A: Yes, eventually. Oak wilt moves through oak roots at about 75 feet /year. The trench is placed 100 feet in front of the advancing oak wilt, which is 1.5 to 2 years ahead of the disease.
The Forest Service has dug up old trenches to see what happens over time. Rather than growing roots across the trench, the roots tend to run up and down the trench in the loosened soil of the trench. The trees that will die, will die long before roots cross the trench or any root grafting (which is a slow process) occurs. There are trenches that are 12 - 13 years old that are still holding, preventing the oak wilt from crossing the trench. Timing is important. You want the disease to challenge the trench relatively soon. If you trench around an oak wilt center, and 3 years later the disease is up to the trench line all the way around, then it is likely that the trench will contain the oak wilt. Where you get in trouble is if you misplace the trench. If inspection after one year shows that the disease is already to the trench line, then it means that the trench was not put in the right place, and the trench might fail to contain the disease. Alternatively, if the disease just sits there and doesn’t move, there is a higher likelihood that the disease will break out of the trenched area.
You can improve the effectiveness of your trench by a few percentage points if you install the trench and then push over all of the oaks inside the trench. This is very difficult for most people to do because they see the buffer zone of live trees. Those trees are alive today, but actually they are dead - they just don’t know it yet. The Forest Service highly recommends pushing over all the trees inside the fence and burning them, but this doesn’t happen very often.
Ultimately, three years after trenching, you hope to see dead trees right up to the trench line, and live oak trees just outside the trench line.