Dutch elm

The leaves on one or more branches of a stricken tree suddenly wilt, turn dull green to yellow or brown, curl, and may drop early. Young, rapidly growing elms may die in one to two months; older or less vigorous trees sometimes take two years or more to succumb. A brown to black discoloration occurs in the white sapwood of wilting branches just under the bark. Because symptoms are easily confused with other diseases, especially elm phloem necrosis and diebacks, positive diagnosis is only possible through laboratory culturing. The fungus can spread up to 50 feet (15 metres) from diseased to healthy trees by natural root grafts. Overland spread of the fungus normally occurs by the smaller European elm bark beetle (Scolytus multistriatus), less commonly by the American elm bark beetle (Hylurgopinus rufipes). Female beetles seek out dead or weakened elm wood to excavate an egg-laying gallery between the bark and the wood. If the fungus is present, tremendous numbers of fungal spores (conidia) are produced in the galleries. When young adult beetles emerge through the bark, many carry the spores on and in their bodies. The infection of healthy elms occurs when beetles feed in the leaf axils and young twig crotches of healthy trees. Some spores are dislodged and get into these trees’ water-conducting vessels (xylem), in which they reproduce rapidly by yeastlike budding. The weakened elm is quickly colonized by hordes of beetles, and the cycle is repeated.