Ecological Status of Juglans cinerea in New Brunswick

Butternut (Juglans cinerea L.), an early successional riparian hardwood species native to Canada and the United States, is under serious threat from a nonnative fungal pathogen, Ophiognomonia clavigignenti-juglandacearum. Since it was first reported in Canada in 1990, this fungal pathogen has spread rapidly and established in New Brunswick in 1997. Apart from the first report in 1997 and another in 2004, no surveys have been conducted to assess the spread of the pathogen in the province. The purpose of this research was to survey butternut throughout its range in New Brunswick, evaluate disease occurrence as well as tree health, and investigate the impact of different topographic and tree health factors on canker incidence. Results showed that the disease has spread throughout the range of butternut in New Brunswick. The disease likely only recently (2007) spread to the northeastern-most populations, given that lower rates of canker occurrence and higher health ratings are found further away from the point of initial occurrence of the disease. Although canker incidence is high throughout the province, tree dieback is minimal, and trees still producing nuts could support opportunities for ex situ conservation. Because of the rate of pathogen spread in the province, implementing a tree improvement strategy might be the only means for maintaining the butternut genome on the landscape.

Assessing the Efficacy of Seedling Planting as a Forest Restoration Technique in Temperate Hardwood Forests Impacted by Invasive Species

The emerald ash borer (Agrilus planipennis Fairmaire; EAB) is an invasive insect that causes mortality of trees in the genus Fraxinus, creating canopy gaps that may facilitate invasion by exotic plants. Planting native tree seedlings under EAB-infested Fraxinus may accelerate succession and preclude invasive plant expansion; however, the effectiveness of this approach has not been experimentally tested. We assessed understory seedling planting of Quercus rubra, Carya laciniosa, and Juglans cinerea in EAB-infested forests, where the invasive shrub Lonicera maackii (Amur honeysuckle) was removed. We tested whether the use of plastic tree shelters (“tree tubes”) or planting season (fall versus spring) contributed to the success of the reforestation plan by measuring growth rates (cm/yr) and survivorship two and seven years after planting. After seven years, seedling survivorship was <25% for all species and planting techniques. Quercus rubra exhibited poor survivorship with one seedling surviving to the conclusion of the experiment. Juglans cinerea and C. laciniosa had higher survivability and growth rates than did Q. rubra after two and seven years. Effects of tree tubes were weak and temporary. After 2 years, Q. rubra seedling survivorship was higher in tree tubes; however, by the end of the experiment 29 of the 30 Q. rubra seedlings in tree tubes had died. Juglans cinerea seedlings grew faster when planted in the fall compared to the spring, but overall survivorship of these seedlings was unaffected by planting season. Neither the use of tree shelters nor the planting season contributed to the growth or survival of C. laciniosa seedlings. In summary, our results indicate that seedling planting of Carya and Juglans may be a useful way to increase biodiversity in regenerating forests; however, the resource-expensive processes of over-wintering seedlings and using tree shelters may not increase the success of reforestation efforts.

L’histoire d’un chancre menaçant d’exterminer le noyer cendré

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.

First Extensive Microscopic Study of Butternut Defense Mechanisms Following Inoculation with the Canker PathogenOphiognomonia clavigignenti-juglandacearumReveals 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.