Mining Herbaria For Clues To the Historic Prevalence of Lily Leaf Spot Disease (Pseudocercosporella inconspicua) On Gray's Lily (Lilium grayi) and Canada Lily (L. canadense)

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Lily leaf spot disease, caused by the fungal phytopathogen, Pseudocercosporella inconspicua, infects Lilium canadense and L. grayi. The disease is currently ubiquitous in populations throughout the range of L. grayi. To determine the historical prevalence of the disease, lily specimens from eight herbaria were examined visually and microscopically, and a search for records of the pathogen was conducted using mycology databases and relevant literature. Of 516 herbarium specimens, two L. canadense and one L. grayi had the characteristic leaf lesions that contained diagnostic conidia of P. inconspicua. All three diseased specimens were collected prior to 1950. Mycological collections included two North American records of P. inconspicua on L. canadense, two on L. michiganense, one on L. philadelphicum var. andinum, and one on a cultivated Eurasian lily hybrid. Interestingly, the earliest diseased herbarium specimens were from the northeastern US with a later appearance in the southern Appalachians, a pattern also present in mycological collections of P. inconspicua. The rarity of historical specimens with disease, the temporal geographic pattern of occurrence, and the ubiquity of P. inconspicua in current populations of L. grayi suggest the spread of lily leaf spot disease in North America may threaten the viability of native Lilium host species.