The Dutch Elm Disease (DED) is fungi related and affects elm trees while elm bark beetles further propagate it. It is believed to be initially from Asia but later found its way and spread to Europe, North America, and New Zealand accidentally. This disease led to the distraction of elm trees that were resistant to the disease across Europe and North America. The name Dutch Elm might lead one into thinking that it is of Dutch origin but the name is because research on the DED was first done by Dutch pathologists named Bea Schwarz and Christine Buisman. The two pathologists worked under Johanna Westerdijk, a Professor. The disease, however, does not explicitly affect the Dutch elm hybrid trees but also the species Ulmus and Zelkova.
Causes And Vectors Of DED
Three types of fungi present in genus Ophiostoma which only grows and reproduces on elm trees cause the Dutch elm disease.One of the fungi types is called Ophiostoma ulmi which in the 1900s destroyed many trees across Europe (Dutch elm pandemic). This type managed to find its way to North America when timber from the affected areas was exported to North America in 1928. In America, measures were put to counter the disease but did not eradicate the DED which attacked the very vulnerable American elm (Ulmus americana) trees. This fungus can both eat the living tissue (parasite) of the tree and also feed on the dead tissue of the tree (saprophyte).
The second type is called Ophiostoma novo-ulmi which was first discovered in the late 1940s in Europe and US. O.novo-ulmi is more aggressive than the O.ulmi, and as a result, elm trees were significantly destroyed, and in 1989 it found its way to New Zealand. New Zealand almost managed to eradicate it entirely, but due to lack of funds, the measure failed. This fungus affects the stems and roots of elm trees. The third type of fungi is Ophiostoma himal-ulmi which was discovered in 1993 and mainly affects the Western Himalayas.
The Dutch elm disease is spread by three bark beetles (vectors). The first vector is the American Hylurgopinus rufipes beetle which is a native elm bark beetle. The second beetle is Scolytus multistriatus which is a small European bark beetle that spreads the disease much faster compared to the former beetle. The last beetle is the Scolytus schevyrewi which is a banded beetle. There are also other known vectors such as the Scolytus sulcifrons and Scolytus pygmaeus. Root grafts are also known to transmit the disease.
These vectors lay their eggs on weak elm trees, and when the fully grown beetles emerge, they go to other healthy trees.
Signs And Symptoms
The first sign of the Dutch elm disease is that the top branches of a tree start to wither and turn yellow, dull green, brown or start to curl during the summer period. This color change takes place months before the autumn leaves begin to fall. The infection then slowly affects the rest of the tree which leads to the death of more branches. The dying branches will eventually develop a brown to black color. It leads to the death of young elm trees in about two months while the older trees may take up to more than two years. The roots of some species like Ulmus procera may even take up to fifteen years to die off. This disease is sometimes confused to be other diseases like elm phloem necrosis, and therefore, lab examination is vital.
Controlling The DED
This involves exclusion of the vectors and burning all the dead and weak elm wood with tight barks or burying them to prevent them from sprouting. The trees can also be sprayed with a spray that coats the tree surfaces (like methoxychlor) which will eradicate a lot of beetles. There are also reports that some fungicides can be injected into the sapwood. These measures, however, tend to protect trees rather than cure them.