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Remarks (public):For a complete description including images see 
Remarks (internal):Dutch Elm Disease is a vascular wilt condition, caused by the infection of the vessels by C. ulmi. Within the tree the fungus forms yeast-like cells which multiply and spread, producing a toxin that poisons the host and stimulates formation of tyloses which partially block the vessels, interrupting the transpiration stream. The first sign of infection is a yellowing and wilting of foliage of affected branches followed by leaf cast. The extremities of the twigs die, forming typical crozier-like hooks. Necrosis may proceed and result in the death of the whole tree within a year. However, this is rare, as spread of the fungus from an annual ring to the next seldom occurs. Final death of the host, therefore, depends on two or three re-infections and may take several years. Control largely rests in the destruction of infected trees and beetle-infested cut timber in the vicinity of elms. Recent insecticide and fungicide formulations (benomyl) show promise of good future control but their use may be expensive. Application is by injection.
Description type:Non-original description 
Description:Ceratocystis ulmi (Buisman) C. Moreau, Revue de Mycologie, Suppl. Colonial 17: 22, 1952.
= Ceratostomella ulmi Buisman, Tijdschrift over Plantenziekten 38: 1. 1932.
= Ophiostoma ulmi (Buisman) Nannf., 1934.
Conidial state: Graphium ulmi Schwartz, University of Utrecht, thesis, 1922.
Perithecia not easily found in nature, in damp conditions superficial on exposed wood chips or appearing partially immersed in fissures of the bark, globose, 90-135 µm diam., with a dark, slender neck, 170-350 µm which terminates in numerous hyaline ostiolar hyphae, 30 x 2 µm; base of the neck 16-24 µm diam., tip 11-16 µm diam. Asci not seen, dehiscing soon after formation. Ascospores crescent-shaped, hyaline, 4,5-6 x 1,5 µm. Cultures on PDA initially pale yellow to light brown, becoming floccose and felted and turning dark grey to greyish-brown or black with the onset of coremial development; often 2-10 hyphae growing together to form strands; aerial mycelium producing masses of conidia after 24 h and the whole mycelium becoming covered with conidia, perithecia rarely developing in cultures. Conidiophores of two types; simple hyphal or lateral cells scarcely differentiated from the aerial hyphae and producing masses of conidia from short sterigmata or lateral pore; the second type is what is referred to as the Graphium state with conidiophores formed of 3-20 erect black agglutinated hyphae several mm tall which flare at the tips to form a hyaline globose head with many ramifications. Conidia hyaline, obovoid to ellipsoid and somewhat variable in shape, 6,5-14 x 2-3 µm; formation is in a drop of mucilage and may form a head up to 200 µm diam.
Hosts: Ulmus spp., Zelkova serrara; U. glabra and U. americana are highly susceptible, U. procera is susceptible, while Asian elms, U. parvifolia, U. pumila and U. pumila pennato-ramosa, have marked resistance. Zelkova serrata is highly susceptible; the reactions of other species of Zelkova are not known.
Disease: Dutch Elm Disease.
Geographical distribution: Europe (throughout, to a northern limit in central Scotland, central Norway, central Sweden. Absent from north Russia). N. America (eastern Canada, central and eastern USA) (CMI Map 36, ed. 4, 1970).
Physiological specialization: Variation in pathogenicity between strains of the fungus has been recorded (25, 85). Gibbs & Brasier (1973) state that isolates of C. ulmi from Britain fall into two groups on the basis of cultural characters. Pathogenicity experiments show one group to be aggressive and the other to be non-aggressive. Some North American isolates resemble the aggressive isolates while isolates from Northern Europe are non-aggressive.
Transmission: By the bark beetles Scolytus scolytus, S. multistriatus and (in N. America) by Hylurgopinus rufipes. The fungus fruits in the larval galleries and spores are carried internally and externally to young shoots where beetles emerge and migrate. Infection of the vessels, which leads to the disease, takes place during the period before breeding, when the beetles feed, cutting grooves and boring in the thin bark of twigs. Transmission by root grafts is known (15, 266) and direct infection by airborne spores is theoretically possible but does not take place under natural conditions.
Literature: Peace, Forestry Commission Bulletin 33, 1960; Gibbs & Howell, Forestry Commission Forest Record 82, 1971; Gibbs & Brasier, Nature, UK 241: 381, 1973.

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