#228: Dutch Elm Disease

Elm trees were once the defining tree of American life, widely planted in cities, suburbs, and farms.  This changed beginning in the early 1900’s when Dutch Elm Disease arrived on the continent.  Caused by the fungi Ophiostoma ulmi and O. novo-ulmi, Dutch Elm Disease blocks off the water transportation tissues in elm trees, leading to wilting and death.  The fungus moves between trees by using elm bark beetles as vectors and by growing from tree to tree, making it a difficult pathogen to control.  The best way to manage Dutch Elm Disease today is by planting resistant cultivars – fortunately, there are many resistant options available.^1,2

Elm Trees

Elms are very popular trees because they are hardy trees that thrive urban areas.  They grow well in compacted soil and tolerate pollution, which makes them ideal street trees.  Additionally, the top of the tree spreads out to create an attractive tree shape with a lot of shade.  In the United States, the American Elm was planted extensively along streets, beside farms to create windbreaks, and around homes for aesthetic value.  Indeed, the American Elm became a symbol of American life from the Great Plains to the Atlantic Ocean (which explains why so many roads are named “Elm Street”).¹

The American Elm was the most widely planted and most susceptible elm in North America, but there are several other native species of elm trees.  All native North American elms and most European elms are susceptible to Dutch Elm Disease, though to varying degrees.^1,2  Asian trees show the most resistance³ because they coevolved with the pathogen.

Identifying elm trees is something I’ve struggled with because I don’t see the trees very often.  I mostly pay attention to the bark and lower trunk, but the leaves, leaf arrangement, flowers, and seeds are also useful.  For all the morel hunters who want to learn to identify dead elm trees, skip to the end of this article.

In natural habitats, elms are mostly found in floodplains and slopes near streams.  They prefer rich well-drained soil, but can survive a range of similar conditions.⁴  Different species of elms have somewhat different soil requirements and ranges.

I mostly find elm trees by looking at their bases (I’m usually looking for mushrooms).  The first thing I look for is how the tree attaches to the ground.  Like most trees, elms get larger toward the base and have a few lobes at the bottom of the trunk.  Unlike most trees, the lobes on elm trees tend to be very well developed and snake out away from the tree as they slowly dive underground.  These are called buttress roots and are common sights on rainforest trees.  Other North American trees form buttress roots, but usually aren’t as well-formed as those of elm trees.  After spotting a buttress root, I check the tree’s bark.  Medium-aged elms typically have rather spongy bark that easily gives if you push your fingernail into it (this doesn’t work well on old or young trees).

If those features are inconclusive, I look for other characteristics.  Elm leaves are alternate, are oval-shaped, and have doubly serrate margins.  They are borne from twigs that have a slight zigzag to them (not many other trees have this feature).  Drooping clusters of small flowers appear on twigs in early spring.  These are eventually replaced with groups of seeds.  Each seed is enclosed in a paper-like circle, giving it the appearance of a tiny flying saucer.⁵  These seeds are probably the most distinctive feature of elm trees.

Dutch Elm Disease

Dutch Elm Disease is a vascular wilt of elm trees caused by the fungi Ophiostoma ulmi and O. novo-ulmi.  O. novo-ulmi is further divided into two subspecies that readily hybridize where their ranges overlap: O. novo-ulmi ssp. novo-ulmi, which infects trees in Europe, and O. novo-ulmi ssp. americana, which infects trees in North America.⁶  O. ulmi is native to Asia and was first introduced to Europe, probably through the transport of infected lumber.^6,7  The fungus then moved from Europe to North America, again likely because of lumber transport.  O. novo-ulmi later appeared in Europe and North America.⁶  Mycologists still do not fully understand where O. novo-ulmi came from or how it arrived in North America and Europe.

O. ulmi was first identified in 1921 causing disease in elm trees in the Netherlands (thus the name “Dutch Elm Disease”).  In the United States, it was first found in Ohio in 1930, but may have arrived earlier and been present in other places.  Dutch Elm Disease finally reached the West Coast in 1973, completing its spread across the continent.  At some point during the second half of the 20^th century, O. ulmi was replaced by the more virulent O. novo-ulmi.¹

O. ulmi and O. novo-ulmi have a simple life cycle.  First, spores of the fungus enter a tree’s xylem through damage caused by an elm bark beetle.  Most of the time, the spores are deposited in the upper parts of the tree.  After germinating, the fungus grows through the vascular system as mycelium.  It also produces conidia inside the xylem.  These small spores easily move with the flow of water, which transports them to new uninfected areas and spreads the disease within the tree.  Elm trees can detect the fungus, but in susceptible species the tree defenses are activated too slowly to stop the fungus.  The trees keep trying to put up walls in the xylem to block out the fungus, but this ends up completely blocking the xylem to upper parts of the tree.  Without access to the xylem network, water cannot reach the upper branches.  Leaves at the ends of infected branches begin to wilt and yellow, then turn brown and die.  Cutting an infected branch in half reveals dark brown streaking of the outer layers of wood.  As the fungus grows down the tree, larger areas of leaves will wilt and eventually the whole tree will die.  Depending on when the tree was infected and how susceptible the tree is, the tree may die after less than a year or up to a few years.  Once the tree dies, the fungus continues growing in the wood as a decomposer.^1–3

The fungus spreads to new trees in two ways: by using elm bark beetles as vectors and by growing from tree to tree through root grafts.  When elm bark beetles are present, the fungus produces spores in tunnels made by the beetles.  The spores can be either conidia on synnemata (bundles of conidia-producing cells) or ascospores in perithecia.  Either way, the sticky spores attach to the beetles and can be carried long distances.  The fungus can also grow directly from tree to tree through root grafts – where roots from two different elm trees grew next to each other and fused.  Since the vascular systems are fused, the fungus can easily grow into the neighboring tree.  Trees infected through root grafts typically die very quickly because the fungus already has access to the base of the vascular system.  On elm-lined streets, the trees were typically planted very close together and had many root grafts, allowing Dutch Elm Disease to spread very quickly in urban areas and avoid management strategies that targeted beetles.^1–3

Elm Beetles

Three species of beetles spread Dutch Elm Disease: the Native Elm Bark Beetle (Hylurgopinus rufipes, native to North America), the Smaller European Elm Bark Beetle (Scolytus multistriatus, native to Europe), and the Banded Elm Bark Beetle (Scolytus schevyrewi, native to Asia).  The Native Elm Bark Beetle looks rather compact, is reddish-brown, is minutely hairy, has short antennae, and grows 2.2-2.5mm long.  The European Elm Bark Beetle looks a little more elongated, is very dark reddish brown, is minutely hairy, has very short antennae, and grows 1.9-3.1mm long. None of these species causes significant harm to elm trees when the fungus is not present.^2,3,8

The life cycle of elm bark beetles begins when an adult burrows into dying elm trees or dead elm logs or pieces with bark still attached.  The beetle digs a straight tunnel called a gallery and lays eggs in the gallery.  The gallery runs along the interface between the innermost layer of bark and the outermost layer of wood.  When the eggs hatch, the larvae begin eating the innermost layer of bark and create new tunnels that extend out from the central gallery.  Together, these galleries form a unique pattern somewhat reminiscent of a centipede with long legs or perhaps a rib cage.^2,3

Eventually, the larvae pupate and emerge from the pupae as adults.  The new adults crawl back through their tunnels to exit the trees.  If the beetles’ host tree is infected with Dutch Elm Disease, the fungus produces sticky spores in the bark beetle galleries.  The spores will stick to or be eaten by the exiting elm bark beetles to be carried to the next tree.  Elm bark beetles reproduce multiple times throughout the year, so the fungus has many opportunities to get its spores onto young beetles.^2,3

Adult elm bark beetles have slightly different feeding and overwintering habits depending on the species.  Native Elm Bark Beetles feed on the layer of wood just below the bark of stems and branches.  In winter, they burrow into elm trunks, which allows them to survive temperatures below -6°F.  Throughout most of the United States, the Native Elm Bark Beetle has been replaced by its European counterpart (the Banded Elm Bark Beetle is slowly spreading from the Great Plains eastward).  However, it is still the most common elm bark beetle in Canada and bordering areas because of its ability to tolerate cold.  The Smaller European Elm Bark Beetles and Banded Elm Bark Beetles feed on wood in the crotches of healthy twigs, so they primarily introduce the fungus to the upper parts of trees.  These beetles overwinter as pupae or adults in their larval galleries and so can’t survive very cold temperatures.  With all three beetles, fungal spores stuck to the beetles can dislodge, germinate, and infect the tree during feeding (mostly in the spring) or during construction of galleries or overwintering burrows.^2,3

Management

The good news about Dutch Elm Disease is that because beetles need to feed on trees with lots of bark, they mostly spread disease to larger trees.  Consequently, all elm species can grow large enough to produce seeds before being attacked by the beetles and fungus.  This means that no elms are in danger of going extinct.¹

However, elms can no longer be used as pretty street trees without intensive management of Dutch Elm Disease.  Management is primarily accomplished by using cultural practices, insecticides, fungicides, and resistant varieties.^1–3

There are a variety of cultural practices designed to reduce the spread of Dutch Elm Disease in a tree and in an area.  For a single tree, you can try to get ahead of the disease by cutting off any infected branches, making sure to cut them five to ten feet below the last sign of dark streaking in the wood.  To reduce the elm bark beetle population, you can remove (and burn, bury, de-bark, or chip) all dead elm wood.  This reduces the amount of habitat suitable for laying eggs.  Root grafts can be disrupted by plowing a trench (at least five feet deep) between nearby trees.  Finally, increasing the diversity of the elms in an area (i.e. using American, slippery, and rock elms instead of just one species) can help slow the progression of the disease.^1–3

Insecticides can be used to control the elm bark beetle population by spraying them on sites where the beetles are likely to feed on or burrow into elm trees.  This strategy did not work very well during the height of the Dutch Elm Disease epidemic.  On top of that, heavy use of insecticides can impact helpful insects and other animals.^1,2  Widespread spraying of elm trees was attempted in the mid-1900’s using DDT, which turned out to be extremely dangerous to birds and toxic to humans.

Fungicides can be used to prevent infection from beetle feeding but can neither treat infected trees nor prevent infection from root grafts.  Several fungicides are available, but they are all expensive and need to be reapplied yearly or up to every three years.  Consequently, preventative fungicides are usually reserved for trees deemed to be “important.”^1–3

The best way to manage Dutch Elm Disease is by planting resistant trees.  Hybrid trees that are resistant to the disease, tolerant of urban environments, and have an attractive shape are readily available.  There are even some clones of resistant American Elm trees that are available.^1,2  For some reason, I haven’t seen resistant trees being widely planted.  Perhaps it’s been long enough that people have forgotten how much they liked elm trees.  Or, maybe people realized the easiest way to prevent Dutch Elm Disease is to not plant elm trees.

Elms and Morels

Dead elms are prime locations to find morels, so Dutch Elm Disease has had a significant impact on morel hunting.  After the arrival of Dutch Elm Disease, dead elms were easy to come by.  Now (at least in the Washington, DC area), most of the elms killed by the disease are well-decomposed and no longer produce morels.  The surviving elms are mostly young, so dead elms are hard to come by.  Combined with the Emerald Ash Borer killing all the ash trees, prime morel-producing habitat has decreased substantially.

So, how do you find a dead elm tree?  First, look in the right habitats.  Elms tend to like wetter environments, valleys around streams and floodplains are good places.  When you find a dead tree, pay attention to the base of the tree and the bark.  Since elms often grow in wet areas, their bases tend to form buttress roots: extensions of the tree base that snake off into the ground (rainforest trees often do this, although at a much larger scale).  Other trees like hickory and beech also do this, although elm tree buttresses tend to have a snakier appearance.  Elm tree bark is also distinctive.  It tends to be very corky, so it will feel like a corkboard when you push your fingernail into it.  After dying, the bark tends to separate from the wood in large pieces that are easy to peel off.  Standing dead elms tend to be barkless (especially near the top of the trunk) and have lots of bark debris scattered around the ground below them.  Lastly, look for the galleries of elm bark beetles – these galleries form in a distinctive shape not found in any other type of tree.  Identifying dead elm trees definitely takes practice, especially since there aren’t many freshly dead elms around anymore.

Taxonomy

O. ulmi and O. novo-ulmi belong in the phylum Ascomycota and the class Sordariomycetes,^9,10 along with most other fungi that produce perithecia.  Elms (trees in the genus Ulmus) belong in the order Rosales,¹¹ which includes many familiar plants such as roses and apples.  Elm bark beetles are true beetles (in the order Coleoptera) but belong to two different genera.^12,13  This is rather surprising for two organisms that are so similar in their ecology, life cycle, and appearance, but the two beetles are still relatively close evolutionarily because both genera belong to the Scolytinae subfamily (the taxonomic rank one level higher).^12,13