Dutch Elm Disease

Physical Description 

DED is caused by pathogenic fungi, O. ulmi and O. novo-ulmi, that lives in the xylem (vascular tissue) of elm trees.  

The mycelium of O. ulmi and O. novo-ulmi is creamy white and is composed of septate hyphae (a branching, filamentous structure with separated cells) containing haploid nuclei. 

Host Tree 

Dutch elm disease is caused by a fungal pathogen that lives in the vascular tissue of elm trees (Ulmus spp). 

Three species of elm are native to Canada: Rock elm (Ulmus thomasii), Slippery elm (Ulmus rubra), and White or American elm (Ulmus americana). Three more species, English elm (Ulmus procera), Scotch elm (Ulmus glabra), and Siberian elm (Ulmus pumila) are naturalized to Canada, meaning they are present and reproducing without human assistance. 

All elm species are susceptible to DED, but the most substantial impacts have been on American elm and Rock elm. 

The fungi which cause DED can spread from tree to tree through two modes of transmission, insect vectors and root grafts.

Elm Bark Beetles 

DED is primarily transmitted through species of elm bark beetle of the invasive genus Scolytus, including the smaller European elm bark beetle (Scolytus multistriatus), the larger elm bark beetle (Scolytus scolytus), and the banded elm bark beetle (Scolytus schevyrewi), and by  the North American native elm bark beetle (Hylurgopinus rufipes).  

Egg-laying female beetles are attracted to weakened trees, such as those infected by DED, where they dig maternal or egg galleries beneath the bark to deposit their eggs. These galleries reach the vascular tissue of the tree, on which the larvae will feed and take about 3 weeks for the female to build. After about one week, the eggs hatch and the larvae begin to dig larval galleries which run perpendicular to the egg galleries. 

The larvae feed on the tree’s vascular tissue for about 30 days before boring a pupal chamber where they undergo metamorphosis into pupae and then about two weeks later into adults. Adults emerge through holes bored out of pupal chambers (Santini and Faccoli, 2015). 

Broods that develop in trees infected by DED will be exposed to the fungus present in the vascular tissue and become carriers. As adults, they emerge covered in fungal spores and fly away to feed on healthy elm trees, creating feeding wounds in the twig crotches which introduce the fungal spores directly to the tree’s vascular tissue.  

Root Grafts 

DED can also be transmitted through root grafts (a natural fusion between roots of compatible plants growing near one another) between infected trees and healthy trees, particularly when the two trees are derived from the same parent plant. Root grafts form naturally between closely spaced elm trees with intertwined roots, and healthy trees growing within 7 meters of infected trees are nearly 100% guaranteed to become infected (D’Arcy, 2000).  

Once inside the tree, the fungus spreads within stems and roots of living elms both by passive transport of spores and by mycelial growth of colonies initiated by the spores that germinate in the vascular tissue. Spores are produced both sexually and asexually. 

Asexual Reproduction 

O. ulmi and O. novo-ulmi have two asexual forms which produce spores called conidia. In dying or dead trees, sticky, nearly colourless conidia form at the tips of 1-2 mm tall synnemata (a hyphal stalk with a sticky conidia head).

These conidia can stick to beetle vectors that tunnel into the wood. In healthy trees, small, white, oval conidia are formed in clusters on short mycelial branches. These conidia are transported through the vascular tissue, spreading the disease throughout the tree (D’Arcy, 2000). 

Sexual Reproduction  

Both DED-causing Ophiostoma species have two mating types (A and B). When the two mating types come together, they form ascospores inside structures called perithecia. These ascospores accumulate into sticky droplets which are discharged from an opening in the perithecia and can stick to and be disseminated by beetle vectors (D’Arcy, 2000).  

DED-causing fungi, O. ulmi and O. novo-ulmi, live in the vascular tissue of elm trees in the leaves, branches, roots, and trunk. All Elm trees native to Canada are susceptible.   

Flagging 

The first sign that an elm has been infected by DED is wilting and yellowing or browning of leaves in the tree’s upper branches which occurs out of season, usually in mid-summer. The fungus growing in the tree’s vascular tissue cuts off the sap supply and poisons the tree with fungal toxins, drying out leaves that wilt and die. This gradually spreads down to the rest of the tree, resulting in eventual death.  

Dead and dried-out leaves usually remain attached to the tree (this is referred to as flagging), however, trees infected later in the summer will usually undergo premature leaf drop. This can sometimes be confused with natural fall leaf drop. In the year following infection, leaves will fail to develop or develop undersized and wither soon after.  

Trees that are infected by a beetle vector will exhibit symptoms first in the upper crown, while trees infected through root grafts will first exhibit symptoms in the lower crown (D’Arcy, 2000).  

Epicormic shoots 

Dying trees may resprout from the base, forming epicormic shoots (shoots produced when a tree is under stress). These epicormic shoots can live for a number of years.

Internal Damage 

The damage caused by DED can also be seen within the tree and infection is often more widespread internally than shown by external symptoms. Healthy elm wood is cream-coloured, but infected branches display ring-shaped, brownish discolorations in the vascular tissue and exposed wood beneath the bark will have numerous brown streaks.  

Signs of Beetle Infestation  

Signs of bark beetle infestation, DED’s primary source of transmission, can also be seen on infected trees. Beetle emergence holes can be found on the bark and are about the size and diameter of a pencil lead. Sawdust on the bark may indicate a burrowing beetle.  

The beetles also leave characteristic galleries beneath the bark which can be used to identify their presence. The shape of the galleries is dependent on the type of bark beetle present. For example, the smaller European elm bark beetle and the banded elm bark beetle leave egg galleries that run vertically along the grain of the wood. Native elm bark beetle egg galleries run horizontally across the grain of the wood. In both cases, larval galleries will run perpendicular to the central egg gallery.  

Bark beetles themselves are dark brown to black, approximately 3 mm long with a rough shell sprouting short, stiff, yellow hairs. European elm bark beetles are smaller and more aggressive, the latter trait allowing them to be more successful colonizers than their native counterparts.  

DED Look-alikes 

Two other elm diseases, Verticillium wilt and Dothiorella wilt, may display similar symptoms to DED but are not as serious. Sampling and testing can be done to determine if DED or one of these look-alikes is present. 

DED emerged in the Netherlands shortly after the first world war. Quickly spreading from Europe, it was established in Great Britain by 1927, in the United States by 1930, and reached Eastern Canada by 1945, transported through infected lumber. The first confirmed case in Ontario was in 1946. During the 1970s and 80s, the majority of elm trees in Eastern Canada succumbed to the disease. By the 1980s, DED had spread west to Manitoba and Saskatchewan, but as of 2021, it has not yet invaded Alberta and British Columbia, where elm trees are present but have been planted beyond their natural range (Hubbes, 1999).  

Ecological Impacts 

As all elm species native to Canada are susceptible to DED, the disease wiped out most of the population in Eastern Canada. However, some young and medium-aged elm trees can still be found in the wild. The most devasting effects can be seen on American elm populations, of which 95% were wiped out (GRIPP 2018).  

Economic and Social Impacts 

DED has had a significant impact on urban elm trees.  In Montreal, nearly 90%  of the city’s elm trees succumbed to DED between 1970-80, and the city of Toronto had lost 80% of its elm population by 1976. Elms play an important role as shade trees in many urban areas. Lethbridge, Alberta estimates that they provide nearly 30% of the city’s shade. Today, the total value of the 700,000 elms that serve as shade trees in Canadian cities is estimated to be over $2.5 billion dollars (Hubbes, 1999).