In the summer of 1904, a forest ranger at the Bronx Zoo in New York noticed that some American chestnut trees were showing signs of illness. This observation marked the start of a devastating decline for the American chestnut, a tree once abundant in the eastern United States. The disease, known as chestnut blight, is caused by a fungus called Cryphonectria parasitica. The fungus was originally from Asia and was accidentally introduced to North America through infected Japanese chestnut plants brought from Asia. These plants were then sold to nurseries across the U.S., spreading the fungus far and wide. In its native regions of China, Japan, and Korea, the Asian chestnut had evolved alongside the fungus, developing a natural resistance. However, the American chestnut had no such history with the pathogen and was completely vulnerable. The blight was first identified at the Bronx Zoo, where a forester named Hermann Merkel noticed unusual symptoms on American chestnut trees. The following year, a mycologist named William Murrill isolated and named the fungus responsible. The fungus spreads by entering a tree through wounds, cracks, or dead bark, and it uses wind, rain, and animals to move its spores. Once inside, it produces oxalic acid, which damages the tree's cells while nourishing the fungus. This leads to the formation of cankers—areas of dead tissue—that gradually strangle the tree. The speed of the blight’s spread has surprised scientists; it moved across the Appalachian Mountains at an average of 80 kilometers per year, with some areas seeing it move up to 95 kilometers annually. Despite efforts to stop it, including quarantine lines and firebreaks, the fungus continued to spread unchecked. Between 1904 and 1940, about 3.5 billion American chestnut trees were lost to the blight, with some estimates suggesting the total could reach four billion. This loss affected nearly 300,000 square miles of forest, where the chestnut once made up a significant portion of the canopy in some regions. The economic and cultural impact was immense. American chestnuts were used for construction, fencing, and poles due to their rot-resistant wood, and they were a staple food for many families, especially around Thanksgiving. Although the American chestnut is now considered functionally extinct—meaning it no longer reproduces naturally in the wild—its genetic material still exists in the form of sprouts growing from the roots of dead trees. These sprouts, however, rarely survive to maturity due to the persistent presence of the fungus. Scientists are working to restore the species by crossbreeding American chestnuts with resistant Asian varieties and using genetic engineering to introduce tolerance genes from other plants. In Europe, where the fungus arrived later, some chestnut groves have survived due to the development of less virulent strains, a natural adaptation that has not occurred in the U.S.