The American beech is an ecologically important species, providing food and habitat to over 40 species of birds and mammals in eastern forests. This iconic tree has already been impacted by beech bark disease, and is now under attack by a new enemy causing beech leaf disease (BLD). Smaller beech saplings afflicted with BLD can die within 2 to 5 years, and there are now reports of mortality in large, mature trees. A break in this mystery came in the fall of 2017 when thousands of microscopic worms, called nematodes, were found on a symptomatic beech leaf. When healthy American beech seedlings were inoculated with nematodes isolated from symptomatic beech leaves, they, too, developed beech leaf disease symptoms. Nematodes extracted from these newly symptomatic tissues were confirmed to be the same species found on diseased trees from the field, providing enough evidence to solve the beech leaf disease mystery. A group of scientists, including several from the NRS, are continuing research to determine the mechanisms involved in the spread and success of nematode infestations responsible for the transition of presumably presymptomatic leaves to symptomatic. A better understanding of this previously unknown pest will help researchers develop management strategies and treatments.