Announcement of Final Thesis Defense Plant Pathology M.S. Degree
April 6, 2026 1:30PM - 2:30PM
Candidate’s Name: Ethan Wachendorf
Date: April 6th, 2026
Time: 1:30 pm
Room: A271 PSSB
Stump susceptibility to infection and quantification of host red oak response to differential inoculum loads of bretziella fagacearum
Members of the Examining Committee and their Department:
- Martin Chilvers - Plant, Soil and Microbial Sciences
- Deborah G. McCullough - Entomology
- Bert Cregg - Horticulture
ABSTRACT
Oak wilt, caused by Bretziella fagacearum, is a devastating vascular wilt of red oaks (Quercus sect. Lobatae), particularly in the Midwest, yet key aspects of infection courts, inoculum thresholds, and host physiological responses remain poorly understood. To investigate these aspects of the pathosystem, we performed field experiments in the northern lower peninsula of Michigan, which assessed (1) the susceptibility of fresh-cut stumps, (2) the role of inoculum concentration in disease development, and (3) the relationship between vascular dysfunction and symptom progression in mature red oaks in northern Michigan. Fresh-cut Quercus rubra stumps (n = 36) were inoculated across two sites to evaluate their potential as infection courts. Real-time PCR confirmed pathogen presence in above and belowground tissues. Symptom development in adjacent trees indicated potential for belowground spread. In complementary inoculation trials on standing trees, higher inoculum concentrations (≥ 5 × 10⁴ spores mL⁻¹) significantly increased disease incidence, canopy wilt progression, and vessel occlusion. Contrary to expectations, vessel size did not predict occlusion, although symptomatic trees exhibited smaller vessels overall. To link external symptoms with internal processes, sap flux in mature oaks was monitored following inoculation. Sap flux declined more rapidly in symptomatic trees and scaled with disease severity, with high-severity trees declining approximately 4-fold faster than asymptomatic individuals. Visual canopy symptoms generally preceded severe hydraulic decline, and sap flux reduction coincided with ~38% vessel occlusion. This research demonstrates that fresh-cut stumps can serve as infection courts, inoculum load influences disease development, and wilting precedes severe hydraulic failure in the main stem. These results improve understanding of oak wilt epidemiology and have direct implications for the management of exposed stumps, long-term monitoring, and early disease detection.
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