First Extensive Microscopic Study of Butternut Defense Mechanisms Following Inoculation with the Canker Pathogen Ophiognomonia clavigignenti-juglandacearum Reveals Compartmentalization of Tissue Damage.

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.

Factors Influencing the Regional Dynamics of Butternut Canker.

Butternut (Juglans cinerea) is an important component of native biodiversity in eastern North America. Of urgent concern is the survival of butternut, whose populations are declining rapidly, in large part due to an exotic pathogen, Ophiognomonia clavigignenti-juglandacearum, that causes butternut canker. The disease presently occurs throughout the range of butternut in North America, causing branch and stem cankers, dieback, and tree mortality. Despite the existential threat posed by O. clavigignenti-juglandacearum to butternut, a detailed understanding of the factors that drive cross-scale disease patterns is lacking. Therefore, we investigated the association of a range of factors, including tree attributes, topography, and weather, with butternut canker spatial dynamics at different scales using data collected in the province of Quebec, Canada. Trunk canker damage and dieback showed distinct geographic patterns. Bark phenotype was not significantly associated with trunk canker damage. Results suggest that open or dominant trees may show less dieback than intermediate or suppressed trees. Probability of the presence of trunk canker and percent dieback were proportional to the tree diameter at breast height. Temperature was positively associated with disease severity at a 1-km2 scale. Our results provide strong evidence that multiple factors, notably weather, influence butternut canker epidemiology.