Dutch Elm Disease
|Dutch elm disease cause leaves to wilt
||Leaves then brown and die
Dutch elm disease (DED) is the most devastating shade tree disease in North America. It is a wilt disease with an extremely high fatality rate among elms.
How Dutch Elm Disease Kills
Dutch elm disease (or DED) is caused by a fungus. After the disease is contracted, spores rapidly reproduce and spread toxins throughout the tree.
The fungus Ophiostoma (Ceratocystis) ulmi attacks various species of elm. It can kill a tree within a few weeks or it can kill it gradually over a period of years. The fungus blocks the water-conducting or vascular system of the tree preventing water and minerals from reaching the branches and leaves. The leaves wilt and eventually the tree dies.
There are two strains of the fungus in North America - the non-aggressive strain (O. ulmi) and the aggressive strain (O. novo-ulmi). While the elm’s natural defense mechanism tries to fight off the fungus, the aggressive strain often moves too quickly for the tree to react without human intervention.
A younger fast-growing tree can die quickly, but some younger trees seem to have a degree of natural resistance to DED. However, this resistance tends to wear off after 15-20 years. Slow-growing older trees can linger for a year or two.
An infected elm tree usually exhibits symptoms soon after infection. Because of the speed with which the disease attacks, detecting symptoms as early as possible is essential for treatment. The first sign of the disease is the sudden wilting of leaves in the upper reaches of the tree. Next, the leaves change colour from green to yellow to brown. They then shrivel and die.
If the infection occurs very late in the season, the leaves will appear to fall normally. However, the following spring the new leaves will be smaller than normal. The tree will often die before mid-summer. In late summer, it may be difficult to distinguish between wilting and natural fall colours.
Control efforts have focused on two areas - prevention and treatment. The spread of Dutch elm disease can be effectively checked with a stringent sanitation program involving surveillance, timely pruning and proper disposal of infected wood. Directly attacking the elm bark beetle population is less effective and often not necessary - only a very small percentage of the beetle are carriers of the disease.
Preparing your elm by improving the soil with mycorrhizal fungi and fertilizer can also boost a tree's strength. Click here for more.
Treatments have so far been based almost entirely on fungicidal injections. There are indications that new treatments resulting from advanced molecular biology research will become available in the future.