Genetic Constitution and Pathogenicity of Lolium Isolates of Magnaporthe oryzae in Comparison with Host Species-Specific Pathotypes of the Blast Fungus

Abstract

Fungal isolates from gray leaf spot on perennial ryegrass (prg isolates) were characterized by DNA analyses, mating tests, and pathogenicity assays. All of the prg isolates were interfertile with Triticum isolates and clustered into the crop isolate group (CC group) on a dendrogram constructed from rDNA-internal transcribed spacer 2 sequences. Since the CC group corresponded to a newly proposed species, Magnaporthe oryzae, all of the prg isolates were designated M. oryzae. However, DNA fingerprinting with MGR586, MGR583, and Pot2 showed that the prg isolates are divided into two distinct populations, i.e., TALF isolates and WK isolates. The TALF isolates were virulent only on Lolium species, whereas the WK isolates were less specific, suggesting that gray leaf spot can be caused not only by Lolium-specific isolates but also by less specific isolates. We designated the TALF isolates as Lolium pathotype. The TALF isolates showed diverse karyotypes in spite of being uniform in DNA fingerprints, suggesting that theyare unstable in genome organization.

Wheat blast disease: danger on the move

“…Rossman et al (1990) merged P. oryzae and P. grisea, grouping all host specific forms under the name P. grisea. Kato and colleagues (Kato et al 2000;Tosa et al 2004) examined pathogenicity, mating compatibility, and RFLPs of Pyricularia isolates from various hosts, and found that isolates from Oryza, Setaria, Panicum, Eleusine, Triticum and Lolium species formed a genetically close, interfertile group (the CC crop isolate group) that was distinct from the crabgrass isolates originally designated P. grisea. They suggested the CC group should become P. oryzae.…”

“…In general, M. oryzae isolates collected in nature are considered as specialized for particular host species, but some isolates do appear to cross-infect other host species (Tosa et al 2004). A confusing picture of host species specificity has emerged in the literature in part because different studies using artificial inoculations sometimes report conflicting results (Urashima et al 1993;Viji et al 2001).…”

“…In addition to the recent emergence of wheat blast, gray leaf spot (GLS), a serious disease on turf grasses used in the golf course industry, emerged in 1991 in Pennsylvania and subsequently spread across the U.S. Magnaporthe oryzae isolates causing GLS on perennial ryegrass (Lolium perenne), annual ryegrass (Lolium multiflorum) and tall fescue (Festuca arundinacea) constitute the distinct Lolium pathotype (MoL) (Viji et al 2001;Farman 2002;Uddin et al 2003b;Tosa et al 2004). This disease also occurs in Japan, presumably introduced through movement of perennial ryegrass seed from the U.S. (Tosa et al 2004;Tosa et al 2016).…”

“…This disease also occurs in Japan, presumably introduced through movement of perennial ryegrass seed from the U.S. (Tosa et al 2004;Tosa et al 2016). In the U.S. in 2011, M. oryzae was isolated from a single diseased wheat spike in a University of Kentucky wheat trial plot in Princeton, Kentucky.…”

“…Subsequent scouting failed to identify a second occurrence on wheat in the U.S. However, this fortuitous finding highlighted the close relationship between the MoT and MoL populations (Viji et al 2001;Farman 2002;Tosa et al 2004;Farman et al 2017), and led to the understanding that native MoL isolates also pose a risk to U.S. wheat production. That is, a subset of U.S. MoL isolates already infect wheat, although they are less aggressive on wheat than MoT strains from the 1980's in Brazil (Viji et al 2001;Farman et al 2017).…”