Competitive Advantage of Geosmithia morbida in Low-Moisture Wood May Explain Historical Outbreaks of Thousand Cankers Disease and Predict the Future Fate of Juglans nigra Within Its Native Range

Bark beetles vector symbiotic fungi and the success of these mutualisms may be limited by competition from other microbes. The outcome of fungal competition is strongly influenced by the physical and chemical conditions of the wood they inhabit. These conditions are in turn subject to climatic variation. In particular, wood moisture content (MC) influences fungal competition and, therefore, could help determine environmental suitability for thousand cankers disease (TCD) caused by Geosmithia morbida and its vector Pityophthorus juglandis. We conducted competition experiments in Juglans nigra wood that was naturally or artificially colonized by G. morbida and other fungi over a range of wood MC expected across prevailing United States climatic conditions. G. morbida outcompeted antagonistic fungi Clonostachys and Trichoderma spp. at <5% equilibrium moisture content. Aspergillus spp. outcompeted G. morbida at low moisture in wood from Indiana. We fit a logistic regression model to results of the competition experiments to predict survival of G. morbida across the United States. Expected survival of G. morbida was highest in historical TCD epicenters and accounted for the low incidence and severity of TCD in the eastern United States. Our results also predict that under future climate scenarios, the area impacted by TCD will expand into the native range of J. nigra. Given its role in emergent forest health threats, climate change should be a key consideration in the assessment of risks to hardwood resources.

Forest and Plantation Soil Microbiomes Differ in Their Capacity to Suppress Feedback Between Geosmithia morbida and Rhizosphere Pathogens of J. nigra Seedlings

Thousand cankers disease (TCD) is hypothesized to have a greater impact on eastern black walnut (Juglans nigra) in urban forests and plantations compared to natural forest stands. Along with other factors, such as resource availability, the phytobiome could partly account for observed differences in disease severity across management regimes. We investigated the extent to which J. nigra-associated soil microbiomes from plantations and natural forests modulate a) the amount of necrosis caused by Geosmithia morbida Kolařík, Freeland, Utley and Tisserat in one-year-old seedlings, and b) relative abundance of rhizosphere endophytes and opportunistic pathogens Fusarium and Rhizoctonia in response to aboveground inoculation with G. morbida. Our results suggest that the microbiome from natural forest soil in central Indiana suppresses Fusarium and is indirectly suppressive of G. morbida. Natural forest soil had a greater ability to reduce the size of necrotic area caused by G. morbida compared to steam-treated soil. Inoculating stems of seedlings with G. morbida induced a shift in fungal community composition in the rhizosphere, including Fusarium and Rhizoctonia, but the direction and magnitude of the shift depended on whether seedlings were amended with forest, plantation, or steam-treated soil. In a companion experiment, necrotic area in G. morbida-inoculated seedlings was twice as high in seedlings grown from seeds that were treated with Fusarium solani relative to those grown from seeds treated with water. Our findings support the hypothesis that TCD severity can be modulated by host-mediated feedback between above- and belowground pathogens, as well as by microbial interactions in the rhizosphere.

Dispersal and colonization risk of the Walnut Twig Beetle, Pityophthorus juglandis, in southern Europe

The Walnut Twig Beetle (WTB), Pityophthorus juglandis Blackman, is a small bark beetle native to Mexico and Southwestern USA recorded for the first time in Europe (NE Italy) in 2013. WTB attacks walnut (Juglans spp.) and wingnut trees (Pterocarya spp.) and is the vector of Geosmithia morbida Kolarík et al., a pathogen causing the thousand cankers disease (TCD). WTB and TCD represent a serious threat for walnut orchards in Europe. Spatiotemporal data of the WTB-TCD infestations recorded from an 8-year-long (2013–2020) monitoring conducted in 106 walnut orchards of NE Italy were used to develop a model in order to analyze: (i) the effective dispersal capacity of WTB, (ii) the factors affecting dispersal and (iii) the colonization risk of healthy walnut orchards. We registered a mean annual dispersal of 9.4 km, with peaks of about 40 km. Pest dispersal is affected by distance of suitable hosts from the nearest infested site, number of walnut orchards in the surroundings (both infested and healthy), orchard size and walnut species in the orchard. Using the model, it was also possible to calculate the colonization risk of a specific walnut orchard according to its characteristics showing, for instance, that a medium-size (5,000 trees) black walnut orchard located at 25 km from the nearest infested orchard has an infestation risk of about 50% of probability.

Geosmithia morbida (thousand cankers disease).

Thousand Cankers Disease is a disease complex native to the western United States that affects many Juglans and Pterocarya species, i.e. walnut and wingnut trees. It is caused by the fungus Geosmithia morbida, which is vectored by the walnut twig beetle (Pityophthorus juglandis), and possibly by other insects. The beetle carries fungal spores that are introduced into the tree during gallery construction, and the fungus then causes cankers in the inner bark that disrupt the flow of nutrients throughout the tree, often leading to its death. In recent years the disease has been reported in several eastern states, and also in Italy. Long-distance spread is thought to be a result of the movement of infected and infested wood.

Eastern Black Walnut (Juglans nigra L.) Originating From Native Range Varies in Their Response to Inoculation With Geosmithia morbida

Thousand cankers disease (TCD) is caused by the walnut twig beetle (Pityophthorus juglandis) vectoring the fungal canker pathogen Geosmithia morbida, which can result in severe dieback and eventual death to species of walnut (Juglans spp.) and wingnut (Pterocarya spp.). This disease is most devastating to the highly valued species J. nigra (black walnut). This species is primarily grown and harvested for timber production in the Central Hardwood Region of the United States, which comprises part of its native range. Management options for TCD are limited; therefore, finding resistant genotypes is needed. Initial studies on black walnut susceptibility to G. morbida documented some genetic variation and suggested potential resistance. Furthermore, G. morbida is thought to be native to the United States, which may have allowed for co-evolution. To capture the representative genetic diversity and screen for resistance to G. morbida, J. nigra families were collected from across the native range. These wild trees, in conjunction with seedlings developed in a black walnut timber improvement program, were planted in a common garden in Fort Collins, Colorado and repeatedly inoculated with G. morbida over the course of four years and three growing seasons. Improved seedlings exhibited larger cankered areas than wild J. nigra of the same provenance. Cankers induced by G. morbida in wild germplasm were smaller on J. nigra collected from the western and central portions of the native range compared to those collected from the eastern portion. Although trees from the western and central part of the range still incurred cankers, our findings indicate that variation in genetic resistance to G. morbida is present in black walnut. This study was performed with G. morbida independent of the walnut twig beetle, but our results suggest the limited G. morbida resistance observed in J. nigra will prevent the full compromise of black walnut to TCD. Results from this study should be taken into consideration in future black walnut breeding programs.

Vacuum Steam Treatment Effectiveness for Eradication of the Thousand Cankers Disease Vector and Pathogen in Logs From Diseased Walnut Trees

Logs of high-value eastern black walnut (Juglans nigra L.) are commonly exported from the United States for production of veneer and lumber. Veneer logs are not debarked to minimize degradation of wood quality and reduce moisture loss. Thousand cankers disease (TCD) is caused by the walnut twig beetle (Pityophthorus juglandis Blackman) and the fungal pathogen, Geosmithia morbida M. Kolarik, E. Freeland, C. Utley and N. Tisserat sp. nov., which colonize the inner bark of Juglans species. Effective eradication of these organisms by heat or chemical fumigation treatment is required for walnut logs prior to export. Because vacuum steam is an effective and efficient means of heating round wood, its use in eliminating the TCD causal agents was evaluated using Juglans logs (12- to 44-cm small end diameter and 1.7- to 1.9-m length) from TCD-symptomatic trees in Oregon and Washington State. Five replicate trials with three logs per load were conducted in a portable vacuum chamber to test two treatment schedules: 60°C for 60 min and 56°C for 30 min. Complete elimination of P. juglandis and G. morbida was achieved when using a minimum of 56°C at 5-cm targeted depth from bottom of bark furrow into the sapwood and held for 30 min. Treatment cycle time ranged from 298 to 576 min depending on log diameter and initial log temperature. Artificial inoculation of J. nigra trees with G. morbida within the TCD range in Pennsylvania was minimally successful in producing adequately colonized logs for experimental trials.

Fungi Associated with Damage Observed on Branches of Juglans nigra in Indiana, Missouri, and Tennessee

Branch and stem cankers caused by Geosmithia morbida associated with mass attack by its primary insect vector (Pityophthorus juglandis) result in thousand cankers disease (TCD) on Juglans and Pterocarya species. Because other fungi and insects can cause visible damage to Juglans nigra, a baseline assessment was performed to document damage types present and to characterize fungi associated with each type. Two branches were collected from trees with visually healthy crowns in TCD-free locations (Indiana and Missouri) and two branches from trees with and without crown symptoms characteristic of TCD within the disease range in Tennessee. In most cases, one of the two branches was girdled at the base 3 to 4 months prior to harvest. Outer bark was peeled from branch subsamples, observed damage characterized, and isolation of fungi from each damage type attempted. Three known pathogens of J. nigra were obtained from different damage types: G. morbida, in Tennessee only; Botryosphaeria dothidea, in Indiana and Tennessee; and Fusarium solani (= members of F. solani species complex), in all three states. The latter two fungi may exacerbate branch dieback and mortality of TCD-affected trees. These results will be of value to plant health specialists monitoring J. nigra in the field and laboratory diagnosticians processing survey samples.