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.
Highlights
Powdery mildew, caused by the obligate biotrophic fungus Blumeria graminis, is a major problem in cereal production as it can reduce quality and yield
Among the 70 isolates used in this study, 63 virulence profiles were identified on the wheat lines with known powdery mildew resistance genes
The present study pursued a combined pathological andgenetic approach to explain the evolutionary processes that led to the host range expansion of powdery mildew from wheat to triticale
Summary
Powdery mildew, caused by the obligate biotrophic fungus Blumeria graminis, is a major problem in cereal production as it can reduce quality and yield. 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. B. graminis is an obligate biotrophic fungus which implies that it depends on living plant cells for survival and reproduction. Penetration and biotrophic colonization of a plant is the result of highly specific recognition mechanisms in the gene-for-gene response involving avirulence (AVR)- and resistance genes [5]. Two AVR genes have been isolated from B. graminis and both have effector activity [8,9]. The fungus has a huge repertoire of homologues of these genes, which may function as effectors and contribute to parasite aggressiveness [10]
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