One in three horse chestnut trees in the Netherlands is sick, the working group Aesculaap announced last week. The group is led by Dr Marijke Dijkshoorn of Applied Plant Research, and it has been hunting the pathogen for months.
‘We think so, but we do not have conclusive evidence yet. We think that it’s a bacteria from the Pseudomonas syringae group. We have found this bacteria in all affected trees, and in laboratory tests it also caused bleeding in chestnut seedlings. But to prove that it is the culprit, according to the rules we have to isolate the pathogen from newly infected trees. That’s why we are still waiting for the results of tests on mature trees so that we can be completely sure. We have been able to exclude a number of other potential causes, though. We have looked very carefully at whether it could be a form of phytophthora. Many experts thought that this was the pathogen, but we have found no trace of evidence for this. We have also examined viruses and fungi. We did find one virus in all infected trees, but this virus is also found in healthy trees. There is nothing to indicate that the pathogen is a virus or a fungus.’
Are there similar pathogens?
‘Yes, there are various strains of pseudomonas that cause disease in plants. There is one that appears to be similar to our suspect, that causes bacterial canker in cherry trees.’
Now that you have identified the cause is this going to help chestnut trees?
‘Not yet. We don’t know anything yet about how the pathogen spreads, nor do we have a good way to treat or control the disease.’
You will present your findings to the Minister of Agriculture in October, and what then?
‘We hope that we’ll get money for follow-up research. We will make recommendations in our report, but whether money is forthcoming is not up to us. That’s up to the Ministry of Agriculture to decide.’ / KV