City forestry staff are bracing for the arrival of an invasive species that could spell big trouble for thousands of Sarnia’s most beautiful trees. Oak wilt is a lethal fungal disease, one similar to the Dutch elm disease that wiped out most of the continent’s American elm trees. It blocks the tree’s vascular system, preventing it from taking in food and water. Wilting starts at the top, works its way down, and within just weeks or months a large and otherwise healthy oak tree is dead. This devastating disease has been heading slowly northward through the eastern U.S. Though it hasn’t been confirmed in Canada yet it has reached St. Clair County in Michigan, just across the river, and Belle Island in the Detroit River.