In 2002, the causal pathogen of GLS of grass species including ryegrasses (Lolium species) and rice blast was identified as a new species, M. oryzae (anamorph P. oryzae). This new species was considered distinct from Magnaporthe grisea (anamorph P. grisea), which is associated with the grass genus Digitaria. The distinction was based on phylogenetic analyses and laboratory mating experiments that showed the two species were not interfertile, although there were no morphological differences between them .
In this chapter, the term “M. oryzae” is used. However, it is important to note that a formal change from M. grisea to M. oryzae has not yet occurred. A proposal for changing the name based on the results of  is allowed under the International Code of Nomenclature for algae, fungi, and plants (Melbourne Code). A proposal will be submitted to and discussed by the Nomenclature Committee for Fungi of the International Association for Plant Taxonomy . A final decision on a name change will be made during the Nomenclature Session of the International Botanical Congress in 2017 .
2.2. Population structure and host specificity
Analysis of genomic DNA using molecular markers is the most powerful method for determining the population structures of the Magnaporthe species. Repetitive DNA elements such as transposons and retrotransposons are often used to generate probes for Southern blotting experiments during DNA fingerprinting [11-15]. This is because of the diversity in copy numbers of elements and the richness of polymorphisms around, within, or among the elements, which might be caused by base substitutions or insertions and deletions. The use of internal transcribed spacer regions between ribosomal DNAs as probes for DNA fingerprinting is also common [12, 13]. Similarly, the internal transcribed spacer regions have been sequenced for population structure analyses . Table 1 lists the repetitive sequences that have been used to analyze the population structure of Magnaporthe species associated with grass weeds, turf grasses, and/or forage grasses in addition to major crops such as rice and wheat (Triticum aestivum) [11-15].
|Sequence||Result of application|
|MGR583||Retrotransposon||[18, 19]||[12, 14]|
|rDNA||Ribosomal DNA||[22, 23]||[12-14]|
Repetitive DNA sequences for DNA fingerprinting of Magnaporthe species associated with grass weeds, turf grasses, and/or forage grasses
In some cases, probes derived from these repetitive DNA sequences cannot clearly distinguish between isolates from different hosts. Restriction fragment length polymorphisms (RFLPs) with single-copy probes derived from long insert-cosmid clones (35–40 kb) are appropriate for the initial comparison of poorly characterized isolates from different hosts . In addition to the repetitive DNA sequences, amplified fragment length polymorphisms (AFLPs) can produce many markers and provide a higher resolution for population structure analyses even within the same Magnaporthe lineage [25, 26].
Population structures can be determined in dendrograms constructed by analyzing genetic distances among isolates, which are reflected by differences in the banding patterns obtained during molecular marker analyses. Dendrograms of ryegrass isolates have often revealed genetic similarities between ryegrass isolates and isolates from wheat [12-14, 25] and tall fescue (Schedonorus arundinaceus) [12, 25].
In artificial inoculation conditions, isolates from ryegrasses, wheat, and tall fescue can cause serious infections in all hosts. Table 2 summarizes the data from six studies on the pathogenicity of Magnaporthe isolates from ryegrasses, tall fescue, wheat, rice, and/or crabgrass [13-15, 25, 27, 28]. The isolates from ryegrasses are generally avirulent, but can be virulent to rice [13, 14]. Conversely, although the rice isolates are thought to be unable to cause serious infections in ryegrasses [13, 14], they are occasionally highly virulent to the plant species . The wheat isolates are avirulent to rice [14, 27], although the rice isolates are virulent to wheat [13, 27]. Some isolates from crabgrass (Digitariasanguinalis) are virulent to tall fescue  and ryegrasses [25, 28], highly virulent to Italian ryegrass  but are avirulent to wheat [14, 25]. Additionally, isolates from perennial ryegrass, wheat, and rice can infect crabgrass, but these are generally not highly virulent to crabgrass . Many isolates from tall fescue are avirulent to crabgrass .
|Original hosta||Inoculated hostb||Reference|
|Perennial ryegrass (PR)||++||++||++||-|||
|Italian ryegrass (IR)|||
|Tall fescue (TF)|||
Pathogenicity and host specificity of Magnaporthe species during artificial inoculations
aAccording to , the crabgrass isolate might be M. grisea and the others might be M. oryzae.
b+: virulent; ++: highly virulent; -: avirulent; +-: virulent but sometimes fails to infect; ++-: highly virulent but sometimes fails to infect.
In addition to the isolates listed in Table 2, during artificial inoculations, ryegrasses are highly susceptible to isolates from weeping lovegrass (Eragrostis curvula) , and susceptible to isolates from finger millet (Eleusine coracana) , St. Augstinegrass (Stenotaphrum secundatum) [25, 28], Alexandergrass (Brachiaria plantaginea) , Pennsylvania smartweed (Polygonum pensylvanicum) , and soybean (Glycine max) .
The cross-infections observed during artificial inoculations suggest that “opportunistic” cross-infections may occur in nature . However, population structure analyses based on molecular marker analyses have revealed that although there are genetic differences even in isolates from the same host species, the population structures are generally associated with host differences. This indicates that the host species is a major selective factor for constructing isolate populations, and cross-infections among hosts might not be detectable in nature . Nevertheless, ryegrasses might be infected by tall fescue isolates because these hosts are congeneric [29-31]. Therefore, the isolates from ryegrasses and tall fescue are genetically quite similar  or belong to the same lineage in some cases . Additionally, wheat isolates are genetically similar to the ryegrass and tall fescue isolates, and all can cause serious infections in wheat, ryegrass, and tall fescue in artificial inoculation conditions (Table 2). However, the wheat isolates are clearly genetically distinct [12, 25]. This might explain why no epidemics of wheat blast caused by the cross-infection of ryegrass isolates and vice versa, have been reported . This may also be the case for weeping lovegrass, in which there are genetic similarities and cross-pathogenicity among hosts . Therefore, isolates from wheat and/or weeping lovegrass may be progenitors of isolates of ryegrasses and tall fescue rather than being directly responsible for GLS in ryegrasses or tall fescue [12, 25].