Fraxinus americana, white ash, is a large native shade tree of eastern and central North American forests, historically prized as one of the best fall-color trees available — its compound leaves reliably turn deep burgundy, purple, and gold, often on the same tree — and for wood so strong and shock-absorbent it was the standard material for baseball bats, tool handles, and hockey sticks for generations. Small, inconspicuous, wind-pollinated flowers appear in spring before the leaves, with no ornamental display.
White ash and its close relatives are now facing a genuinely dire threat: the emerald ash borer (Agrilus planipennis), an invasive Asian beetle first detected in Michigan in 2002, has since spread across most of the tree's eastern and midwestern range and kills the overwhelming majority of untreated ash within a few years of infestation — hundreds of millions of trees have already died, and white ash is now classified as Critically Endangered by the IUCN. A healthy-looking tree today can be dead within 3–5 years of the borer's arrival in an area. Planting a new white ash without a realistic, ongoing plan for systemic insecticide treatment (a certified arborist can apply trunk injections on a 1–2 year cycle) is planting a tree with a firm expiration date in most of this species' range; where EAB is already established, treating an existing mature specimen is usually a far better use of resources than planting a new one.
Verticillium Wilt
Lower leaves yellow and wilt, often on one side of the plant, with brown streaking in the stem and crown tissue when cut open. Plants are stunted and decline over the season.
Remove and destroy affected plants. Avoid planting in soil that has grown infected tomatoes, peppers, or other susceptible crops. Rotate beds and improve drainage; there is no cure once a plant is infected.
Fungal leaf spot
Brown or purplish spots on the leaves, often ringed with a yellow halo, that spread and merge in warm, humid, or wet weather; heavily spotted leaves yellow and drop early.
Pick off spotted leaves and clear fallen debris to remove overwintering spores. Improve air circulation by thinning, and water at the base so foliage dries quickly. If it persists, apply a copper- or sulfur-based fungicide at the first sign.
Emerald Ash Borer
Agrilus planipennis →Larval tunneling under the bark girdles the tree's phloem, cutting off nutrient transport; causes canopy dieback progressing from the top down, bark splitting, and death of the majority of untreated ash within 3–5 years of first infestation.
Systemic insecticide trunk injection (emamectin benzoate) by a certified arborist, repeated on a 1–2 year cycle, for trees caught early enough to be worth saving. Biological control via released parasitoid wasps (Tetrastichus planipennisi, Oobius agrili, Spathius spp.) is underway in many states as a longer-term population suppression strategy, not a rescue option for individual trees.