Date of Award

Summer 2022

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Jeffrey R Garnas

Second Advisor

Heidi Asbjornsen

Third Advisor

Isabel A Munck

Abstract

This work explores the present-day interactions of a long-ago established tree disease complex known as beech bark disease (BBD) that afflicts the American beech tree (Fagus grandifolia) in northeastern North America. Disease development initiates when an exotic scale insect known as the beech scale (Cryptoccocus fagisuga) infest susceptible American beech and feed on live parenchyma cells in the outer bark. The interactions that occur between the beech scale and living bark predispose the host to infection by two independently vectored species of pathogenic fungi: Neonectria faginata and Neonectria ditissima. Infection by these fungi causes necrosis of bark and meristematic tissue that can girdle and kill the host if infections are severe enough. However, disease development can be highly variable and host trees can survive many decades under varying levels of chronic disease. Physical responses of the host to disease cause heavy alterations in bark structure including fissures, cankers, necrosis, and other defects in a complex mosaic that forms and changes with host growth and infection accumulation. As the primary symptom of the disease, variation in altered bark structure matches the high variability observed in disease expression within and between hosts. In long-affected stands, the interactions between the beech scale, Neonectria fungi and physical bark response on individual hosts to current and prior infection appear to shift and cause changes in disease severity and expression over time.

We test the hypothesis that host-mediated feedbacks associated with altered bark structure occur between the insect and fungal disease agents in the system. Such feedbacks represent a potential explanation for the high level of variability in disease agent populations, for the lack of a strong correlation between putative mutualists/commensals in aftermath forests, and unpredictability associated with disease dynamics in long-affected stands. To test the importance of bark type, we designed and carried out two experiments and an observational study that investigate different angles of the interactions occurring between insect and host and fungi and host. Hosts were stratified by bark structure (rough and smooth) so that we could test how differences in disease altered bark structure may interact within the larger framework of host-mediated feedbacks between insect and fungi.

Our findings indicate that physical changes in bark structure have impacts both on scale insect establishment rates and fungal growth. The two primary bark tissue types induced by the disease – necrophylactic periderm (syn. wound periderm) and wound rhytidome – are found to be dichotomous in quality for scale insect feeding and protection. However, the spatial amalgamation between these induced tissue types across the bark surface can form a refugia for scale insect. The two pathogenic Neonectria fungi were found to be impacted differently by altered bark structure. N. faginata was largely unaffected in terms of growth while N. ditissima appeared to have better growth within hosts with altered bark structure. Lastly, the beech scale was found to have a moderately antagonistic relationship with Neonectria growth when it was present at low densities within the immediate area (radius) of the bark tissue that was experimentally inoculated with Neonectria fungi.

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