Oak Decline in Switzerland: New Root Disease Discovered

Five years ago, the bacterial complex disease known as acute oak decline was also discovered in Switzerland.

In 2023, a new root disease on oak trees was also discovered, as Waldschutz Schweiz reports in the new Waldschutz Aktuell bulletin. It is particularly concerning when both diseases affect oak trees simultaneously.

Oak as a Tree of the Future Under Threat

Oak trees are considered tree species of the future because, compared to other common broadleaf species, they suffer relatively little from the consequences of climate change such as heat or drought. In recent years, however, the oak has been affected by a disease called acute oak decline, which is triggered by various species of bacteria. Extreme events such as prolonged periods of drought may potentially favour the disease. The bacterial species involved are native.

Since acute oak decline was first detected in Switzerland in 2017, Waldschutz Schweiz has received numerous reports of this disease from all parts of the country. It is currently the primary reason for reports of oaks with slime flux on the trunk and sparse crowns. Waldschutz Schweiz also examines all reported oak trees for root and trunk diseases of the genus Phytophthora, as the symptoms of these diseases are similar to those of acute oak decline. Until now, all oak trees affected by acute oak decline had been free of Phytophthora. In spring and summer 2023, however, Waldschutz Schweiz detected Phytophthora in two cases on oak trees with acute oak decline. Both cases originated in urban areas and in both cases the species Phytophthora cinnamomi was present, which had previously only been known as the pathogen causing ink disease on sweet chestnut trees in Ticino.

What Are the Effects of a Dual Infection?

Not only for Switzerland is it new that oak trees are simultaneously affected by acute oak decline and Phytophthora cinnamomi are affected. To date, there have been no reports worldwide of both diseases being detected together on the same tree. Many questions therefore remain open. The most important is what impact this has on oak trees. Are the symptoms more severe and do oaks die more quickly when infected by both diseases rather than just one? To address this, Waldschutz Schweiz, together with the Phytopathology group at WSL, is launching a new research project at the start of 2024.

Phytophthora cinnamomi: a heat-loving pathogen

Of particular concern is the fact that one of the cases originates from the northern side of the Alps. It was previously assumed that this pathogen could not establish itself on the northern side of the Alps, as winter temperatures there are generally too low. This also explains why the pathogen has long been present in Ticino as the causative agent of ink disease, yet has been unable to spread to the northern side of the Alps. Phytophthora cinnamomi originates from South to East Asia and is spread primarily through the plant trade. Climate change, particularly mild winters, could result in the pathogen becoming more prevalent on the northern side of the Alps in the future. Since only isolated diseased trees have been found there to date, Waldschutz Schweiz recommends always reporting oaks showing bleeding cankers and other symptoms. Both diseases, Phytophthora cinnamomi and acute oak decline, can only be reliably identified through laboratory analysis.