Tree disease that has wiped out Kadriorg elms is spreading across Estonia

Elm tree in Kadriorg park.
Elm tree in Kadriorg park. Source: Ken Mürk/ERR

Liina Jürisoo, a forest pathologist at the Estonian University of Life Sciences (EMÜ), warns that the fungus causing the pandemic in Kadriorg has already spread throughout Estonia and neighboring countries. Climate change is the leading cause of the epidemic.

Jürisoo said the spread of deadly Dutch elm disease in Estonia has reached alarming proportions, even though it only came to the attention of the general public because of the problem in Kadriorg. "I have been mapping the affected areas for years; they are already everywhere," Jürisoo told ERR.

"The most infected areas of Tallinn are in the city's northern region: the Stroomi seashore area and the Kopli cemetery park. In addition, there are cases near the main bus station, where plants were not removed promptly enough. As the infected branches were drenched in water, there was some hope that the trees would survive. In reality, however, there is no chance of recovery from this," the scientist said.

Jürisoo said that the situation in neighboring countries is sometimes even worse. "The situation in Sweden is so dire that all three native elm species are now under protection. They are already considering their reintroduction. I myself have been to Russia; the scenery in St. Petersburg is so bad that 5,000 trees are felled down annually. There are miles and miles of lime alleyways," she said.

Jürisoo said that the afflicted trees in Kadriorg should be removed immediately. "We must cut the trees! If we are to have any hope of containing the disease's spread we must eliminate them as quickly as possible. Obviously, not all 1,000 elms in Kadriorg should be removed, but if we don't act quickly, the spread will be quite rapid, necessitating the removal of many more trees," the pathologist emphasized.

Jürisoo said that the disease has not yet spread widely throughout the park. We may be able to save something if we act immediately. "I only noticed one or two infected elm trees in the Kadriorg Park, but there are a couple of dozen infected trees in the kindergarten area next to it and that's where it came from," she said.

"The city should issue cutting permits quickly and allow a trustworthy party to dispose all the infected trees. There is nothing that can be done; we must sacrifice some trees in order to save the others. This is something that the City of Tallinn must realize. They need to direct this funding not to bringing in new trees and new diseases, but to dealing with existing disasters," Jürisoo added.

How does Dutch elm disease spread and what causes it?

Dutch elm disease (DED) is a fungal disease caused by a member of the sac fungi (Ascomycota) affecting elm trees, and is spread by elm bark beetles. "They hibernate beneath the foliage of dying or diseased trees until the temperature is appropriate," Jürisoo said. The first symptom of infection is usually an upper branch of the tree starting to wither and yellow in summer. "But nobody is concerned if a few branches dry up." This morbidity then spreads throughout the tree, with further dieback. Eventually, the roots die, starved of nutrients from the leaves.

At the same time, Jürisoo said, they are coated with pathogen spores. If the insect makes an excision in the tree in order to feed on it, there is a chance of further infection. From there, however, a rapid reproduction process begins that resembles a yeast infection.

"Because the blight happens very quickly, the tree knows something is wrong. The tree then starts to produce chemical compounds to inhibit the spread of the fungal disease, but it lags behind the speed of yeast-like reproduction," Jürisoo admits.

In other words, the plant's defenses pursue the fungal pathogen while blocking essential pathways. The tree begins to quickly dry out as the nutrient-carrying water is unable to move around. "As a result of the rapid drying, the foliage is beginning to droop," the scientist explained.

The American elm that has been introduced to Estonia is the most susceptible to the fungal disease. However, native to our climate, the common elm is also susceptible. Jürisoo said that the European white elm, or Russian elm, is considerably more resilient. "A few of them have died, but for the most part, they are resilient. Possibly only one branch disappears," he said.

Climate change gives a boost to the pest

"In the old days, it was said of our climate that Dutch elm disease wasn't so dangerous here because there were fewer insects to spread the disease. In particular, their distribution and abundance was limited by the northern weather," Jürisoo continued.

"As the climate has warmed, disease-carrying insects have become increasingly comfortable in this region. Insect scientists also say the spread is explosive. When I was out picking insects in 2016, I found two new species that are potential vectors."

So the climate change increases the prevalence of the elm disease, but also the likelihood of hybrid diseases is on the rise.

"There are two subspecies of the elm disease pathogen: the American and the European strain. The American strain is much more aggressive. When the two get together, hybrids are formed. I've been growing their mycelium on petri dishes. I can tell you that the rate of mycelium growth is many times faster in hybrids than in the American and European variants," Jürisoo said.

There are currently no good solutions to stop the spread of it, he said. "If a tree is infected, only swift removal will save the surrounding trees. The disease has been studied for a hundred years and no effective measures have yet been discovered. Cutting it down does not eliminate the pest, but slows its spread. There are, however, some promising avenues for research," Jürisoo said.

"There is one fungus that could be injected into a tree trunk and spreads throughout the tree. It should definitely be injected before the insects arrive. The effects of the fungus are still being studied, but it boosts the tree's immune system. The results in Sweden are quite promising: the mortality rate for trees is somewhere between three and four percent; compared to one hundred percent, this is a very good result," he said hopefully.

Unfortunately, the current vaccine only strengthens the immune system for one year. This means that it would have to be repeated every growing season. "However, for valuable trees in our cities, this vaccination should be considered," he added.


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Editor: Kristina Kersa

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