American Elm

The quickest tell is the leaf base. The Morton Arboretum points out that one side of the leaf is shorter than the other where it meets the stalk, and the edge is double-toothed. From a distance it is the shape. The silvics manual notes that an open-grown elm usually forks near the ground into wide, arching limbs, and that gives the classic vase: narrow at the base and wide and flat at the top.

Two fungi cause the disease, Ophiostoma ulmi and O. novo-ulmi, according to the University of Wisconsin. They travel two ways. Elm bark beetles breed in dying elms and carry spores to healthy twigs, and root grafts carry the fungus underground between neighbours. The first sign, Morton says, is usually in early summer: leaves on one upper branch curl, turn grey-green or yellow, then brown. Morton suggests checking a valuable elm weekly from early May through July and monthly through September, and getting a lab diagnosis, because the disease is hard to confirm by eye. Sampling and symptoms are covered step by step in our Dutch elm disease guide.

Resistance is not immunity. Both Morton and Wisconsin say so. A resistant elm can still be infected, but the disease moves more slowly, which leaves time to prune or inject. The silvics manual adds that American-elm selections, even ‘American Liberty’, still trail cultivars bred from Asian or European elms.

Plant them as part of a mix. One species down a whole street is how the elm era ended and how the ash era ended too. Set the gaps for a 70-foot vase with the planting spacing calculator.

The silvics manual describes a second killer with nearly the same first symptom. Elm phloem necrosis also shows up as flagging or browned leaves. The difference is under the bark: the inner bark turns butterscotch-coloured and smells of wintergreen. A leafhopper spreads it, it also moves through root grafts, and trees usually die within a year of symptoms. Verticillium wilt causes dieback that looks like Dutch elm disease too. The Morton Arboretum adds elm leaf beetle and leaf miner to the list, and UF/IFAS notes that American elm hosts Ganoderma butt rot, a decay at the base of the trunk that matters to anyone judging whether a big elm will stand.

That is why both Morton and Iowa State send you to a lab before anyone condemns the tree. The size of the stakes explains the caution. On good forest sites the silvics manual records elms of 98 to 125 feet and 48 to 60 inches DBH, and UF/IFAS describes the wood as very hard. Removing one is a large job, and so is losing one by mistake.

Wisconsin lists pruning among the ways people spread the disease, and the beetles that carry it are active through the growing season. Morton notes they run two generations a year in the Midwest. Wisconsin's general rule is to prune deciduous trees from late fall into winter, when insects and pathogens are inactive. It names elm among the trees that bleed sap if cut in late winter, and says late spring and summer are poor times to cut anything because pathogens are about. The bleeding looks bad but does not harm the tree. The date that matters for elm is the beetle date, and the pruning calendar keeps the elm row empty all summer.

A diseased elm left standing becomes beetle habitat, so removal is part of the treatment. Wisconsin's order of work is to break the root grafts to nearby elms first, with a trencher or vibratory plow, then fell. Morton says this matters when elms stand within 25 to 50 feet of each other. Skip that step and the removal itself can push the fungus into the neighbour.

The wood is the second half of the job. Burn it where your ordinance allows, bury it, or chip it. If you keep logs, Wisconsin says to debark them and cover the pile with a heavy tarp, edges buried. Morton warns against keeping elm firewood with the bark on. Elm is also stringy and hard to split, as the firewood chart notes. Price the removal in the tree removal cost calculator, and check permit rules first. Wisconsin's burning advice comes with “where allowed by local ordinance” for a reason.

The causes, the two spread routes and the replanting plan are covered at length in Dutch elm disease: symptoms, spread and sanitation.