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.

Lily leaf spot is an emerging disease of the globally rare Lilium grayi S. Wats., a species endemic to the southern Appalachian Blue Ridge, USA. The species is considered Threatened or Endangered in the three states where it naturally occurs (North Carolina, Tennessee, Virginia). Disease leads to premature senescence of aboveground tissues and curtailment of sexual reproduction. Spore morphology, completion of Koch’s postulates, and DNA sequence data showed the causative agent of lily leaf spot to be Pseudocercosporella inconspicua (G. Winter) U. Braun, a Lilium-specific basidiomycete. Diagnostic disease symptomatology includes amphigenous necrotic lesions with tan to green margins encircling a white to grey powdery mass of conidia. Studies conducted in the field at Roan Mountain, NC/TN, in the largest known population, showed that a visual disease diagnosis based on morphology can be highly accurate in predicting P. inconspicua infection, and that high concentrations of conidia of P. inconspicua are strongly associated with infected L. grayi, but are largely absent or few on uninfected L. grayi and other species. Field inoculation trials using infected L. grayi leaf tissue as inocula resulted in transmission of disease and induction of premature senescence under natural conditions.

A population of the rare Southern Appalachian endemic species Lilium grayi, (Gray’s lily) Roan Mountain, TN/NC was monitored for 2 years to determine the cause and impact of an early season collapse. High concentrations of the Lilium spp. host-specific fungal phytopathogen, Pseudocercosporella inconspicua (G. Winter) U. Braun were associated with 19/20 symptomatic and 0/30 asymptomatic plants. Strength of the association between pathogen and disease and the replication of disease symptoms in 4/4 healthy hosts showed that P. inconspicua was the causal agent of the disease referred to as lily leaf spot. Disease had a severe impact on the population with 59% of mature and 98% of adolescent plants undergoing early senescence. Only 32% of mature plants produced capsules and they were frequently diseased. A recurring spatiotemporal pattern typical of an infectious disease suggested that the lily leaf spot disease is capable of causing sequential annual epidemics of unknown long-term consequences to the stability of the host population.