Scientist have now ascertained that a bacterium is the primary cause of bleeding canker in horse chestnut trees. This is the outcome of further research in which large trees were artificially infected. This study, which started October 2005, has been coordinated by the Aesculaap work group. Already in 2005 the work group suspected the disease to be caused by a bacterium from the Pseudomonas syringae group.
Although the precise way the bacterium enters the tree is not yet known, it has become clear that damages or wounds of horse chestnut trees present a risk of infestation. The Aesculaap work group calls the situation serious because effective control agents are not available at the moment. Aesculaap therefore thinks that control of this bacterial disease in these characteristic trees will not be an easy matter. The recently started follow-up study will according to the work group show how the disease can be attacked. This study addresses questions such as: how does the bacterium enter the tree and how does it disseminate? Do splashing water, insects, rubbing branches or frost damage play a role? It is also investigated whether stress factors such as drought or water logging play a role or whether a relationship exists between the place where the tree grows and the extent to which the tree is infected.
Based on the study results so far Aesculaap advises municipalities and private owners to leave horse chestnut trees alone as much as possible and to continue following the advice as given on the website www.kastanjeziekte.wur.nl
Bacterial studies
After tests in 2005 with horse chestnut seedlings in which suspicion fell on the bacterium, Plant Research International of Wageningen UR and the Plant Protection Service performed extensive studies in which older trees were artificially infected. These studies showed beyond doubt that the bacterium in question is the primary cause of bleeding canker in horse chestnuts. It is still being investigated whether a new species is involved or an existing variant of a bacterium from the Pseudomonas syringae group.
The scientists also found that the infestation of the tree is often further advanced than expected at first sight. A seemingly healthy bark with no more than a small wound often hides a serious infestation.
National survey
A new survey among Dutch municipalities was recently started. Until now about half of the municipalities, evenly distributed over the Netherlands, have said to be prepared to participate. Although only a small amount of data have been processed until now, the first impression is that the number of infested trees has increased since 2005 and that there is a shift from light infestation to heavier infestation. The survey among Dutch municipalities in 2005 showed that almost a third of the number of investigated horse chestnuts was infested by bleeding canker.
Aesculaap
The following partners are collaborating in the work group Aesculaap: Applied Plant Research (PPO) of Wageningen UR (coordination), Groenadvies Amsterdam BV, de Plantenziektenkundige Dienst (PD) of the Ministry of Agriculture, Nature and Food Quality, Alterra of Wageningen UR and the municipalities The Hague, Utrecht, Haarlemmermeer and Houten. Plant Research International (PRI) and the Chair Groups Plant Cell Biology and Plant Physiology of Wageningen UR are participating in the studies as well.