2004-08-25 WS & CC Meeting - Laserfiche WebLink

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Fungus Transmission by Beetles In the U.S. the fungus is spread by the smaller European elm bark beetle, Sco/ytus multistriatus, and the native elm bark beetle, Hylurgopinusrufipes (figure 2). The European beetle is the primary vector (disease carrier) in the southern portion of Minnesota, including the Twin Cities, while the native beetle is the major vector in the northern portion of the state. Native and European elm bark beetles use dead or dying elms for breeding. In areas where disease is present, most dead elm wood is already infested with the Dutch elm disease fungus. Beetle breeding tunnels in infected wood become filled with fungus spores, resulting in contamination of the bark beetles. Emerging adult beetles carry fungus spores inside and outside their bodies. Thus, when beetles breed in infested elm material, It is the first step toward the spread of Dutch elm disease to healthy trees. Figure 2. European elm bark beetle (left) and native eim bark beetle (right). Both are about 118 Inch long. The second step occurs when the beetles feed on healthy trees. The European beetles fly to healthy elms during this phase and feed in smalllwig crotches, and the native beetles feed on larger branches. The water-conducting vessels (the xylem) are exposed to the beetle-carried spores as the insects feed through the bark. The tree responds to the fungus by plugging Its vessels. This defense mechanism results in flagging (wilting) of one or more of the upper branches. Once established, the fungus can move throughout the tree's water-conducting system, resulting in systemic infection. The initial fungus spread, however, is often relatively slow and on occasion may not become systemic. Such slow-developing infections are not very apparent until the fungus moves into the larger branches and rapid wilting occurs. Fungus Transmission Through Root Grafts The fungus also can spread from tree to tree through root grafts (roots naturally fused together), especially if spacing is less than 30 feet between elms. Root grafting may occur between larger trees up to 60 feet apart (figure 3.) Death of a tree infected through root grafts is much more rapid (a few weeks or less) than that caused by beetles. Figure 3. The Dutch elm disease fungus can spread through root grafts. 3