What is canker stain of plane trees ?
Canker stain of plane trees is a disease caused by a fungus, Ceratocystis platani, that exclusively affects plane trees. Native to the United States, it was introduced into southern France and Italy during World War II. The most common assumption is that it was brought over on contaminated wooden packaging transported by American troops. It has since gradually spread through Europe.
The fungus has been classified by the European Union as a quarantine pest: its introduction and spread are prohibited in Europe and measures to control this pathogen are mandatory.
What are the consequences of an infection ?
The fungus infects the roots and aerial parts of plane trees. It colonises the vessels that carry raw sap, limiting the infected tree's supply of food. Whole branches of the tree’s leaves turn yellow. The bark of trees infected with C. platani is also stained dark orange-brown to blue or black in an elongated flame shape. The tissue beneath these stains is necrotic. Depending on its state of health, a contaminated plane tree can die within a few months or a few years.
Where is the disease present and how does it spread ?
In Europe, canker stain of plane trees has been detected in Italy, Greece, Switzerland, Turkey, Albania, and France. The most affected regions of France are Provence-Alpes-Côte d’Azur, Occitanie, Auvergne-Rhône-Alpes, Corsica, and Nouvelle-Aquitaine. Outbreaks have also been reported in Pays de la Loire and Île-de-France.
The disease spreads easily along streams and rivers. Transported directly in water, the fungus enters the tree through wounds in the roots that may have been caused by passing boats. It can also spread through the roots of trees that are in direct contact with neighbouring trees, contaminated sawdust, or poorly disinfected pruning tools.
What control methods are used ?
There is no treatment for canker stain of plane trees. The only way to eradicate the disease is to fell and uproot the infected tree before incinerating it.
To prevent the dispersal of canker stain of plane trees, it is advisable to carefully disinfect tools used for maintaining plane trees as well as tools used for road work when this work is being carried out near such trees.
Moreover, certain varieties of plane trees have been selected to be more resistant to the disease, and others are currently being developed. However, the level of resistance of these varieties currently available is not sufficient to prevent them from becoming contaminated in areas where canker stain is widespread. Selecting new, even more resistant varieties of plane trees remains a possible medium-term option. In the short term, replacing plane trees with other plant species seems the most accessible solution.
Lastly, certain synthetic fungicides and beneficial micro-organisms may be effective against this fungus. However, no data have been identified from studies conducted directly with plane trees that demonstrated the ability of these control methods to kill C. platani or curb its growth.
What role does ANSES play in combating canker stain of plane trees?
Coordinating efforts to detect the fungus
ANSES's Plant Health Laboratory is the National Reference Laboratory (NRL) for fungi affecting plants. As such, it is responsible for coordinating the detection of canker stain of plane trees in France.
It developed and validated the official analytical method for detecting the pathogen. This method is used by an analytical laboratory approved by the French Ministry of Agriculture. The NRL performs second-line analyses to confirm the results when the fungus is detected for the first time in a previously disease-free region. Lastly, the NRL ensures that the analytical method is correctly implemented.
Since 2019, ANSES has also been the European Union Reference Laboratory (EURL) for fungi and oomycetes. Its role is to coordinate the work conducted by the NRLs of the EU Member States with regard to these pathogens and make sure they are capable of detecting them.
Issuing recommendations to better control this fungus
ANSES has undertaken two expert appraisals at the request of the Directorate General for Food. One of them, published in 2019, was based on a study that had monitored the Platanor tree variety in the Occitanie and PACA regions of France. The objective was to identify what had caused this variety to become infected despite it being known as resistant to the disease.
The other study, published in 2022, assessed the most appropriate control methods in these same regions, where the disease's eradication no longer seems possible given how widespread it has become in certain areas. The aim was to propose control strategies to slow down the spread of the disease in various contexts: in urban environments, along major roads, and along rivers and streams.