Germany’s gardens and parks are threatened by an introduced fungus: For the first time, scientists have detected the aggressive plant pest Peronospora aquilegiicola on columbines in Lower Saxony. In this country he specializes exclusively in columbines and causes “downy mildew”. Affected plants die off. As the pest is known for its rapid spread, experts are warning of a mass infestation across Europe.
“Peronospora aquilegiicola belongs to the Oomycetes,” explains Marco Thines from the Senckenberg Biodiversity and Climate Research Center. “Although they look like mushrooms, these organisms are more closely related to brown algae and diatoms.” The spores can either hibernate in the ground or spread freely through the wind by penetrating the stomata of plant leaves. There they form a fungal network between the cells, extract their nutrients from the plant and thereby damage it.
Contrary to what is often assumed, the pest does not specialize in entire plant families, but in individual species. This affects, for example, species of the buttercup, foxtail or carnation family. Researchers have also identified host leaps. Downy mildew spreads mainly under warm, humid conditions in the field and also in greenhouse crops. There have already been infections that are widespread around the world, for example on basil plants.
First find on German soil
The discovery of such a fungal infestation on columbine plants is now causing a sensation in Germany: In a garden in Lower Saxony, the first symptoms of a possible plant pest were noticed as early as 2019. This year the gardeners noticed a clear fungal infestation, which made itself noticeable by “downy mildew” on their columbines. With their pink to violet-colored flowers, the long-stemmed columbines are a real eye-catcher, especially in spring. In a columbine that is infected with Peronospora aquilegiicola, the leaves initially turn yellow and later turn purple. The leaf margins roll outward. In the long term, columbines affected by Peronospora aquilegiicola die off.
A research team led by Thines has now carried out a closer analysis to further examine the pest: They collected some of the infected specimens and dried them. The researchers then sprinkled the surface of the infected leaves with a 70 percent aqueous lactic acid solution. Under a microscope with 400x magnification, they finally found that a beige to purple coating of oomycete spores was forming on the underside of the leaves of the plants. These spread through wind or rainwater splashes. However, it was still unclear what type of pest the fungal infestation was in Germany. In order to find out how the pest got to Germany, the scientists finally carried out genetic analyzes of the leaf samples.
The origin in view
The result: The examined gene sequences showed that, surprisingly, it is the species Peronospora aquilegiicola. This corresponds to the same species that found its way into Western Europe seven years ago. This columbine species is originally from East Asia and has only appeared in Europe in 2013 in Great Britain and Wales. “Since then, it has spread rapidly on the British Isles and a large part of the columbines in public parks on the conscience there,” reports the expert Thines. There it could be destroyed on a large scale. Nevertheless, the plant pest, which has hitherto not been native to Germany, seems to have made the leap to continental Europe.
But how did the fungus make this leap? At the place where Peronospora aquilegiicola was first recorded in Germany, no new columbines have been planted in the last twenty years. The researchers suspect that gardeners in the neighborhood imported plants from England or Wales that the pathogen used as a Trojan horse. Where exactly is currently still in the dark. According to estimates by the researchers, the infected plants – supported by the wind – sown themselves in the garden in Lower Saxony. The pest spreads very quickly via infesting seeds, as the example in Great Britain showed. Since they survive even over long distances, the columbine species, which are popular as ornamental plants among gardeners, are in danger.
The plant pest is not only a threat to commercially grown plants – wild columbine species can also be damaged. Due to their rarity in Germany, they are all under nature protection. The scientists therefore plead for increased attention to the harmful immigrants. “Everyone who has columbine species in their garden or park should pay attention so that Peronospora aquilegiicola cannot spread in Germany and continental Europe,” says Thines. “Otherwise we run the risk of it, like other plant pests, such as the ‘downy mildew’ of basil, spreading globally and causing several million euros in damage. Our goal must therefore be to identify and eliminate as many sources of infection as possible.
Source: Senckenberg Research Institute and Nature Museums; Technical article: Mycological Progress, doi: 10.1007 / s11557-020-01596-2