Population genomic analysis of an emerging pathogen Lonsdalea quercina affecting various species of oaks in western North America

Understanding processes leading to disease emergence is important for effective disease management and prevention of future epidemics. Utilizing whole genome sequencing, we studied the phylogenetic relationship and diversity of two populations of the bacterial oak pathogen Lonsdalea quercina from western North America (Colorado and California) and compared these populations to other Lonsdalea species found worldwide. Phylogenetic analysis separated Colorado and California populations into two Lonsdalea clades, with genetic divergence near species boundaries, suggesting long isolation and populations that differ in genetic structure and distribution and possibly their polyphyletic origin. Genotypes collected from different host species and habitats were randomly distributed within the California cluster. Most Colorado isolates from introduced planted trees, however, were distinct from three isolates collected from a natural stand of Colorado native Quercus gambelii, indicating cryptic population structure. The California identical core genotypes distribution varied, while Colorado identical core genotypes were always collected from neighboring trees. Despite its recent emergence, the Colorado population had higher nucleotide diversity, possibly due to its long presence in Colorado or due to migrants moving with nursery stock. Overall, results suggest independent pathogen emergence in two states likely driven by changes in host-microbe interactions due to ecosystems changes. Further studies are warranted to understand evolutionary relationships among L. quercina from different areas, including the red oak native habitat in northeastern USA.

Temperature Limits for the Brown Wheat Mite, in Colorado

Brown wheat mites, Petrobia latens (Müller 1776, Acari: Tetranychidae), are sporadic yet economically damaging pests of winter cereals. In Colorado, their life history is closely tied to the development of winter wheat, where they are present in the field from crop planting in late September through harvest in early June. In order to withstand winter months, these mites are able to survive cold temperatures. However, the mechanisms of cold hardening and their temperature limits are unknown. This research documents the seasonal supercooling points of the brown wheat mite. Their seasonal average supercooling point stayed consistent throughout the year, never varying more than a degree from the overall average supercooling point of -17°C. The greatest variation in supercooling point was seen in the spring, during which supercooling point temperatures ranged from -9.2 to -25.5°C. We also documented the upper and lower lethal temperatures for the brown wheat mite. When comparing small nymphs to large nymph and adult stages, small nymphs were slightly more cold tolerant (lethal temperature estimates required to kill 99% of the population [LT99] were -30.8 and -30.6°C, respectively), but less heat tolerant (LT99 was 50 and 56°C, respectively).

Insects Visiting Drippy Blight Diseased Red Oak Trees Are Contaminated with the Pathogenic Bacterium Lonsdalea quercina

The focus of investigation in this study was to consider the potential of arthropods in the dissemination of the bacterium involved in drippy blight disease, Lonsdalea quercina. Arthropod specimens were collected and tested for the presence of the bacterium with molecular markers. The bacterium L. quercina was confirmed on 12 different insect samples from three orders (Coleoptera, Hemiptera, and Hymenoptera) and eight families (Buprestidae, Coccinellidae, Dermestidae, Coreidae, Pentatomidae and/or Miridae, Apidae, Formicidae, and Vespidae). Approximately half of the insects found to carry the bacterium were in the order Hymenoptera. Estimates of the insects that are contaminated with the bacterium, and likely carry it between trees, is conservative because the documented insects represent only a subset of the insect orders that were observed feeding on the bacterium or present on diseased trees yet were not able to be tested. The insects contaminated with L. quercina exhibited diverse life histories, where some had a facultative relationship with the bacterium and others sought it out as a food source. These findings demonstrate that a diverse set of insects naturally occur on diseased trees and may disseminate L. quercina.

Virulence of Genetically Distinct Geosmithia morbida Isolates to Black Walnut and Their Response to Coinoculation with Fusarium solani

Geosmithia morbida is well documented as the causal agent of thousand cankers disease of black walnut trees. However, it is not well understood how G. morbida strains differ in virulence and how their interactions with co-occurring pathogens contribute to disease severity. In this study, we systematically investigated virulence of genetically distinct G. morbida strains. Overall, we found varying degrees of virulence, although differences were not related to genetic groupings. Furthermore, the pathogen Fusarium solani is also commonly isolated from thousand canker-diseased trees. The degree of disease contribution from F. solani is unknown, along with interactions it may have with G. morbida. This research shows that coinoculation with these pathogens does not yield a synergistic response.

The effect of temperature on survival of Pityophthorus juglandis (Coleoptera: Curculionidae)

The walnut twig beetle (Pityophthorus juglandis Blackman) vectors Geosmithia morbida, the causal agent of thousand cankers disease in Juglans, and is particularly damaging to Juglans nigra L. (black walnut). Native hosts of P. juglandis are distributed in the southwestern United States where winter temperatures tend to be higher than those found within the native range of black walnut. To better understand temperature effects on survival of P. juglandis, we initiated studies to determine: 1) seasonal variations in cold tolerance, as measured by the supercooling point (SCP), and 2) upper and lower lethal temperatures (LT). In the lower LT study, Xyleborinus saxeseni (Ratzeberg) was tested for comparison. Insects were either exposed to increasing or decreasing temperatures and then checked for survival. Upper and lower LTs were estimated using a logistic model. For the SCP study, data were grouped into seasons. Seasonal mean SCPs were highest in summer (-15.4°C) and lowest in fall (-18.1°C). The upper lethal limit estimations required to kill 99% of the population (LT99) for adults and larvae were 52.7 and 48.1°C, respectively, and lower limit LT99 estimations for adults and larvae were -18.1 and -18.7°C, respectively. The lower median LT (LT50) of X. saxeseni was -24.7°C. These studies, as well as beetle survival in infested Colorado trees where temperatures reached -29°C in February 2011, suggest P. juglandis could survive the winter in much of the native range of black walnut, but may be limited in trees where temperatures regularly exceed the lower LT.