Pyricularia graminis-tritici, a new Pyricularia species causing wheat blast

Abstract

Pyricularia oryzae is a species complex that causes blast disease on more than 50 species of poaceous plants. Pyricularia oryzae has a worldwide distribution as a rice pathogen and in the last 30 years emerged as an important wheat pathogen in southern Brazil. We conducted phylogenetic analyses using 10 housekeeping loci for 128 isolates of P. oryzae sampled from sympatric populations of wheat, rice, and grasses growing in or near wheat fields. Phylogenetic analyses grouped the isolates into three major clades. Clade 1 comprised isolates associated only with rice and corresponds to the previously described rice blast pathogen P. oryzae pathotype Oryza (PoO). Clade 2 comprised isolates associated almost exclusively with wheat and corresponds to the previously described wheat blast pathogen P. oryzae pathotype Triticum (PoT). Clade 3 contained isolates obtained from wheat as well as other Poaceae hosts. We found that Clade 3 is distinct from P. oryzae and represents a new species, Pyricularia graminis-tritici (Pgt). No morphological differences were observed among these species, but a distinctive pathogenicity spectrum was observed. Pgt and PoT were pathogenic and highly aggressive on Triticum aestivum (wheat), Hordeum vulgare (barley), Urochloa brizantha (signal grass), and Avena sativa (oats). PoO was highly virulent on the original rice host (Oryza sativa), and also on wheat, barley, and oats, but not on signal grass. We conclude that blast disease on wheat and its associated Poaceae hosts in Brazil is caused by multiple Pyricularia species. Pyricularia graminis-tritici was recently found causing wheat blast in Bangladesh. This indicates that P. graminis-tritici represents a serious threat to wheat cultivation globally.

Keywords: Triticum aestivum; cryptic species; host adaptation; phylogenetics; systematics.

Fig. 1

Phylogeny inferred by Bayesian Inference from the combined sequences of 10 partial loci (actin, BAC6, β-tubulin, calmodulin, CH7-BAC7, CH7-BAC9, chitin synthase 1, translation elongation factor 1-α, MPG1 hydrophobin, and nitrogen regulatory protein 1) from isolates of Pyricularia spp. The 50 % majority-rule consensus tree is shown. The numbers above the branches are the Bayesian posterior probabilities (BPP) for node support with BPP > 0.95. Pyricularia grisea and P. pennisetigena were used as outgroups. The original host of the isolate can be distinguished by the colour of the isolate number: black = wheat; green = other poaceous hosts; and orange = rice. The asterisk (*) indicates the isolates listed in the Taxonomy section as specimens examined.

Fig. 2

Phylogeny inferred by Bayesian Inference from the sequences of the MPG1 hydrophobin locus from isolates of Pyricularia spp. The 50 % majority-rule consensus tree is shown. The numbers above the branches are the Bayesian posterior probabilities (BPP) for node support with BPP > 0.95. Pyricularia grisea and P. pennisetigena were used as outgroups. The original host of the isolate can be distinguished by the colour of the isolate number: black = wheat; green = other poaceous hosts; and orange = rice. The asterisk (*) indicates the isolates listed in the Taxonomy section as specimens examined.

Fig. 3

Boxplot distribution of leaf blast severity of seedlings of five poaceous hosts in response to inoculations with isolates of P. graminis-tritici (Pgt, N = 7), P. oryzae pathotype Triticum (PoT, N = 7), and P. oryzae pathotype Oryza (PoO, N = 4). Boxplots represent blast severity as mean disease index assessed 7 d after inoculation using an ordinal scale from 0 to 5, and based on lesion type (Urashima et al. 2005). Disease index means with the same letter are not significantly different according to Dunn’s All Pairs for Joint Ranks non-parametric test (P > χ² ≤ 0.05). a. Inoculation tests on seedlings of wheat (Triticum aestivum); b. barley (Hordeum vulgare) cv. BRS Korbell; c. signal grass (Urochloa brizantha, ex Brachiaria brizanta) cv. Marandú; d. signal grass cv. Piatã; e. oats (Avena sativa) cv. EMBRAPA 29; f. oats cv. IAPAR 61; g. rice (Oryza sativa) cv. IRGA 409.

Fig. 4

Boxplot distribution of blast severity observed on heads of wheat (Triticum aestivum) cv. Anahuac after inoculations with isolates of P. graminis-tritici (Pgt, N = 7), P. oryzae pathotype Triticum (PoT, N = 7), and P. oryzae pathotype Oryza (PoO, N?=?4). Heads were not detached from the plant. Boxplots represent = 4). Heads were not detached from the plant. Boxplots represent blast severity as mean disease index assessed 7 d after inoculation as percentage wheat head affected by blast using Assess v. 2.0 Image Analysis software. Head tissue was considered diseased when it was chlorotic and/or covered in pathogen spores. The test was conducted twice, and replicates (experiment 1 and 2) were analysed independently (a, b). Disease index means with the same letter are not significantly different according to Fisher’s protected Least Significant Difference test at P ≤ 0.05.

Fig. 5

Blast symptoms on leaves and heads of poaceous host after inoculation with Pyricularia species. Inoculated hosts: a and f. wheat (Triticum aestivum); b. barley (Hordeum vulgare); c. signal grass (Urochloa brizantha, ex Brachiaria brizantha); d. oats (Avena sativa); e. rice (Oryza sativa). Pyricularia species: Pyricularia graminis-tritici (Pgt), P. oryzae pathotype Triticum (PoT), and P. oryzae pathotype Oryza (PoO). Control plants (Ctr) were inoculated with sterile deionized water amended with Tween 80 (2 drops/L). Plants were assessed for disease symptoms 7 d after inoculation.

Fig. 6

Pyricularia graminis-tritici. a–j. Cultures of isolate 12.1.037 grown for 7 d at 12 h photoperiod and 25 °C in CMA (a, f), MEA (b, g), OA (c, h), PDA (d, i), and SNA (e, j) media; k–l. sporulation on SNA on sterile barley seeds; m–o. scanning electron micrographs of conidiophores and conidia; p–x. bright field microscopy images of conidiophores and conidia. — Scale bars = 10 μm.

Fig. 7

Pyricularia oryzae pathotype Triticum. a–j. Cultures of isolate 12.1.291 grown for 7 d at 12 h photoperiod and 25 °C in CMA (a, f), MEA (b, g), OA (c, h), PDA (d, i), and SNA (e, j) media; k–l. sporulation on SNA on sterile barley seeds; m–o. scanning electron micrographs of conidiophores and conidia; p–v. bright field microscopy images of conidiophores and conidia. — Scale bars = 10 μm.

Fig. 8

Pyricularia oryzae pathotype Oryza. a–j. Cultures of isolate 10880 grown for 7 d at 12 h photoperiod and 25 °C in CMA (a, f), MEA (b, g), OA (c, h), PDA (d, i), and SNA (e, j) media; k–l. sporulation on SNA on sterile barley seeds; m–o. scanning electron micrographs of conidiophores and conidia; p–t. bright field microscopy images of conidiophores and conidia. — Scale bars = 10 μm.