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How to Identify and Manage Dutch Elm Disease Page 7 of 16 Figure 4. The disease cycle of Dutch elm disease is closely linked to the life cycle of elm bark beetles. (Artwork by Julie Marlinez, Scientific Illustrator, St. Paul, MN) Ei1l!!fe 4. Full-scale image Tbe elm host. Native species of North American elms vary in their susceptibility to OED, even within species. American elm (Ulmus americana L.) is generally highly susceptible. Winged elm (U alata Michx.), September elm (U serotina Sarg.), slippery elm (U rubra Muh!.), rock elm (U thomas;; Sarg.), and cedar elm (U crossifolia Nutt.) range from susceptible to somewhat resistant. No native elms are immune to OED, but some individuals or cultivars have a higher tolerance (and thus may recover from or survive with infection) or resistance to OED. Many European and Asiatic elms are less susceptible than American elm. In addition to genetic factors present in some cultivars and species, physical factors affect tree susceptibility. These factors include time of year, climatic conditions (such as drought) and vitality of the tree. Water conducting elements are most susceptible to infection as they are being produced in the spring, thus elms are most susceptible to infection after earliest leafing out to midsummer. Trees are less susceptible under drought conditions. Vigorously growing trees are generally more susceptible than slower growing trees. Tbe Dutch elm disease fungus. OED can be caused by either of two closely related species offungi: Ophiostoma ulmi (Buism.) Nannf. (formerly called Ceratocystis ulmi) and Ophiostorna novo-ulmi Brasier. The latter, which is more aggressive in causing disease, was recently recognized as being a separate species. The OED fungus was first introduced to the U.S. on diseased elm logs from Europe prior to 1930. It is unknown when the more aggressive species became established in the U.S.; however it was possibly present as early as the 1940's- 1950's, and most likely caused much of the devastating mortality through the 1970's. The less aggressive species is becoming increasingly rare in nature, and the aggressive species is thought to be responsible for most of the current mortality. Although some local resurgence of OED has been observed, there is no evidence that it is due to a change in the pathogen. Localized resurgence is more likely due to the following: (1) a decrease in vigilance in monitoring and sanitation, (2) a build-up in populations of the insect vectors, or (3) ingrowth of susceptible host trees in the wild. Spread by elm bark beedes. Overland spread of OED is closely linked to the life cycles of the native elm bark beetle (Hylurgopinus rufipes Eich.) and the smaller European elm bark beetle (Sco/ytus multistriatus Marsh.) (figure 5). Both beetles are attracted to stressed, dying or dead elm wood to complete the breeding stage of their life cycle. The adult beetles tunnel into the bark and lay their eggs in tunnels (called galleries) in the inner bark. The eggs hatch and the larvae feed in the inner bark and sapwood. The larvae mature into adults and emerge 3 mm or 0.12 inches from the elm wood. If the OED fungus was http://www.na.fs.fed.uslspfolpubslhowtoslht_dedlht_ded.htm 8/2512004