A variety of diseases and infestations have affected, and continue to affect, resources at Minute Man National Historical Park (park). The most notable of these diseases may be the "Chestnut Blight", which devastated populations of American chestnut in the early 1900s. The chestnut blight is an Asian fungus that typically does not kill trees, which are able to resprout after topkill (micro-organisms in the soil prevent the fungus from killing tree roots). However, the blight usually prevents trees from flowering and reproducing and have prevented re-establishment of healthy populations of American chestnuts throughout their range. The park supports several populations of American chestnut, all of which are affected by chestnut blight. However, working in cooperation with the American Chestnut Society, the park has treated individual trees, which may enable production of viable fruit and/or pollen. The park also has been invaded recently by hemlock wooly adelgids, which continue to move north after becoming established in more southern areas of the United States. Hemlock wooly adelgids, another Asian invasive, sap strength and eventually may kill infected trees when adelgid populations have reached critical levels. In addition, many ash trees, particularly white ash, at and in the vicinity of the park have been infected by "ash yellow", which has resulted in extensive die-offs and branch kills.