Hidden treasures—historical specimens from the late blight pandemic discovered in the Herbarium of the State Museum of Natural History Karlsruhe

Phytophthora infestans (Peronosporaceae, Oomycota) is the causal agent of late blight of potato (Solanum tuberosum) and a native to Central America. When introduced to Europe, it rapidly spread in 1845, triggering the Irish Potato Famine, which claimed millions of lives and led to an exodus of Europeans to North America. The spread of the species was recently traced using historical specimens from various herbaria. However, there are critical spatial and temporal gaps in the documentation of the early spread of the species. Within the framework of a digitalization and restoration project of the mid-nineteenth century fungus collections of the herbarium of the State Museum of Natural History Karlsruhe, several specimens of Phytophthora infestans from North-East Germany collected in 1853, 1855 and 1856 were discovered. In addition, we revised already deposited material and identified a specimen of Ph. infestans that was collected no later than 1852. These specimens are among the oldest from Central Europe and are now available to the scientific public. Further, we searched for thus far overlooked specimens, using online catalogues. We found specimens from 23 European countries, with the oldest material from western Europe and almost no data from eastern Europe, south-eastern Europe and southern Europe. Our results emphasize the need for archiving and digitizing natural history collections in order to document the historical spread of agricultural and forest pathogens and to better understand current-day epidemic spreads.