DNA-barcoding the Powdery Mildews – sampling herbarium specimens in the National Mycological Herbarium (DAOM).

The obligate plant pathogens known as Powdery Mildews (Erysiphales) comprise a morphologically diverse group of fungi producing conspicuous symptoms of white mycelium covering mainly leaves. These fungi infect a wide range of plants, causing premature leaf fall, hypertrophy and deformation, and consequently significant economic losses in agricultural, horticultural and viticulture industries. Since the description of the first powdery mildew by Linnaeus in 1753, a comprehensive system for species identification gradually developed based on various biological characters and more recently, incorporating molecular characters. Although using DNA sequences is an efficient and effective approach for diagnoses by regulatory departments and pest diagnostic agencies, considerable data gaps still exist for DNA-based identifications and species complexes remain unresolved. In a metagenomics study of environmental samples, only 3% OTUs matched to the genera Podosphaera and Erysiphe could be identified to species. To fill gaps in reference sequences, 305 historical specimens of 149 species in 15 genera from DAOM were sampled to amplify ITS region for the pathogens and rbcLa for the hosts, using published primers from various sources. The preliminary data set generated included 105 ITS sequences of 35 species and 63 partial rbcLa of 15 species. Based on analyses supplemented with data from GenBank, the identifications of 82 DAOM specimens were revised in light of the new phylogeny-based classifications. No primer set was found to be universally specific for all species sampled and some also amplified non-target fungal contaminants co-occurring on the specimens. Primer design and optimization is a priority for this fungal group.