Lily leaf spot, a fungal foliar disease caused by Pseudocercosporella inconspicua, leads to premature senescence of aboveground tissues in Lilium grayi. At Roan Mountain, North Carolina/Tennessee, the disease was most prevalent and most severe in seedlings and juveniles. In the two growing seasons assessed, 59 and 70% of mature plants experienced disease-induced premature senescence. Plants with disease lesions on seed capsules matured fewer capsules and had fewer seeds per capsule, and seeds had reduced viability. Disease prevalence over the growing season followed a sigmoidal pattern typical of polycyclic epidemics. Plants with low and high disease severity occurred in clusters whose locations were stable across growing seasons. Prior to the recent description of lily leaf spot, L. grayi was already considered threatened or endangered in each of the three states where it naturally occurs. Therefore, this infectious disease poses conservation and management difficulties because increases in plant density can be expected to lead to enhanced disease transmission. Lily leaf spot of L. grayi is best characterized as an annually recurring epidemic because of high prevalence rates, strong impacts on all life stages, and reductions in seed production and viability.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

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.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Le criocère du lis, Lilioceris lilii (Coleoptera : Chrysomelidae), un ravageur de lis et de fritillaires d’origine eurasienne, a été observé pour la première fois en Amérique du Nord en 1943 sur l’Ile de Montréal au Canada. Après y avoir été confiné pendant environ 25 années, ce coléoptère a par la suite progressé rapidement sur le territoire nord-américain. Actuellement, on l’observe dans huit provinces canadiennes et huit états américains. Cette étude a investigué les routes d’invasion utilisées par le criocère du lis au Canada et aux États- Unis avec l’aide de marqueurs génétiques AFLP. Pour ce faire, 516 individus parmi 34 sites en Amérique du Nord et en Europe ont été échantillonnés et analysés. Le premier objectif était de déterminer, en analysant la structure génétique des populations nord-américains, s’il y avait eu une ou plusieurs introductions en provenance d’Europe. Le deuxième objectif était d’identifier l’origine de la ou des populations introduites en Amérique du Nord. Finalement, le troisième objectif consistait à proposer un scénario d’invasion de L. lilii en Amérique du Nord basé sur les données de première mention et de structure génétique des populations échantillonnées. Les résultats démontrent une signature génétique distincte entre les criocères du lis du Canada et ceux des États-Unis, suggérant ainsi deux sources d’introductions indépendantes en Amérique du Nord, soit une première introduction à Montréal, Québec, dans les années 1940 et une seconde aux États-Unis au début des années 1990 à Cambridge, Massachusetts. De plus, les deux populations nord-américaines semblent provenir de différentes régions du nord de l’Europe, ce qui est conséquent avec le scénario suggérant deux sources d’introductions indépendantes. Chacune des populations aurait par la suite progressé respectivement dans leur pays d’introduction selon une dispersion de type stratifiée. En effet, la progression continue de L. lilii dans certaines régions suggère une dispersion naturelle de l’espèce sur le territoire nord-américain, alors que la progression rapide sur de longues distances semble être causée par le transport anthropique de lis contaminés. / The lily leaf beetle, Lilioceris lilii (Coleoptera : Chrysomelidae), a Eurasian pest of lilies and fritillaries, was first observed in North America in 1943 on the Island of Montréal, Canada. After being confined to Montréal for approximately 25 years, the beetle quickly progressed in North America, and is currently present in eight Canadian provinces and eight American states. During this study, we have investigated the routes of invasion followed by L. lilii in North America, using AFLP markers. We sampled and analysed 516 individuals from 34 sites across North America and Europe. Our first objective was to characterize the genetic structure of North American L. lilii populations to determine if they originated from a single or form multiple introductions from Europe. The second objective was to identify the geographical origin of the invasive population(s). Finally, the third objective was to trace back the geographical routes of invasion of L. lilii in North America, using information from both the dates of first observations and the genetic structure of sampled populations. Our results showed clear genetic difference between individuals from Canada and the USA, suggesting at least two different sources of introductions of L. lilii in North America. A first episode of introduction took place in Montréal, Canada in the 1940’s while a second introduction occurred in Cambridge, Massachusetts, around 1990. Also, both North American populations seem to originate from different populations in northern Europe, which support the hypothesis of two distinct sources of introduction. Each population next progressed in its respective country following a stratified dispersal. Indeed, continuous progression of the beetle in some regions of North America suggests a pattern of natural dispersion, while human-mediated carrying of infected lilies seemed to be responsible for the long-range movement of the beetle.