[8] Since its discovery in 1910, new forms of the fungus, specifically Ophiostoma novo-ulmi, have emerged and appear to be more resistant to control measures and more aggressive in their infection.

In North America, Ulmus americana, U. thomasii, U. alata, U. serotina and U. rubra are listed as highly susceptible to Dutch Elm disease, while U. crassifolia is less threatened.

Trees that have been infected by a vector will exhibit symptoms of leaf wilting and yellowing on branches and twigs that have been colonized by the Scolytid beetle.

Symptoms of brown streaking that runs in the direction of the grain of the wood, and tylosis formation by the tree as a reaction to the fungal infection are characteristic of this disease.

This tissue is characterized by long xylem vessels with relatively thin walls, making it the ideal habitat for the pathogen.

[9] The pathogen enters its host with assistance from the Scolytid beetle, and will colonize the tunnels, or breeding galleries, made by the insect.

Because of this, it is common for the fungus to avoid branches with small diameters and localize in areas with thick bark, high moisture, and abundant nutrients.

[7] Ophiostoma ulmi can reproduce asexually by overwintering in both the bark and upper layers of dead or dying elm wood as mycelia and synnemata.

At the same time, the fungus secretes enzymes that break down the cell walls of the tree and allow the mycelia to grow into the xylem tissue.

[7] As the fungus grows it creates blockages in the vascular system of the tree, causing the characteristic symptom of wilting in the leaves.

As new beetles bore through the xylem tissue, they come into contact with conidia in the sap which stick to their bodies and can be transmitted to other trees that they feed on.

The perithecia can form singly or in large groups, and typically will have a long neck like structure with a black ball at the top.