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Disease Cycle and Epidemiology

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Disease cycle
The Dutch elm disease pathogens overwinter in the bark and outer wood of dying or recently dead elm trees and in elm logs as mycelia and synnemata with conidia. The fungi are spread from these sites by their vectors - elm bark beetles (Figure 9). Two beetle species spread the pathogens in North America: the smaller European elm bark beetle (Scolytus multistriatus) and the native elm bark beetle (Hylurgopinus rufipes). The adult female beetle bores through the bark of dead or dying elm trees and elm logs and creates a tunnel in the wood as she feeds. She lays eggs in the tunnel behind her. The eggs hatch into larvae (Figure 10) that begin to feed, creating tunnels at right angles to the maternal tunnel.


Figure 9	Figure 10

The resulting pattern of tunnels is called a gallery (Figure 11). The larvae pupate and emerge through the bark as adults (Figure 12). If the fungi are present in the tree or log, the emerging adults carry thousands of sticky conidia on their bodies.


Figure 11	Figure 12

Newly-emerged S. multistriatus adults feed in the twig crotches of elm branches (Figure 13); newly emerged H. rufipes adults tunnel in the bark of elm branches and trunks. As the beetles feed, fungal spores are deposited. The beetle vectors only feed on healthy elms for a few days. Then they fly to dying or recently dead elm trees or to freshly cut elm wood to feed, create galleries, and lay eggs.

The spores dislodged from elm bark beetles in feeding wounds and tunnels germinate and produce mycelium that grows into the xylem. The mycelium produces millions of small, white, oval conidia that spread through the xylem sap.


Figure 13	Figure 14

The fungi also produce enzymes and probably toxins that degrade plant cell walls and kill xylem parenchyma cells. In addition, the fungi induce hormonal imbalance that leads to the formation of tyloses (Figure 14), overgrowths of parenchyma cells that push into and block the water-conducting xylem cells. The blockage of the xylem by tyloses and gums (thought to be products of plant cell wall breakdown) causes one of the diagnostic symptoms of Dutch elm disease, wilting of leaves. The killing of xylem parenchyma cells causes another diagnostic symptom, brown discoloration just under the bark.

Epidemiology
Infections that take place in the spring or early summer involve “springwood” which has very long xylem vessels. In these vessels the fungi can spread rapidly throughout the tree, which then may die quickly. Later in the season, the fungi are restricted to the much shorter vessels of the “summerwood,” and the fungi spread much more slowly in the tree. Localized infections often result, and the tree is likely to survive longer.

Figure 15

Healthy elm trees can become infected by the feeding of spore-contaminated elm bark beetles or through the development of grafts between their roots and the roots of infected trees (Figure 15). Trees infected via beetle vectors often first develop symptoms in an upper section of the crown, whereas trees infected via root grafts often first develop symptoms lower in the crown. When the fungi are introduced through a root graft, they can be quickly distributed throughout the tree in the vascular system, and the entire tree may soon wilt and die. Root grafts form naturally between closely spaced elm trees with intertwined roots. Large elms growing within 7 meters (20 feet) of each other have almost 100% chance of becoming infected through root grafts. The likelihood of spread is lower when the elms are at least 13 meters (40 feet) apart.

The severity and rate of spread of Dutch elm disease depend on the species of the pathogen, how rapidly the elm bark beetles reproduce, the level of susceptibility of the elm hosts, and the environment. Temperatures around 20°C (68°F) favor the formation of conidia, whereas perithecia are induced at temperatures of 8-10°C (46-50°F).

In the absence of effective disease management, Dutch elm disease increases exponentially until an affected elm population is greatly depleted. Seedlings and many saplings escape and live long enough to reproduce, so even the most susceptible elm species have never been threatened with extinction by Dutch elm disease. Wild elm populations in the eastern and Midwestern U.S. have increased in recent decades, and this increase has led to renewed prominence of Dutch elm disease in landscapes