Ophiognomonia clavigignenti-juglandacearum endangers the survival of butternut (Juglans cinerea) throughout its native range. While screening for disease resistance, we found that artificial inoculations of 48 butternut seedlings with O. clavigignenti-juglandacearum induced the expression of external symptoms, but only after a period of dormancy. Before dormancy, compartmentalized tissues such as necrophylactic periderms (NPs) and xylem reaction zones (RZs) contributed to limiting pathogen invasion. Phenols were regularly detected in RZs, often in continuity with NPs during wound closure, and confocal microscopy revealed their presence in parenchyma cells, vessel plugs and cell walls. Vessels were blocked with tyloses and gels, particularly those present in RZs. Suberin was also detected in cells formed over the affected xylem by the callus at the inoculation point, in a few tylosis walls, and in longitudinal tubes that formed near NPs. Following dormancy, in all inoculated seedlings but one, defensive barriers were breached by O. clavigignenti-juglandacearum and then additional ones were produced in response to this new invasion. The results of this histopathological study indicate that trees inoculated in selection programs to test butternut canker resistance should go through at least one period of dormancy and that asymptomatic individuals should be dissected to better assess how they defend themselves against O. clavigignenti-juglandacearum.

Summary The use of a molecular assay for quantifying conidia of Ophiognomonia clavigignenti‐juglandacearum, the fungal pathogen responsible of butternut canker, was investigated. Purified DNA from conidia collected on glass fibre filters of a passive rain collectors was quantified using a TaqMan real‐time quantitative polymerase chain reaction (qPCR) assay. The qPCR assay could specifically discriminate the target species from all other North American known species of Ophiognomonia, and it was sensitive enough to repeatedly detect one conidium. A linear relationship between numbers of conidia and qPCR Ct values was determined, and used to assess the sporulation of the pathogen under trees that were released to promote their vigour. In total, 977 samples of field‐captured conidia from 49 trees, at two locations, and from two successive growing seasons were analysed. No significant difference of sporulation was observed under control and release treatments. However, our results demonstrated that qPCR assay was reliable for detecting and quantifying O. clavigignenti‐juglandacearum from environmental samples, which will be useful to assess further control methods for this disease.

Le noyer cendré (Juglans cinerea) est considéré depuis 2005 en voie de disparition au Canada; un chancre, causé par le champignon Ophiognomonia clavigignenti-juglandacearum, est la principale cause du statut précaire de l’espèce. Cette maladie, qui semble avoir été introduite en Amérique du Nord, a été initialement rapportée en 1967 au Wisconsin. Au Canada, c’est au Québec qu’on l’observe en premier en 1990. Les symptômes se présentent sous forme de mort en cime et, très souvent, de chancres au tronc qui finissent presque toujours par tuer les arbres de tous âges. Nous décrivons entre autres certains de nos travaux qui recherchaient une meilleure compréhension de cette maladie, mais surtout qui visaient à repérer des arbres potentiellement résistants au chancre, à les multiplier et à évaluer leur résistance, et finalement à les protéger en stimulant leur vigueur par des travaux de dégagement. Nous avons pu montrer, par exemple, qu’à la suite d’inoculations artificielles en serre, quelques noyers cendrés montraient une certaine résistance au chancre, et qu’ils se défendaient en compartimentant les tissus envahis par l’agent pathogène. Ces individus potentiellement résistants constitueraient ainsi une source unique de matériel pour rétablir l’espèce sur certains sites.