In the most recent study of grassland biodiversity by Mommer et al. (2018), the authors isolated 27 species of fungi from symptomatic roots of Anthoxanthum odoratum (Poaceae) and the forb Leucanthemum vulgare (Asteraceae). Among the isolates, Magnaporthiopsis panicorum and Paraphoma chrysanthemicola caused host specific adverse effects on seedlings (Mommer et al., 2018). Hence, root inhabiting pathogens can cause a considerable loss on the biomass of natural grasslands (Ampt et al., 2019).

Although the mechanisms of soil-borne plant pathogenic fungi affecting the spatiotemporal dynamics of natural grassland ecosystems are not well known, three main pieces of experimental based evidence have been identified (Ampt et al., 2019). First, next generation sequencing (NGS) has been used to identify a very high diversity of soil-borne fungal community growing on roots of grassland plants. Second, some soil-borne fungi have been observed to negatively affect plant growth in bioassays. Third, the relationship between plant diversity and plant productivity is affected by the sterilization of the soil and by application of fungicides (Ampt et al., 2019). However, the interactions of soil-borne fungi and their effects on plants within the larger context of biodiversity are yet to be discovered.

In natural grasslands, strategies that account for different productivities are plant-plant interactions, resource partitioning, and facilitation between plants. In species-rich grasslands, soil-borne fungi play an immense role in increasing the productivity of grasslands. However, in less-diverse grasslands, soil-borne fungi reduce productivity. Hence, stable grasslands are in proper equilibrium with host species and soil-borne fungal communities. In highly diverse grasslands soil-borne fungal diseases are rarely observed due to the low disease incidence and the natural replacement of poorly performing species. However, in general, high species or genotypic diversity of grasses leads to high diversity of soil-borne pathogens, although disease expression depends on the traits of host plants. Furthermore, the monocots and dicots in grasslands share the same soil-borne pathogens.

The studies on above ground fungal pathogens of grasses mentioned above were primarily studies based on agricultural monocultures. Above ground fungal pathogens in complex natural grassland ecosystems are much less studied. In the following sections, several well-studied fungal pathogens found in agricultural monocultures will be discussed.

There are many studies of pathogens on grasses. Although the pathogen-related studies are mainly focused on economically important monocultures. They provide invaluable information about aspects of populations involved as well as their genomes. As such, in this section we describe several studies on selected well studied pathogens to understand how important it is to carry out a proper study on pathogenic fungal population in natural grasslands. Herein we are describing four well studied species: Bipolaris sorokiniana, Colletotrichum graminicola, Fusarium graminearum, and Pyrenophora tritici-repentis. Each species was selected in order to explain significant characteristics of fungal pathogens relevant to the Poaceae. The race structure of Pyrenophora tritici-repentis and its relatedness to economically important crops as well as certain instances where it is relevant to the natural grassland systems along with data on genomes relevant to the pathogenicity have been intensively studied. Fusarium graminearum has been well studied for its mycotoxin production and its involvement with pathogenicity. Bipolaris sorokiniana is a well studied pathogen noted for its variations in pathogenicity, DNA polymorphism, and adaptability. Colletotrichum graminicola has been well studied for the infection mechanism and involvement of proteins in inducing diseases. Some of the common details of those pathogens are given in Table 1 .

The natural grasslands are complex ecosystems of plants and animals (Gibson, 2009). Their interactions are shown to be highly variable (Gibson, 2009). Hence, the interactions between animals and grassland ecosystems cannot be neglected. Grasslands shelters an immense number of vertebrate and invertebrate animals and their interactions are vital for the grasslands (Gibson, 2009). Therefore, identification and understanding of fungal diseases on animals living in grasslands are important.

A direct study regarding fungal pathogens on grassland animals is not reported. Hence the linkage of current knowledge regarding domestic and industrial farms such as cattle farms can be taken to get an insight. A well-studied disease chronic facial eczema on sheep is caused by a grassland fungus Pithomyces chartarum. Pithomyces chartarum is a well-known saprobe in monocots including grasses (Laven et al., 2020; Munday et al., 2020) and it causes facial eczemas in ruminants and camelids (Laven et al., 2020). Pithomyces chartarum produces a mycotoxin known as sporidesmin which causes edema, ulceration and crusting dermatitis on face and ears in ruminants and camelids (Munday et al., 2020). Sporidesmin concentrates in bile and causes damages to liver through the bile duct (Laven et al., 2020).

Fungal diseases on humans relevant to grasslands have not studied directly and no published data is available on direct fungal infections on humans caused specifically by grassland fungi. But diseases caused by mycotoxins through the cattle industry was reported (van Egmond, 1983; Kemboi et al., 2020). Mycotoxins are hazardous to cattle as well as humans who consume dairy products (van Egmond, 1983; Kemboi et al., 2020). Main mycotoxin categories relevant to the dairy industry are Aflatoxins (AF) produced by Aspergillus species, deoxynivalenol (DON) produced by Fusarium species, fumonisins (FUM), ochratoxin A (OTA) produced by Penicillium and Aspergillus species, T-2 toxin (T-2) and zearalenone (ZEN) (Kemboi et al., 2020). The well-known aflatoxin M1 is a carcinogen (van Egmond, 1983; Kemboi et al., 2020).

Fungal pathogens of humans and other animals in grasslands are least studied, although, such pathogens are economically highly important. There should be emerging animal and human pathogens in grasslands around the world. Hence comprehensive studies on pathogens in grasslands are important.

Natural grasslands are a vital component of several different types of terrestrial ecosystems but are not yet sufficiently studied (Gibson, 2009). Grasslands are highly complex ecosystems comprised of perennials and are dominated by members of the family Poaceae (Risser, 1988). With the high density of plants and the high density of below ground root systems, grasslands provide an ambient environment for microbial growth (Mommer et al., 2018; Ampt et al., 2019). The composition of the flora of the grasslands in various regions is affected by unique biological and ecological factors (Risser, 1988; Gibson, 2009). Hence, the interaction between the microbiota and their hosts becomes unique in each grassland system based on the floral composition.

There is a wide range of fungal lifestyles in nature, but our concern in this paper was focused on the pathogenic lifestyle of the fungi associated with natural grasslands. The fungal- plant interactions in grasslands can be described using two approaches—host specificity based and density dependent based. Research on fungal pathogens can be based on either phylogeny or pathogenicity (Janzen, 1970; Connell, 1971; Bever et al., 2015). Currently, there have been many taxonomy and phylogeny based studies on the fungi associated with grasses in different ecosystems, which are subject to change since many novel taxa are continuously being introduced. This situation provides the basis for developing an insight for resolving the taxonomic placement of identified and unidentified fungal taxa. In many cases these data cannot be used in accordance with the pathogenicity related studies unless the life mode is confirmed, specifically through the confirmation of the pathogenicity through the Koch postulate. Grasslands also cause disease of humans and other animals.

Grassland ecosystems are characterized by complex interactions between pathogens and their hosts. Although the majority of the grassland plants are monocots, there are dicots present among the monocots plants (Herben et al., 1993; Van Der and Sykes, 1993; Gibson, 2009). Furthermore, there is a high diversity of species in the family Poaceae. Hence, grasslands cannot be considered as a monoculture, and the interactions between monocots as well as between monocots and dicots need to be considered (Herben et al., 1993; Van Der and Sykes, 1993).

The fungal community below ground and above ground are highly diverse and contrasting. Furthermore, the distribution of fungal pathogens above and below ground in grasslands are complex (Van Ruijven et al., 2003; Van Ruijven and Berendse, 2005; Van Ruijven and Berendse, 2009; Cong et al., 2014; Cappelli et al., 2020). Comparatively speaking, the ecology-based studies of fungal pathogens below ground are much more common than the ecology based studies of fungi above ground in natural grasslands. Studies of the fungal pathogens of members of the Poaceae are mainly focused on agricultural monocultures. Hence, there is a dearth of information on above ground fungal pathogens rather than those below ground. In certain cases, these data are applicable to natural grasslands as far as the same species is concerned. However, the effects of complex ecological intereactions in the natural grasslands cannot be neglected.

Fungal pathogens of animals in grasslands are poorly studied. Natural grasslands provide grazing sites for large ruminants (Gibson, 2009). In many instances those grazing ruminants directly or indirectly involve human activities and the economy. Hence it is important to study relationships between grassland animals and fungi. Chronic facial eczema caused by Pithomyces chartarum shows how important it is to understand fungal communities in grasslands (Laven et al., 2020; Munday et al., 2020). In a way, grasslands can be a reservoir of fungal pathogens.

In this review, we have provided four examples where a complete study has been carried out on the pathogenicity and genomics of a particular pathogen. They are based on agricultural monocultures with the help of natural grassland based data. There is a necessity of exploring the natural grasslands to identify novel fungal species. In the meantime, confirming the pathogenicity is also very important. Agronomically, it is important to study the pathogenic community in grasslands as a reservoir of fungal pathogens and understanding the dynamics of the fungal community further helps to understand the effects of fungal pathogens in agricultural multi-lines. The study of Pyrenophora tritici-repentis demonstrated just how important it is to perform a thorough study even on a well-known pathogen, since there are many races with different life modes and attributes. The behavior of the pathogen in natural grassland is also important to observe in order to understand changes in gene expression and pathogenicity.

Plant diversity of grasslands is highly variable around the world. However, the stable natural grasslands are rich in host diversity. The belowground host components shapes the total fungal community and the above ground fungal community highly affects the productivity and the biomass of the grassland. Hence, the stability of grasslands is highly dependent upon the balanced interactions of those factors together with other biotic and abiotic factors. The behavior of specialist pathogens and generalist pathogens in grasslands is crucial for the stability of these communities. Heterospecific neighbors in grasslands disturbs the spread of specialist pathogens but also could facilitate the spread of generalist pathogens. However, fungal disease spread in grasslands depend on huge number of biotic and abiotic factors. Different fungal pathogens act differently on the hosts. Meantime, host respond is highly variable. Studies of Pyrenophora tritici-repentis on grasses provide good insight on how pathogenic response of different host genotypes vary towards the same pathogen. In addition, different hosts or genotypes of grasses can induce host-fungal pathogens without causing symptoms. Hence, in a way, grasslands can act as a reservoir for fungal pathogens. Even though pathogenic mechanisms of fungi have been well explained from monocultures, the virulence of the fungus varies with the high diversity in grass lands. Many factors such as heterospecific neighbors, tightly and closely arranged roots, effects of root exudates and root microbiomes, and different host traits can control disease spread.

The knowledge of host-pathogen interactions in grasslands can be used for agricultural purposes. Strategic grasslands can be used among the crop fields to control the spread of diseases. Grasslands in between crop fields can act as dense heterospecific host sites for many pathogens and this can immensely reduce the disease spread among the adjusent crop fields. Furthermore, the presence of highly diverse grasslands can reduce the disease insidence of the specialist pathogens. Grass heterospecific neighbor effects of the soil-borne fungal pathogens can be explained in two ways. First, neighbor effects though plant traits or root exudates. Second, neighbor effects through root microbiome. However the loss of dominant plant species in grasslands can increase the extent of disease in the system (Mitchell et al., 2002). Hence, proper understanding of the dominant species in the strategic grasslands and maintainance of the dominant species is important for disease control.

Grasslands are an ecologically and economically important component of the earth’s vegitation. Fungal communities in grasslands play a huge role on the stability of grasslands. Having more complete knowledge of fungal pathogens in grass lands is important for developing an understanding of grassland ecology. In addition, understanding behaviour of fungal pathogens in highly diverse grasslands may provide novel insights towards being able to control the diseases in commercial crop fields. In this review, we address the effects of fungal pathogens in grasslands and discussed their complex interactions.

AK, ST, KH and SCK designed the review. SL, JK and SK provided the grant. AK wrote the manuscript with ST, CN, SCK and SLS. RJ, SA, JX, KH, JK and SL reviewed and edited the manuscript. All authors reviewed and approved the final manuscript. All authors contributed to the article and approved the submitted version.

International Postdoctoral Exchange Fellowship Program (number Y9180822S1), CAS President’s International Fellowship Initiative (PIFI) number 2020PC0009 and young staff under the grant number: 2020FYC0002, China Postdoctoral Science Foundation, National Science Foundation of China (NSFC) under the project codes 31850410488 and 31851110759, the Yunnan Human Resources, and Security Department Foundation and Thailand Research Funds for the grant “Impact of climate change on fungal diversity and biogeography in the Greater Mekong Subregion (RDG6130001)” are thanked for funding. This research work was partially supported by Chiang Mai University.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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