Date of Award

2025

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Forest Ecology and Management (MS)

Administrative Home Department

College of Forest Resources and Environmental Science

Advisor 1

Tara Bal

Committee Member 1

Molly Cavaleri

Committee Member 2

Carsten Külheim

Committee Member 3

Sharon Reed

Abstract

Oak wilt is a fatal fungal pathogen to oak trees, caused by the fungus Bretziella fagacearum. This fungus targets oak species (Quercus spp.), where members of the red oak group (sect. Lobatae) are at greatest risk of mortality. Oak wilt is widespread throughout the eastern United States, currently present in 24 states. Recently in 2023, oak wilt was detected in trees for the first time in Canada. Although removed from the locations of the first Canadian detections, there is continual concern that the range of oak wilt is at risk for expansion northward. This is further evident due to the detections in Ontario, Canada, and by recent detections in 2024 in Upper Michigan, USA, in Marquette County. Pruning and harvest restrictions are currently in place from April 15th to July 15th in Michigan to reduce the risk of overland disease spread by the nitidulid vector beetles (Nitidulidae: Coleoptera), which are attracted to fresh wounds on susceptible oak trees. These current recommendations are based on growing degree days, which influence nitidulid flight activity and host tree conditions, with similar recommendations imposed in surrounding states (MN and WI) with oak wilt. To better understand the risk of oak wilt moving north, potential vector species behaviors with hosts and traps as well as forest stand environmental factors were assessed. We established a large collaborative sampling effort in the spring of 2023 and 2024 across 11 sites in Upper Michigan, Ontario, and New Brunswick, Canada. To assess the potential relationship between nitidulid activity in fresh wounds and variables such as spring bud phenology, nitidulids were collected in artificial oak wounds and baited flight traps to determine the timing of flight vs. wound entry, in addition to assess the communities of nitidulids found in fresh oak wounds (the most likely to be potential vectors) in areas north of the current disease range. In total, we collected almost 40,000 nitidulid beetles in wounds or traps between the two years across the wide geographic range of our study. Only two Nitidulidae species in neighboring states are confirmed as vectors that can carry the fungal spores and spread B. fagacearum to healthy trees. However, our research suggests that multiple species in different genera may be important vectors of the disease if it moves into northern regions. Additionally, a stronger relationship was observed between the timing of nitidulids entering wounds and spring bud phenology than with nitidulids entering baited flight traps. This suggests that bud break and spring host phenology may be a more precise time indicator when considering springtime transmission risk via overland spread. Models incorporating bud phenology in addition to growing degree days and site variables may provide the most accurate estimations for the timing of risk, allowing recommendations to be made for potential refinement of the high-risk period. These studies combined help us to be better prepared and understand the risks and factors associated with potential oak wilt transmission in areas north of the current disease range.

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