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<br />How to Identify and Manage Dutch Elm Disease <br /> <br />Page 8 of 16 <br /> <br />present in the wood that the beetles <br />infested, the fungus produces sticky spores <br />in the beetle galleries. Spores of the DED <br />fungus are eaten by or stick to the adult <br />beetles as they emerge from diseased trees. <br />Adult beetles then visit healthy trees, feed <br />in twig crotches or branch inner balk, and <br />introduce the fungus into or near severed <br />wood vessels as they feed. <br /> <br />Figure 5. Overland spread of DED is closely tied <br />to the life cycles of the Native elm bark beetle <br />(top) and smaller European elm baIk beetle <br />(below). Note that the smaller European elm baIk <br />beetle is actually lsrger than the native elm bark <br />beetle. <br />(.4rtwork by Julie Martinez, Scientific IllU3tratar, <br />St. Paul. MN) <br /> <br />The importance of the two bark beetle species as vectors ofDED varies across the range of <br />elms. In northern areas (northern parts of Minnesota, Wisconsin, Maine, New York and <br />New England and most of Canada, where winter temperatures below _60 F are common), <br />the native elm bark beetle is the predominant vector. In other parts of North America, the <br />smaller European elm bark beetle predominates. The life habits of the adults of the two <br />species differ considerably, which has implications for management opportunities. These <br />differences are described below. <br /> <br />Smaller European elm bark beetles overwinter as larvae or adults within the stem of the tree <br />where they hatched. They emerge as adults in spring to feed in twig crotches of healthy <br />trees, where they can introduce spores of the DED fungus to the crown. High numbers of <br />beetles frequently will feed in a single tree, resulting in multiple points of infection. The <br />cycle is repeated when beetles then seek out diseased and dying wood to breed in <br />throughout the growing season, completing two or more generations per year. They have <br />the potential to rapidly build up high populations. <br /> <br />Adult native elm bark beetles tunnel into the bark on the lower stems of healthy elms to <br />overwinter. In spring they emerge to feed in the inner bark of elm branches and small stems <br />before beginning their breeding cycle. They repeat their life cycle as previously described. <br />They can transmit the DED fungus to healthy trees during the construction of overwintering <br />sites in fall, or, more commonly, during feeding in spring. <br /> <br />Once the DED fungus is introduced into the upper crown of healthy elms by bark beetles, it <br />slowly moves downward, killing the branch as it goes. Disease progression may occur <br />rapidly, killing the tree by the end of the growing season, or may progress gradually over a <br />period of two or more years. It is also possible that the tree may recover. The success and <br />rate of progression within the tree depends on tree size, time and location of infection in the <br />tree, climatic conditions, and response of the host tree. <br /> <br />Spread through grafted roots. Roots of the same or closely related tree species growing <br />near each other often cross each other in the soil and eventually fuse (become grafted) to <br />each other. <br /> <br />The DED fungus can move from infected trees to adjacent trees through these grafted roots. <br />Infections that occur through root grafts can spread very rapidly throughout the tree, as the <br />fungus is carried upward in the sapstream. Root graft spread ofDED is a very significant <br />cause of tree death in urban areas where elms are closely spaced (figure 6). <br /> <br />http://www.na.fsfed.us/spfo/pubslhowtoslht_dedlht_ded.htm <br /> <br />8/25/2004 <br />