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P 3bo Dutch Elm Disease (Plate 177) gc. This famous disease is caused by Ceratocystis ulmi,* which entered beetles are contaminated with spores of C. ulmi. These germinate in North America in elm logs from Europe before 1930. Carried by elm egg galleries that the beetles excavate in the bark, and the fungus bark beetles (see Johnson and Lyon, 2nd ed., Plate 116) and in elm grows saprophytically. If the breeding site is a branch or trunk killed by wood by people, the fungus spread within 5 decades from multiple Dutch elm disease, C. ulmi is already on hand in the outer sapwood points of introduction in eastern states and provinces to most places and cambial region when the insects arrive. The fungus produces where elms grow in North America. spores in the beetles' galleries, and as the insects molt to adulthood, Infection usually begins when leaves reach full size, and symptoms they become contaminated. On emergence from the site of breeding, develop rapidly during a period of 4 -6 weeks. The primary symptoms they carry the fungus either to sites of feeding, where new infections are internal: death of xylem parenchyma cells, loss of water- conduct- begin, or to breeding sites where saprophytic growth of the fungus ing ability of xylem vessels, and browning of infected sapwood in resumes. narrow streaks that follow the grain. The streaks are closely associated The severity and pace of a Dutch elm disease epidemic depend with vessels in the innermost part of the youngest annual sheath of upon the strain of C. ulmi, the reproductive success of the vectors, and wood. Wood produced after vessel formation, closer to the bark, the level of elm susceptibility. C. ulmi varies in aggressiveness, or the remains normally colored for a time. Infected wood in the current ability to colonize and damage elm trees. Aggressive strains, which season's shoots may show diffuse brown color associated with the have replaced nonaggressive ones in most North American and Euro- scattered arrangement of xylem vessels. Secondary symptoms are the pean localities, kill elms rapidly and thus favor vector populations and familiar wilting and browning early in the season, yellowing and fast epidemic development. The origins of aggressive versus nonag- defoliation during summer, and branch death either during the grow- gressive Vector tra of C. ulmi are unclear. ing season or during dormancy. to high levels in early years of an epidemic Symptoms usually arise on one branch and gradually involve the and remain high until nearly all large elms in a locality are gone. entire crown. All branches may die within a few weeks or during Natural enemies of the vectors are not significantly effective while several years. In a tree that is somewhat resistant, branch death maybe breeding sites are abundant. Reproduction of the smaller European localized and intermittent as the tree sustains and resists repeated elm bark beetle is limited by severe winters in parts of the northern infections. In a branch or trunk that survives infection, normal sap- United States and in Canada. wood forms outside the sheath containing the brown streaks. Trees Elm species vary from extremely susceptible to nearly immune. that sustain repeated infections have brown streaks in more than one Highly susceptible species include American (or white), Belgian, En- annual sheath of wood. The pathogen remains alive in these streaks for glish, red, rock, September, European white, and winged elms. Those years. Symptoms are caused in part by toxins (three identified here- of intermediate susceptibility are cedar, European field (or smooth - tofore) secreted by the fungus. In western areas where summers are leaf), and wych (or Scots) elms. Chinese, Japanese, and Siberian elms dry, symptoms of Dutch elm disease are often masked by those of heat as well as many lesser -known Asiatic species are resistant. Even within stress and water shortage. Yellowing, premature defoliation, and susceptible species, however, resistant individuals occur. In addition, branch dieback are common there, whether caused by C. ulmi or by tree breeders have developed several resistant clones suitable for other factors. temperate regions of North America. These include, but are not lim- Structures of the pathogen are inconspicuous and vary with its life ited to, the cultivars 'Dynasty', 'Groeneveld', 'Homestead', 'Jacan', stage and environment. In xylem vessels in early stages of disease, C. 'Pioneer', 'Regal', 'Sapporo Autumn Gold', 'Thompson', and 'Urban', ulmi exists mainly as spores that reproduce by budding. During all derived from Asiatic and European elm species. A group of Ameri- saprophytic growth as mycelium, it produces conidia on simple con- can elms collectively called 'American Liberty', introduced in 1985, idiophores or on black - stalked synnemata (H) in galleries made by elm have less resistance than the cultivars listed above but may be useful in bark beetles. Synnemata consist of multiple conidiophores that pro- areas where elm yellows (Plate 188) does not occur. Elms resistant to duce spores in slimy drops up to 1 mm across. Sometimes perithecia C. ulmi are able to restrict the pathogen to small brown streaks near the develop among or following synnemata, but these are seldom noticed. point of introduction. Perithecia produce ascospores that, like the conidia, are in a mucilagi- Control of Dutch elm disease in susceptible elms depends mainly nous drop and are thus well suited to adhere to bark beetles during upon denying elm bark beetles places to breed. This means a continu- dispersal. Diagnosis usually depends on isolation and identification of ing program of detection, removal, and burial or destruction of all the fungus on a laboratory medium. dying or recently dead elm wood in and near elms that are to be C. ulmi is distributed long distances in elm logs and firewood. Elm preserved. Once this basic requirement is met, many additional tech - bark beetles transmit it locally and for distances up to several kilome- niques can be applied for higher levels of protection. Elms resistant to ters. The native elm bark beetle, Hylurgopinus rufipes, and the smaller C. ulmi do not require protective measures. European elm bark beetle, Scolytus multistriatus, are the vectors in References: 144, 145, 240, 280, 302, 317, 626, 628, 725, 1048, 1395, North America. S. multistriatus, inadvertently introduced in elm logs 1435, 1606, 1788, 1804, 1909, 1966 -68, 2054, 2129 from Europe before 1900, predominates in most areas. H. rufipes is *Note added for the 1989 printing: Ophiostoma ulmi is now the preferred more important in Canada and in the northern parts of border states. The insects are attracted to healthy elms by volatile chemicals pro- name. duced by the trees. Beetles bore into the inner bark, cambial region, and outermost sapwood and, while feeding, deposit spores of C. ulmi that cause infection. S. multistriatus feeds in twig crotches; thus most Dutch elm disease in American elm. infections begin in twigs. H. rufipes bores in the bark of branches and A, B. Wilting, yellowing, browning, and dieback. small trunks, initiating infections that involve major branches from the C. early Wilting, and browning, most common in late spring and outset. From the point of inoculation, the fungus moves upward and appar- D. A diseased branch at midsummer, with droopy leaves that may ently also downward by two alternating modes: passive transport of either wilt and die or turn yellow and drop. spores in liquid within xylem vessels and growth as hyphae between E. Streaks in outer sapwood of a small trunk, exposed by removing vessels after spores germinate at points of vessel contact. C. ulmi bark (A —E: NY, Jun —Jul). regularly reaches the roots of large trees within the 1st season of F. A slant cut through a twig that became infected in its 4th year of infection. The fungus proliferates in the roots and then ascends the growth. Discolored streaks appear as brown dots in the most re- trunk in a wave of systemic infection that kills the tree or a major part of cently formed wood. it. Where elms are spaced 10 meters or less from one another and their G. A slant cut through a diseased shoot in its first season of growth, roots are grafted together, C. ulmi may move from one tree into the H revealing Magnified diffuse brown f Ceratocystis color in xyl elm wood. Synnemata are As next through o of an roots. parts o elm die, whether from Dutch elm disease or other up to 1 .5 mm tall. Hyphae have a granular appearance because of au <es, elm bark beetles aggregate and tr. items or branches 3 c m or Ihe presence of conidiphores with clusters of colorless conidia (F- ■ , vv r ��.