Why does the elm dry out? How to identify and treat graphosis disease

In the 1935 degree course in agricultural sciences in Milan, 5 students out of the 50 who graduated presented research on the same topic: elm graphosis, a disease that had caused a real modification of the landscape with thousands of withered elm trees in the Lombardy countryside. The graphosis that still afflicts elm trees today, as we will see, is caused by a “vein parasite” of the tree. The Anglo-Saxon slang term to identify this disease is “Elm Dutch Disease” and we will soon understand why! So let’s find out why the elm dries out due to graphosis.

What is Elm graphosis

Elm graphosis is a genus-specific disease Elmcaused by a parasitic fungus that colonizes the xylem, the vascular tissue responsible for transporting water and mineral salts from the roots to the leaves. The occlusion of the xylem vessels caused by the fungus prevents the correct supply of liquids to the plant, causing deterioration and, in many cases, death.

The pathogen responsible for elm graphosis isElm Ophiostoma (formerly called Graphium ulmi). This ascomycete induces tracheomycosis, the plant’s defensive response, characterized by the production of gums and phenolic compounds, paradoxically aggravates the occlusion of the vessels, accelerating the deterioration of the elm.

The first obvious sign is a sudden withering of a branch, which extends leading to the complete drying of the leaves and the death of the entire branch or even the tree. If the infection originates from the roots, the tree can die within a single season. On the contrary, if the infection starts from a branch, the decay is more gradual, since the fungus must climb up the woody vessels against the current, requiring more time to reach the root system.

Why does the elm dry out: how to check for graphosis infection

To confirm the presence of the fungus, a diseased branch can be cut. Inside, you will notice dark streaks in the woody vessels, caused by the formation of tille and the deposition of gums and phenolic compounds in response to the fungal infection.

Tillae are a very curious formation, we could define them as the extrusion, in the form of a vesicle, of some cells of the woody parenchyma into the cavity of an adjacent vessel. This results in the occlusion of the xylem, in the plant’s attempt to compartmentalize and isolate the problem in a defined area, while sacrificing the affected parts.

A bit like when in films the hatches are closed to block the entry of water following damage to the hull… usually there is always the poor guy on duty who remains on the other side of the glass and the hero who cries on the other.

The death of the plant (or the infected branch) usually occurs in midsummer in hot periods, while in spring even the infected branches manage to vegetate.

The disease is mainly carried by bark beetles of the genus Scolytus which live subcortically and carry spores of the fungus from a diseased plant to a healthy one.

The second vector of the disease is the activity of pruners who do not know the pathogen: in this case the spores of the fungus are carried via work tools.

Why does the elm dry out: treatment against elm graphosis

Unfortunately, the methods of containment of the disease are not effective and are limited to preventive systems that avoid infection or recognize it quickly in order to eliminate the infected plant.

It must be said that bark beetles are typical “worms” of elm trees with lymphatic weakness and therefore any factor that helps the tree to be in good lymphatic tone keeps the insect away.

On infected plants that show limited portions of withered foliage, cutting well below the browned area of ​​wood could (with the conditional!) remove the infected part, but no one can give guarantees in this regard also because when the dead part is cut, a new large wound is opened in the wood of the tree.

Only if the infection has not compromised more than a third of the foliage, it is possible to intervene by removing all the symptomatic wood, extending the cut at least one meter below the area with chromatic alterations.
In an infected row, the rapid progression of the disease makes it essential to immediately cull all symptomatic individuals in order to prevent the spread of the infection through the roots.

For new plantations there is the possibility of using resistant exotic species such as: Wilson elm (U. wilsoniana), Chinese elm (U. parvifolia), Siberian elm (U. pumila).

Asian species, having always lived with this disease, have developed a certain degree of resistance.

Why does the elm dry out: the history of elm graphosis

The history of elm graphosis began in Asia, this fungal disease was accidentally introduced to Europe, America and New Zealand, where it found fertile ground in completely defenseless elm populations. Discovered and described in the Netherlands (hence the origin of the name “Dutch elm disease”) in 1921, graphosis accelerated with the arrival of a more aggressive strain in the 1960s. This new pathogen, rapidly spreading across the continent, has caused the death of millions of elm trees, transforming entire landscapes. It is believed that its introduction into Europe occurred via timber imports from North America.

The data is relentless: in Great Britain, between the 1970s and 1990s, there was an 85% reduction in the elm population. In Edinburgh, numbers have plummeted from 35,000 to 5,000 in just a few decades. Even in Paris, the situation was dramatic, with a loss of over 90% of the elm trees. Fortunately, thanks to more careful management policies and the introduction of resistant cultivars, such as the “Nanguen” elm, signs of recovery are being observed in some areas.

The “decline of the elm”

Fossil pollen holds a secret: the sudden decline of elm trees in Europe, which occurred in two main phases, around 4000 and 1000 BC. For a long time it was believed that man, with his ax and his plow, was mainly responsible for this disappearance. However, recent discoveries on the origin of elm graphosis have opened a new perspective. Analysis of ancient woody finds and fossil insects suggests that a similar disease may have affected elm trees as early as the Neolithic, weakening them and making them more vulnerable to anthropogenic pressures.