Hidden treasures of herbaria - even small collections contain a wealth of diversity: the powdery mildews of the North Carolina State Larry F. Grand Mycological Herbarium

The occurrence of cryptic species is well documented in fungi but the extent of their diversity is not fully understood. This study assessed the fungal diversity within a part of the Larry F. Grand Mycological Herbarium (NCSLG), a small, well-maintained collection at North Carolina State University, with a focus on the powdery mildew fungi (Erysiphaceae). Erysiphaceae were selected due to their economic impact as plant pathogens and availability of extensive DNA sequence data for multiple barcode loci.

Our research objectives included determining the number of phylogenetic species compared with those identified morphologically, and to identify undescribed species. We generated sequence data for 220 of the 299 powdery mildew specimens (73 % success rate) in the herbarium, which represented 60 species in 10 genera, collected from 134 host plant species. Our analyses revealed that ~ 83 % (183 / 220) of the sequenced specimens had identifications that were incorrect and / or outdated based on current genus / species concepts.

Additionally, four new species are described: Erysiphe amphicarpaeicola, E. ulmi-alatae, E. quercus-virginianae, and Takamatsuella grandii. A specimen deposited at NCSLG is designated as an epitype for Phyllactinia liriodendri, and a species of Phyllactinia identified on Carpinus caroliniana, as well as multiple species infecting Quercus spp., likely represent additional undescribed species that require more data. This research highlights the critical role of herbarium collections in uncovering fungal biodiversity, and underscores the importance of preserving these valuable resources, particularly with the growing trend to discard herbaria due to financial and space constraints.