Oaks affected by two diseases at the same time

Five years ago, the bacterial complex disease acute oak dieback was also discovered in Switzerland. In 2023, a new root disease was also discovered on oaks, as reported by Forest Protection Switzerland in the new Forest Protection News. It is particularly worrying when both diseases affect oaks at the same time.

Oaks are considered to be trees of the future because, compared to other common deciduous tree species, they suffer less from the consequences of climate change such as heat or drought. For some years now, however, the oak has been affected by a disease called acute oak dieback, which is caused by various types of bacteria. Extreme events such as long periods of drought may favor the disease. The bacterial species involved are indigenous.

Since acute oak dieback was first detected in Switzerland in 2017, Forest Protection Switzerland has received numerous reports of this disease from all parts of the country. It is currently the main reason for reports of oaks with slime flow on the trunk and sparse crowns. Swiss Forest Protection also examines all reported oaks for the root and trunk diseases of the genus Phytophthora, as the symptoms of these diseases are similar to the symptoms of acute oak dieback. So far, all oaks suffering from acute oak dieback have been free of Phytophthora. In spring and summer 2023, however, Forest Protection Switzerland detected two cases of Phytophthora in oaks with acute oak dieback. Both cases came from urban areas and in both cases the species Phytophthora cinnamomi was present, which previously only occurred as the causative agent of ink disease on sweet chestnut in Ticino.

It is not only new for Switzerland that oaks are simultaneously affected by acute oak dieback and Phytophthora cinnamomi. Worldwide, there have not yet been any reports of the two diseases being detected together on the same tree. Many questions therefore remain unanswered. The most important one is what impact this will have on the oaks. Are the symptoms more severe and do oaks die more quickly if they are infected by both diseases instead of just one? Swiss Forest Protection and the WSL Phytopathology Group are launching a new research project on this topic at the beginning of 2024.

It is worrying 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 the winter temperatures are generally too low for it to do so. This circumstance also explains why the pathogen has long been present in Ticino as the causative agent of ink disease, but has not been able to spread on the northern side of the Alps. Phytophthora cinnamomi originally comes from South to East Asia and is mainly spread through the plant trade. Climate change, especially mild winters, could lead to the pathogen becoming more common on the northern side of the Alps in the future. As only individual diseased trees have been found here so far, Forest Protection Switzerland recommends that oak trees with mucilage flow and other symptoms should always be reported. Both diseases, Phytophthora cinnamomi and acute oak dieback, can only be reliably diagnosed with a laboratory analysis.

Forest Protection Switzerland is the specialist unit for forest protection issues at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. Together with the cantonal forestry services, Forest Protection Switzerland surveys the occurrence and extent of biotic and abiotic damage to forests, provides information on current forest protection events, advises those affected on forest protection issues, and is involved in the further training of forestry services, the green sector, students and other specialist personnel. As coordinated international action is becoming increasingly important, we also maintain an exchange of information with colleagues in Germany and abroad. In this way, we represent an interface between research, practice and authorities, a platform for information.