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Blumeria graminis is a fungus that causes powdery mildew on grasses, including cereals (called barley powdery mildew or corn mildew). It is the only species in the genus Blumeria. It has also been called Erysiphe graminis and (by its anamorph) Oidium monilioides or Oidium tritici.
Previously B. graminis was included within the genus Erysiphe, but molecular studies have placed it into a clade of its own. Thus since 1975, the species graminis was moved into the new taxa Blumeria of which is the only species. Blumeria differs from Erysiphe in its digitate haustoria and in details of the conidial wall. As well Blumeria is considered to be phylogenetically distinct from Erisiphe as it is a plant pathogen that hosts solely on the true grasses of Poaceae.
Eight special forms or formae speciales (ff.spp.) of B. graminis have been distinguished, each of which is parasitic on a particular genus of grasses. Those that infect crop plants are B. graminis f.sp. tritici, which causes powdery mildew of wheat and infects other grasses in the genera Triticum and Aegilops, f.sp. hordei on barley, f.sp. avenae on oats and f.sp. secalis on rye. Other formae speciales are pathogenic on wild grasses, including agropyri on grasses in the genera Agropyron and Elymus, bromi on Bromus spp., poae on Poa spp. and lolii on Lolium spp. (ryegrass).
The mycelium can cover the plant surface almost completely, especially the upper sides of leaves. Ascocarp is dark brown, globose with filamentous appendages, asci oblong. Ascospores hyaline, ellipsoid, 20-30 x 10-13 µm in size. Anamorph produces on hyaline conidiophores catenate conidia of oblong to cylindrical shape, not including fibrosin bodies, 32-44 x 12-15 µm in size. Haustoria are palmate.
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Blumeria graminis disperses by scattering conidia and ascospores. It is biotrophic, and does not grow on synthetic media. Relatively cool and humid conditions are favourable for its growth. Its relatively great genetic variability enables it often to infect previously resistant plant varieties.
The genome of Blumeria graminis f. sp. hordei has recently been sequenced. , as well as the genome of Blumeria graminis f. sp. tritici  Sequencing of the genome of the wheat powdery mildew Blumeria graminis f. sp. tritici, has allowed to infer important aspects of its evolution. It has been seen that it is the most repetitive fungal genome sequenced with 90% transposable elements. Additionally, 6540 genes were annotated, from which 437 encoded candidate secretor proteins and 165 for non-secreted candidate secretor proteins. These were shown to be subject to positive selection, due to their implication in the gene-for-gene relationship to defeat plant disease resistance. The ability to infect tetraploid as well as domesticated hexaploid wheat, was seen to be the result of mildew genomes being mosaics of ancient haplogroups that existed before wheat domestication. This has allowed wheat powdery mildew to maintain genetic flexibility, variability and thus a great potential for pathogen variation. It is hypothesized that this mosacisism can be maintained through clonal reproduction in population with small effective size or quasi-clonal reproduction in populations with large effective size.