Evaluating effectiveness of girdle-herbicide containment of below-ground spread of oak wilt (Bretziella fagacearum)

Oaks (Quercus spp) are one of the most important sources of timber, mast for wildlife, and ecosystem services across the eastern US. Increasingly, this genus is at risk from diseases including oak wilt, which is one of the most serious threats to oaks, caused by the fungus, Bretziella fagacearum. The upper Midwest has over 5 million ha of oak forests, much of which is on rocky glaciated soils where traditional methods of containing below-ground spread of oak wilt (e.g., vibratory plow lines) are not feasible. We evaluated an alternative containment method of girdling and herbicide (GH) of oak wilt infected trees as well as neighboring oak trees likely connected via root grafts. Our results demonstrated that GH was effective at controlling below-ground spread of oak wilt (overall success rate: 55 %). Best control was achieved when infection centers were small (<= 4 newly infected trees), where GH was 81 % effective at containing oak wilt. Containment was only 29 % in larger infection centers (>= 5 newly infected trees). The best predictor of success was the number of newly infected trees (p = 0.02) even when considering other factors that could dictate the size of infection centers (e.g., diameter of trees, or number of neighboring trees treated). Our results illustrate the importance of early and rapid management of oak wilt infections and offer a starting place for continued improvement of the GH methodology.