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Keeping an eye on reed decline

The reed zones around Europe’s lakes are becoming increasingly smaller. The biologist Dr. Jan Nechwatal at the University of Constance is investigating the causes of reed decline and has identified a previously unknown plant pest.

Dr. Jan Nechwatal from the University of Constance is studying the causes of reed decline. © Martina Keller-Ullrich

Although the decline of reeds is not as spectacular as forest dieback in terms of the interest it generates amongst the general public, conservationists and limnologists all over Europe have been alarmed for many years. Many causes of reed decline are discussed, for example the wash caused by boats, over-fertilisation, insect damage or the increasing number of Canadian Geese. The reeds around Lake Constance particularly suffered during the 1999 floods. The reason for this decline is still unclear because it is generally assumed that plants like reeds should be able to tolerate a higher water level on a temporary basis. Biologists at the University of Constance have come up with a new idea to explain the progressive decline of reeds. Dr. Jan Nechwatal of the "Lake Constance Littoral" collaborative research centre focuses his research in particular on the decline of reed zones and has made some fundamental discoveries.

Nechwatal suspected that so-called oomycetes, organisms that are known to destroy other plants, are responsible for the decline of reeds around Lake Constance. Oomycetes are not fungi in the strictest sense but instead are more closely related to algae. Phytophthora infestans causes potato blight, which led to the great famine in Ireland in the 19th century. Many other oomycete species are known as agricultural pests and they are also assumed to play a role in forest dieback. Dr. Jan Nechwatal explains that there are between 80 to 100 different Phytophthora species worldwide, including a particularly dreaded species that is known to cause "sudden oak death" in America. In order to prevent the introduction of this pest into Europe, strict importation regulations and quarantine measures have been put in place for trees and ornamental plants. The pest is still not found in the wild in Europe. But Nechwatal believes that it is already present in tree nurseries.

Unknown species identified

The researchers from Constance have since shown that the oomycetes are the cause of reed decline in Europe. They have also identified the particular species. They did so by collecting a large number of samples at different sites around the lake and subsequently flooding them in order to make the potentially present spores float to the surface. The researchers placed reed leaves on the water surface as “bait” and then characterised the oomycetes that had been collected morphologically and using molecular biology methods. The result clearly showed that a previously unknown species, Pythium phragmitis, was present in all the water samples investigated.

Pythium phragmitis is a close relative of Pythium arrhenomanes which infests maize and corn. In order to prove that it is not the higher water levels themselves that lead to the decline of reeds, but the Pythium that destroys the plants, the biologists submerged reed plants either completely or partially in water. They even cut back the plants. This experiment showed that the presence of Pythium in water causes the reeds to decline and die off. The experiment also showed that most of the plants survived high water levels.

Since Pythium phragmitis was found in all Lake Constance water samples, the question arises as to whether the reeds are particularly prone to damage at high water levels. Dr. Jan Nechwatal also has an explanation for this observation: the leaves of reeds are the part that is particularly susceptible to the pathogens, because the leaves tend to come into closer contact with the oomycete spores that are present in the water. On the other hand, the roots and rhizomes of the reeds seem to be relatively resistant to the pathogen and the researchers assume that they protect themselves against the pest with their hydrophobic surface.

Oomycete-affected leaves in the laboratory experiment.
Oomycete-affected leaves in the laboratory experiment. © Jan Nechwatal

The investigation of Pythium phragmitis has also given rise to further interesting findings. For example, the biologists have found out that the oomycetes release their spores without requiring a cold shock in the same way as the potato blight pathogens do. Quite in contrast, Pythium phragmitis proliferates particularly well at temperatures of 25 to 30°C.

Since the ongoing climate change also leads to an increase in temperature and extreme weather conditions and floods will become more frequent, the researchers assume that this will further reduce the reed zones. Since floods usually occur in summer, this creates optimal infection conditions. The reduction of the reed zones in turn affects the stability of the banks of lakes and rivers as well as the diversity of species, as they are important breeding places for birds and fish.

Development of hybrids

Typical damage caused by Pythium phragmitis. © Carolin Bogs

However, it is not only Pythium phragmitis that endangers the reeds. Molecular biology investigations have led the Constance researchers to identify a hybrid that is a genetic and morphological hybrid of P. phragmitis and P. arrhenomanes. While P. arrhenomanes does not pose a risk for reed plants and is mainly found in drier areas, the hybrid turned out to be even more dangerous for reeds than P. phragmitis. It attacks the subterranean plant parts far more aggressively. Although the hybrid has not been around for a long time, it has already spread extensively and is more frequent than its P. phragmitis parent. Dr. Jan Nechwatal states: "The hybrid is very successful and most likely on the way to establishing itself as a separate species." This raises two questions: one, whether this increases the danger for the reeds, and second, where the hybrid comes from. The biologists assume that it comes from the maize fields located close to the banks of Lake Constance.

It is assumed that global warming and the introduction of new species through global trade activities will lead to the development of other hybrids. Additional danger for the reeds might arise from other fungi, namely true fungi. The fungus Stagonospora neglecta lives endophytically in reeds. At present, Jan Nechwatal is trying to make the plants more resistant to this fungus. "Maybe the plants will develop a basic resistance to this pest," explains the researcher.

Further information:

University of Constance
Faculty of Biology
Universitätsstraße 10
78457 Konstanz

Tel: +49 (0)7531 88-2105
Fax: +49 (0)7531 88-3035
E-mail: jan.nechwatal(at)uni-konstanz.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/keeping-an-eye-on-reed-decline