The American beech (Fagus grandifolia) is native to forest regions east of the Mississippi River and is a key tree species in the United States. In particular, it is very abundant from Ohio to the New England states. The tree is also found in the southernmost portions of eastern Canada, especially southern Ontario. Since 2012, though, American beech trees have been distressed by a lethal leaf disease—termed beech leaf disease—that has spread rapidly from Ohio into the other aforementioned geographical areas. Most cases of the disease have been detected in forest regions, but landscaped locations also have been affected. The external symptoms of the disease include striped banding and shriveling of tree leaves, aborted budding, reduced production of leaves, and premature leaf drop. More importantly, beech trees with beech leaf disease often die within a relatively short span of time after being inflicted by the disease—within 2 to 3 years for saplings and within 6 years for larger and older trees. See also: Beech; Forest; Forestry; Leaf; Tree; Tree diseases
Plant pathologists believe that beech leaf disease alters the photosynthetic processes of affected trees. Although the exact method is undetermined, the disease likely interrupts the photosynthetic pathway that supports normal leaf structure and production. Prior to 2019, the cause of the disease was completely unknown. In 2019, though, plant researchers pinpointed an invasive nematode—Litylenchus crenatae—as the probable culprit. This worm is typically found in certain beech trees (Fagus orientalis) in Asia, but it does not damage the trees on that continent. However, it does appear to harm the American species of beech, and scientists are alarmed by the disease's epidemic-like spread and severity. See also: Invasion ecology; Invasive species; Nemata (Nematoda); Photosynthesis
In addition, the American beech is often affected by a serious fungal disease that attacks the tree bark. Beech bark disease is already prevalent in many locations where beech leaf disease is present. As such, plant pathologists fear that the viability of American beech trees may be reduced dramatically. Questions also remain as to the exact nature of the nematode's ability to cause or spread beech leaf disease. For example, some researchers believe that the feeding of the worm is the direct cause of the disease, whereas others suspect that the disease is transmitted by a microbial pathogen that is carried by the worm. As for the method of how the disease extends into new geographical regions, investigators surmise that the worm is disseminated across great distances by birds and/or mites. See also: Plant pathology; Population ecology; Population viability
