Phylogeography and virulence structure of the powdery mildew population on its 'new' host triticale

Abstract

BackgroundPowdery mildew, caused by the obligate biotrophic fungus Blumeria graminis, is a major problem in cereal production as it can reduce quality and yield. B. graminis has evolved eight distinct formae speciales (f.sp.) which display strict host specialization. In the last decade, powdery mildew has emerged on triticale, the artificial intergeneric hybrid between wheat and rye. This emergence is probably triggered by a host range expansion of the wheat powdery mildew B. graminis f.sp. tritici. To gain more precise information about the evolutionary processes that led to this host range expansion, we pursued a combined pathological and genetic approach.ResultsB. graminis isolates were sampled from triticale, wheat and rye from different breeding regions in Europe. Pathogenicity tests showed that isolates collected from triticale are highly pathogenic on most of the tested triticale cultivars. Moreover, these isolates were also able to infect several wheat cultivars (their previous hosts), although a lower aggressiveness was observed compared to isolates collected from wheat. Phylogenetic analysis of nuclear gene regions identified two statistically significant clades, which to a certain extent correlated with pathogenicity. No differences in virulence profiles were found among the sampled regions, but the distribution of genetic variation demonstrated to be geography dependent. A multilocus haplotype network showed that haplotypes pathogenic on triticale are distributed at different sites in the network, but always clustered at or near the tips of the network.ConclusionsThis study reveals a genetic structure in B. graminis with population differentiation according to geography and host specificity. In addition, evidence is brought forward demonstrating that the host range expansion of wheat isolates to the new host triticale occurred recently and multiple times at different locations in Europe.

Virulence Structure of the Powdery Mildew (Blumeria graminis) Population Occurring on Triticale (x Triticosecale) in Poland

“…Troch et al. () came to similar conclusions. In the cited studies the researchers examined 70 B. graminis f. sp.…”

“…As well as if isolates were pathogenic on triticale, their virulence profiles on triticale cultivars did not differ significantly among the different sampled regions (Belgium, France and Poland; Troch et al. ). Probably the triticale cultivars from different European breeding companies have the same very narrow genetic background and carry the same R genes for powdery mildew resistance (Walker et al.…”

“…Troch et al. () tested 70 isolates on wheat lines with known powdery mildew resistance genes and identified 63 virulence profiles.. However, among 40 isolates pathogenic on triticale, only 14 displayed unique virulence profiles on the triticale cultivars.…”

“…; Troch et al. ). The colonization of a plant is the result of highly specific recognition mechanisms in the gene‐for‐gene response involving avirulence and resistance genes (Flor , ).…”

Hybridization of powdery mildew strains gives rise to pathogens on novel agricultural crop species

“…This contradicts the current hypothesis that the host range of B.g. tritici expanded to include triticale through mutation of a few genes 2,6,7 . Instead, our analysis shows that B.g.…”