Erysiphe Aesculi‐sylvaticae sp. nov. Infecting North American Aesculus Species: A New Cryptic Species Provides Insights Into the Invasion Dynamics of an Important Fungal Pathogen

Aesculus is a genus of woody plant species that contains multiple ecologically and ornamentally important species. Powdery mildew is common on this host genus and is particularly virulent on the economically significant horse chestnut, Aesculus hippocastanum . Previously, all Erysiphe species found on Aesculus spp. were identified as Erysiphe flexuosa . Recent genomic research has indicated that powdery mildew species frequently show a high degree of host specificity which was not captured by traditional morphological approaches. As such, we proceeded to collect and sequence multiple regions of the powdery mildew genome to determine the causal agents of disease on different Aesculus spp. The results of our multilocus phylogenetic analyses revealed the existence of a genetically distinct species, which has been found only on Aesculus spp. native to North America. Erysiphe aesculi‐sylvaticae sp. nov. is proposed for this cryptic species. In the past 25 years, there have been multiple reports of an E. flexuosa epidemic on Aesculus hippocastanum in Europe, while E. aesculi‐sylvaticae has yet to be identified outside of the United States. The discovery of this new cryptic powdery mildew species as well as analysis of additional sequences from collections of E. flexuosa from North America, including from the type specimen collected in 1872, are discussed in detail with an emphasis on the implications for the co‐evolution, worldwide spread, and invasion dynamics of these powdery mildews. Future research should continue to collect and sequence powdery mildews on different Aesculus spp. to better understand the diversity and spread of these economically important pathogens.