Reader asks: What to do with ailing ash and elm trees?
A reader of ERR's Novaator science portal noticed diseased ash and elm trees in their home yard and asked what was causing it and what could be done to help the trees.
According to Rein Drenkhan, professor of forest pathology at Estonian University of Life Sciences, the attentive reader most likely noticed two common pathogens: ash dieback and Dutch elm disease. "The pathogens affecting the two species are different: ash dieback is caused by a sac fungus called Hymenoscyphus fraxineus, while Dutch elm disease is caused by Ophiostoma novo-ulmi, formerly also known as the new elm bark beetle fungus," he said.
Drenkhan explained that the pathogen causing ash dieback is an invasive and dangerous exotic species that likely arrived here with biological material imported from East Asia. The pathogen behind Dutch elm disease, meanwhile, has been causing damage in Estonia for more than two decades and a new outbreak has now begun. "What makes the situation with elms even more complicated is that two subspecies of the Dutch elm disease pathogen have formed a hybrid. Such a hybrid is often far more aggressive than the parent species," the scientist explained.
The symptoms of the diseases are fairly similar in both trees. "As a result of ash dieback infection, the leaves darken and fall off, while shoots with necrotic symptoms remain behind. In the case of Dutch elm disease, the leaves emerge and after some time begin to wilt and curl, giving the impression that someone has scorched or burned them. Eventually the leaves fall off and dying shoots without leaves remain," Drenkhan described.
So what should be done with diseased trees? The scientist emphasized that unfortunately trees cannot be cured. However, people can still do everything possible to prevent the diseases and stop them from spreading. "The most important proactive step is not to bring tree saplings of unknown origin into the garden and to tell neighbors the same thing. That is the best protection for the trees and shrubs in our home gardens," Drenkhan said.
"If we see blackened or curled leaves and dying shoots, the tree should be monitored for some time to see what happens. If the situation keeps getting worse, there is no other effective way to control Dutch elm disease than removing the tree," the scientist explained. The best time to fell diseased trees is early spring before the flight season of elm bark beetles, which are major carriers of the disease.
"Because of ash dieback, we still have quite resilient European ash specimens that are resistant to the disease. It is possible that some trees recover and nothing further happens to them. Trees that continue to deteriorate, however, should be felled just like elms," Drenkhan explained.
According to the scientist, there is another useful way to limit the spread of ash dieback: in autumn, all ash tree leaves in gardens and parks should be raked together and burned. The fruiting bodies of the ash dieback fungus grow on fallen leaf stalks from the previous year, so removing them takes away the fungus's food source.
Warmth-loving pathogens
According to Rein Drenkhan, climate change is having a major impact on the spread of both new invasive pathogens and native ones. On the one hand, increasingly warm temperatures are attracting new and previously unknown diseases to these latitudes; on the other, the problem lies in more frequent weather extremes.
"Climate change brings with it serious, rapid and extreme changes in weather conditions. Take last year, for example, when autumn remained warm until December and then a fairly cold winter suddenly arrived. Such abrupt changes stress trees and affect their health. Because the trees are under stress, they become more susceptible to various diseases," Drenkhan said.
At the same time, last winter was not favorable for root diseases either. The scientist pointed to root rot, which affects spruce and pine trees and whose spread is limited by frozen soil. "In unfrozen soil, root rot pathogens can continue spreading and causing damage quite happily. During a mild winter, pathogen spores can spread even in the middle of winter, for example in January, which previously was not possible during a normal cold winter. This in turn means that native pathogens are also thriving more than ever before," Drenkhan said.
According to the scientist, Norway spruce is currently the biggest concern in Estonia's forests. "The problem is that Norway spruce cannot tolerate drought or sudden extremes, especially a dry growing season. For this reason, we do not recommend that foresters and forest owners establish pure spruce stands because spruce in monoculture suffers from both root rot and spruce bark beetles," the scientist explained. Spruce should not be avoided entirely, however; instead, it is sensible to establish mixed stands together with species such as birch, which are less affected by diseases and pests.
Finally, the scientist stressed how important it is to systematically monitor the condition of Estonia's forests. "We have created a nationwide network of permanent sample plots where we systematically collect disease samples from different tree species every year. This gives us the opportunity to understand how pathogens spread, how aggressive they are and what the future trend in damage may look like. Without systematic data, it is impossible to assess future risks," he said.
Drenkhan also considers it very important that people themselves pay attention to what is happening in nature and take an interest in it. "The readers' interest itself is very positive. It shows that Estonians are a people close to nature and that they care about the health of trees."
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Editor: Johannes Peetsalu, Marcus Turovski
