Mycobiota associated with insect galleries in walnut with thousand cankers disease reveals a potential natural enemy against Geosmithia morbida

Characterizing Pathogen-Induced Changes in Black Walnut Volatile Organic Compounds Following Inoculation with Geosmithia Morbida, The Causal Agent of Thousand Cankers Disease

Thousand cankers disease (TCD) is a pathosystem comprised of Juglandacea spp., a pathogenic fungus Geosmithia morbida, and an insect vector, the walnut twig beetle (WTB) (Pityophthorus juglandis). Of the North American Juglans species, Juglans nigra is the most susceptible to TCD and has resulted in significant decline and mortality of urban and plantation trees in the western United States. Geosmithia morbida causes necrotic cankers in the phloem, and infected trees may release an array of volatile compounds that act as important chemical cues to WTB. Here, we aimed to determine how J. nigra volatile profiles respond to G. morbida infection as these changes can offer valuable insights into plant defense mechanisms and potentially influence WTB behavior, thus impacting disease transmission dynamics. In this study, we collected a series of bark and leaf volatiles from J. nigra seedlings inoculated with one of three isolates of G. morbida and a sham-inoculated control. Our results suggest J. nigra bark responds to G. morbida infection, with the western United States isolate (RN-2) eliciting a distinct volatile response compared to other treatments. We identified six out of fourteen compounds that contribute to 80% of the dissimilarity between RN-2 and sham-inoculated control trees. Inoculation with isolate RN-2 elicited the largest change in volatile profiles and resulted in the smallest cankers in the phloem, suggesting these compounds my play important defensive roles in J. nigra against the fungal pathogen that causes TCD.

Pan-genome and transcriptome analyses provide insights into genomic variation and differential gene expression profiles related to disease resistance and fatty acid biosynthesis in eastern black walnut (Juglans nigra)

Walnut (Juglans) species are used as nut crops worldwide. Eastern black walnut (EBW, Juglans nigra), a diploid, horticultural important woody species is native to much of eastern North America. Although it is highly valued for its wood and nut, there are few resources for understanding EBW genetics. Here, we present a high-quality genome assembly of J. nigra based on Illumina, Pacbio, and Hi-C technologies. The genome size was 540.8 Mb, with a scaffold N50 size of 35.1 Mb, and 99.0% of the assembly was anchored to 16 chromosomes. Using this genome as a reference, the resequencing of 74 accessions revealed the effective population size of J. nigra declined during the glacial maximum. A single whole-genome duplication event was identified in the J. nigra genome. Large syntenic blocks among J. nigra, Juglans regia, and Juglans microcarpa predominated, but inversions of more than 600 kb were identified. By comparing the EBW genome with those of J. regia and J. microcarpa, we detected InDel sizes of 34.9 Mb in J. regia and 18.3 Mb in J. microcarpa, respectively. Transcriptomic analysis of differentially expressed genes identified five presumed NBS-LRR (NUCLEOTIDE BINDING SITE-LEUCINE-RICH REPEAT) genes were upregulated during the development of walnut husks and shells compared to developing embryos. We also identified candidate genes with essential roles in seed oil synthesis, including FAD (FATTY ACID DESATURASE) and OLE (OLEOSIN). Our work advances the understanding of fatty acid bioaccumulation and disease resistance in nut crops, and also provides an essential resource for conducting genomics-enabled breeding in walnut.

Thousand Cankers Disease in Walnut Trees in Europe: Current Status and Management

Thousand cankers disease (TCD) is a new deadly disease in walnut trees (Juglans spp.), which is plaguing commercial plantations, natural groves, and ornamental black walnut trees (Juglans nigra) in their native and invasion areas in the US and, more recently, in artificial plantations and amenity trees in the newly-invaded areas in Europe (Italy). This insect/fungus complex arises from the intense trophic activity of the bark beetle vector Pityophthorus juglandis in the phloem of Juglans spp. and the subsequent development of multiple Geosmithia morbida cankers around beetles' entry/exit holes. After an analysis of the main biological and ecological traits of both members of this insect/fungus complex, this review explores the options available for TCD prevention and management. Special focus is given to those diagnostic tools developed for disease detection, surveillance, and monitoring, as well as to existing phytosanitary regulations, protocols, and measures that comply with TCD eradication and containment. Only integrated disease management can effectively curtail the pervasive spread of TCD, thus limiting the damage to natural ecosystems, plantations, and ornamental walnuts.

Characterization and microsatellite marker development for a common bark and ambrosia beetle associate, Geosmithia obscura

Symbioses between Geosmithia fungi and wood-boring and bark beetles seldom result in disease induction within the plant host. Yet, exceptions exist such as Geosmithia morbida, the causal agent of Thousand Cankers Disease (TCD) of walnuts and wingnuts, and Geosmithia sp. 41, the causal agent of Foamy Bark Canker disease of oaks. Isolates of G. obscura were recovered from black walnut trees in eastern Tennessee and at least one isolate induced cankers following artificial inoculation. Due to the putative pathogenicity and lack of recovery of G. obscura from natural lesions, a molecular diagnostic screening tool was developed using microsatellite markers mined from the G. obscura genome. A total of 3256 candidate microsatellite markers were identified (2236, 789, 137 di-, tri-, and tetranucleotide motifs, respectively), with 2011, 703, 101 di-, tri-, and tetranucleotide motifs, respectively, containing markers with primers. From these, 75 microsatellite markers were randomly selected, screened, and optimized, resulting in 28 polymorphic markers that yielded single, consistently recovered bands, which were used in downstream analyses. Five of these microsatellite markers were found to be specific to G. obscura and did not cross-amplify into other, closely related species. Although the remaining tested markers could be useful, they cross-amplified within different Geosmithia species, making them not reliable for G. obscura detection. Five novel microsatellite markers (GOBS9, GOBS10, GOBS41, GOBS43, and GOBS50) were developed based on the G. obscura genome. These species-specific microsatellite markers are available as a tool for use in molecular diagnostics and can assist future surveillance studies.

First report of Diplodia corticola causing dieback of white oak (Quercus alba) in Tennessee

Diplodia corticola is a fungal pathogen causing oak dieback in Quercus (oak) spp. in parts of North America, northern Africa, and Europe (Ferreira et al., 2021; Smahi et al., 2017; Tsopelas et al., 2018). In August 2021, a single mature white oak (Q. alba) exhibiting wilt symptoms, vascular discoloration, and interveinal chlorosis was observed in Cove Lake State Park in Campbell County, Tennessee, U.S.A. Small sections of phloem tissues were cut from the margins of discolored vasculature of a single wilt symptomatic branch with a sterile scalpel and surface sterilized following Parra et al. (2020). Surface sterilized wood chips were plated onto potato dextrose agar amended with antibiotics (PDA++) following Gazis et al. (2018). Three days after plating, we recovered a single fungal isolate from wood chips that when grown in ½ PDA resembled D. corticola, having irregular margins and white aerial mycelia that progressively turned greyish-black 15 days after sub-culturing (Alves et al., 2004). Total genomic DNA was extracted from the isolate following Gazis et al. (2018). The internal transcribed spacer (ITS) was then amplified using the ITS1 and ITS4 primers and the subsequent PCR product was sequenced. Resulting reads were assembled into a consensus sequence and identity was assigned using BLAST on the NCBI nucleotide database. The assembled sequence (accession OM716006) had a 100% identity match with D. corticola type culture CBS 112549 (accession NR_111152). To complete Koch's postulates and identify potential host range, 5 red oaks (Q. rubra; 2-3 yrs old; caliper 14.7 ± 2 mm) and 5 white oaks (Q. alba; 2-3 yrs old; caliper 22.8 ± 2.3 mm) were inoculated with D. corticola (isolate DC_2.5). Trees were inoculated 15 cm above the soil line in a greenhouse with a 3 mm diameter plug of a 10-day old culture of D. corticola grown on PDA following Sitz et al. (2017). As a negative control, 5 red and 5 white oaks were inoculated with a 3 mm diameter plug of PDA. For each species, trees were sampled when seepage was observed from D. corticola inoculated sites (15 days post-inoculation for red and white oaks). At time of sampling, bark adjacent to inoculation sites on each tree was removed and cankers were photographed. Using a sterile scalpel, four wood chips were cut from canker margins and placed onto PDA++. For all trees, canker areas were measured using ImageJ software (Rasband, 2012). Recovered isolate identities were confirmed by extracting total genomic DNA as described above (Gazis et al. 2018) and PCR amplification of the ITS, large ribosomal subunit (LSU), and elongation factor 1-α (ef1-α) following (Ferreira et al., 2021). Diplodia corticola was reisolated from wood chips of D. corticola inoculated red (5/5 trees) and white (5/5 trees) oaks and ITS (accession OM716954), LSU (accession OM716955), and ef1-α (accession OM752198) sequences matched D. corticola type culture 112549 ITS (100% identity), LSU (99.76%-100% identity; accession KF766323), and ef1-α (98%-98.9% identity; accession XM_020275852). All D. corticola inoculated trees exhibited seepage from inoculation sites with streaking present in vasculature. Cankers were significantly larger in D. corticola inoculated red (2.34 ± 1.36 cm; P=0.042) and white (2.96 ± 0.52 cm; P=0.00029) oaks compared to agar inoculated trees. To the best of our knowledge, this is the first report of D. corticola causing decline of oaks in Tennessee.

Conventional Gel Electrophoresis and TaqMan Probes Enable Rapid Confirmation of Thousand Cankers Disease from Diagnostic Samples

Thousand cankers disease (TCD) is caused by the fungal pathogen Geosmithia morbida and vectored by the walnut twig beetle Pityophthorus juglandis. In infected walnut and butternut (Juglans spp.) hosts and wingnut species (Pterocarya spp.) hosts, tree decline and death results in ecological disruption and economic losses. A rapid molecular detection protocol for TCD using microsatellite markers can confirm the presence of insect vector or fungal pathogen DNA, but it requires specialized expensive equipment and technical expertise. Using four different experimental approaches, capillary and conventional gel electrophoresis, and traditional polymerase chain reaction (PCR) and quantitative PCR (qPCR), we describe simplified and inexpensive processes for diagnostic confirmation of TCD. The improved and rapid detection protocols reported in this study reduce time and equipment costs associated with detection of molecular pest and pathogen DNA by (1) using conventional gel electrophoresis or TaqMan molecular probes to elucidate the detection limits for G. morbida and P. juglandis DNA and (2) identifying resources that allow visualization of positive test results for infected host plant tissue samples. Conventional gel electrophoresis and TaqMan molecular probe protocols detected presence of DNA from TCD-associated fungal and insect samples. These procedural improvements can be readily adopted by diagnostic end-users and adapted for use with other complex disease systems to enable rapid pest and pathogen detection.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Fungi associated with galleries of the emerald ash borer

The emerald ash borer (EAB) is an exotic forest pest that has killed millions of ash trees in the United States and Canada, resulting in an ecological disaster and billions of dollars in economic losses of urban landscape and forest trees. The beetle was first detected in Michigan in 2002 and has spread through much of the Eastern and Midwestern U.S., reaching Minnesota in 2009. Since then, it has spread across the state and poses a great risk to the more than 1 billion ash trees in Minnesota. The larval stage of EAB creates wounds on trees as they feed on the inner bark, causing disruption of water and sap flow that results in tree death. The fungal community associated with EAB larval galleries is poorly understood and the role these fungi may play in tree death is not known. This study describes fungi isolated from EAB larval galleries sampled throughout the main geographic areas of Minnesota where ash is affected by EAB. Fungal cultures were identified by extracting genomic DNA and sequencing the ITS region of the rDNA. Results from 1126 isolates reveal a diverse assemblage of fungi and three functional guilds comprised of canker pathogens, wood decay, and entomopathogenic fungi. The most common canker-associated genera were Cytospora followed by Phaeoacremonium, Paraconiothyrium, Coniothyrium, Nectria, Diplodia, and Botryosphaeria. Fungi in the Basidiomycota were nearly all wood decay causing fungi and many were species of pioneer colonizing genera including Sistotrema, Irpex, Peniophora, Phlebia and Ganoderma. Some of these fungi seriously affect urban trees, having the potential to cause rapid wood decay resulting in hazardous tree situations. Several entomopathogenic genera with the potential for biological control of EAB were also isolated from galleries. Purpureocillium was the most commonly isolated genus, followed by Beauveria, Clonostachys, Lecanicillium, Akanthomyces, Cordyceps, Microcera, Tolypocladium, and Pochonia. The results identify important fungal functional guilds that are occupying a new niche in ash trees resulting from EAB and include fungi that may accelerate decline in tree health, increase hazard tree situations, or may provide options for biological control of this destructive invasive insect.

Assessment of Alternative Candidate Subcortical Insect Vectors From Walnut Crowns in Habitats Quarantined for Thousand Cankers Disease

Thousand cankers disease (TCD) results from the combined activity of the fungal pathogen, Geosmithia morbida Kolařík, Freeland, Utley, and Tisserat and its principle vector, Pityophthorus juglandis (Blackman) (Coleoptera: Curculionidae: Scolytinae) in Juglans L. spp. and Pterocarya Kunth spp. host plants. TCD has been reported from the eastern and western United States. To evaluate potential for other beetle species to vector the fungus in east Tennessee, specimens were collected using ethanol-baited traps that were suspended beneath crowns of TCD-symptomatic trees. Associations of G. morbida with insect species collected in traps were assessed in an unsuccessful, preliminary culture-based fungal assay, and then with a molecular-based detection method. For culture-based assays, rinsate from washed, individual insects was plated on nutrient media and growing colonies were subcultured to obtain axenic G. morbida cultures for identification. For the molecular-based method, G. morbida presence was detected by amplifying the previously developed, species-specific microsatellite locus GS004. Capillary electrophoresis was used to detect the amplified amplicons and representative reactions were validated using Sanger sequencing. Eleven beetle species were found to carry G. morbida, including Cnestus mutilatus (Blandford), Dryoxylon onoharaensum (Murayama), Hylocurus rudis (LeConte), Monarthrum fasciatum (Say), Monarthrum mali (Fitch), Xyleborinus saxesenii (Ratzeburg), Xylosandrus crassiusculus (Motschulsky), Xylosandrus germanus (Blandford) (all Coleoptera: Curculionidae: Scolytinae), Stenomimus pallidus (Boheman) (Coleoptera: Curculionidae: Cossoninae), Oxoplatypus quadridentatus (Olivier) (Coleoptera: Curculionidae: Platypodinae), and Xylops basilaris (Say) (Coleoptera: Bostrichidae). These findings raise concerns that alternative subcortical insect species that already occur within quarantined habitats can sustain incidence of introduced G. morbida and contribute to spread within the native range of black walnut, Juglans nigra L., in the eastern United States.