According to Partridge (1997), Ophiostoma ulmi s.l. is a complex fungus. It has four spore types: conidia produced on mycelium, conidia borne on a mycelial stalk (synnema), yeast like spores that are variable in size, and ascospores, which are produced in a black fruiting body (perithecium) which ooze through the long neck of the perithecium, and accumulate at the tip in sticky mass.

Please see PaDIL (Pests and Diseases Image Library) Species Content Page Fungi: Dutch elm disease\r\n for high quality diagnostic and overview images

Ophiostoma ulmi s.l. was first discovered and studied in the Netherlands by seven women researchers, hence the name Dutch elm disease (Partridge, 1997).

Ophiostoma ulmi s.l. has three asexual phases: a yeast phase, a Sporothrix -like stage, and a Graphium stage (Anacker, 2001).

Hosts: Host trees include all the Euro-American native elms as field elm (Ulmus minor), wych elm (Ulmus glabra), european white elm (Ulmus laevis), American elm (Ulmus americana), red or slippery elm (Ulmus rubra), rock elm (Ulmus thomasii), and cedar elm (Ulmus crassifolia). Asian elm species are generally much less susceptible than Euro-American native elms. The Chinese elm (Ulmus parvifloia) is occasionally infected by the disease but is much less susceptible than American elm. Vectors: O. ulmi is vectored by a number of bark beetles, the efficiency of which is dependant on thei body dimension. In America the main species are Hylurgopinus rufipes, the American elm bark beetle and Scolytus multistriatus, the introduced smaller European elm bark beetle, while in Europe the main vector is the large European bark beetle Scolytus scolytus, and S. multistiatus, S. pygmaeus and S. kirschii are also active (see also Webber, 2000).

Spores of the Dutch elm disease fungus are carried on the bodies of beetles and deposited in egg galleries and tree wounds (Stack et al., 1996).

Stack et al. (1996) notes that the observable symptoms and the progression of Dutch elm disease differs among trees that are inoculated through beetle feeding and those that are infected through root grafts. Trees infected by beetles first show wilting, curling and yellowing of leaves on one or more branches in the upper portion of the tree. Large trees may survive and show progressively more symptoms for one or more years. In addition, because of their position, the infections arising from native elm bark beetle inoculations have a head start in spreading through the tree. Frequently, by the time first symptoms are noted, the fungus has already reached scaffold branches or the main trunk of the tree. Once the fungus is established within a tree, it spreads rapidly via the water-conducting vessels. The tree forms gums within these vessels in response to the presence of the fungus, causing the tree to wilt and eventually die. Trees infected through root grafts wilt and die rapidly; this frequently occurs in the spring soon after the trees have leafed out and progresses from the base of the tree upward.

Preventative measures: According to Stack et al. (1996), Dutch elm disease cannot be eliminated once it begins. A year-round community sanitation program is the key to slowing the spread of the disease. The most available control is removing infected trees and promptly destroying the wood. If infected wood is to be used as firewood, it should first be debarked. Trenching to disrupt root grafts is also recommended to protect healthy elm trees near diseased ones. In urban situations, insecticide spraying of high value trees has been effective in keeping bark beetles from attacking susceptible trees. In ornamental plantings, suggested control measures include planting trees further apart to prevent root grafts or choosing mixed tree species. The use of resistant selections for new plantations is strongly recommended.

The American Phytopathological Society (APS) offers on its website illustrated lessons to introduce the symptoms and signs, pathogen biology, disease cycle, epidemiology, disease management, and scientific, economic and social significance of major plant diseases. Please follow this link \r\nDutch elm disease for details.