EPPO Global Database

EPPO Reporting Service no. 06 - 2009 Num. article: 2009/117

A new disease of horse chestnut caused by Pseudomonas syringae pv. aesculi: addition to the EPPO Alert List

In the Northwest of Europe, increasing numbers of declining and dying horse chestnut trees (Aesculus hippocastanum) have been observed since the early 2000s in urban environments, woodlands and nurseries. Affected trees show bark splits and bleeding cankers on the trunk, defoliation, general decline eventually followed by tree death after 2 or 3 years. Although the presence of a Phytophthora species was first suspected, investigations have showed that a bacterium, Pseudomonas syringae pv. aesculi, was consistently associated with the disease. This pathovar of Pseudomonas syringae was originally observed from Aesculus indica in India, but no further information could be found on the damage it may cause to Aesculus spp. trees in India or in other countries. Molecular studies showed that the sequences of gyrase B gene obtained from P. syringae pv. aesculi isolated in India (on A. indica foliage) and from a P. syringae strain isolated in the United Kingdom (on A. indica showing leaf spot symptoms in Surrey in 2005) were identical. Inoculations studies carried out in the United Kingdom confirmed the pathogenicity of P. syringae pv. aesculi to A. hippocastanum. 5-years old A. hippocastanum saplings were inoculated with P. syringae pv. aesculi and developed necrotic bark lesions and bleeding; the same bacterium could then be re-isolated from inoculated plants.
Considering the severity of the disease and its current spread in Europe, the EPPO Panel on Phytosanitary Regulations recommended adding Pseudomonas syringae pv. aesculi to the EPPO Alert List.

Pseudomonas syringae pv. aesculi (a new disease of horse chestnut)
Why: Since the early 2000s, general decline and bleeding cankers of horse chestnut trees (Aesculus hippocastanum) have increasingly been observed in several European countries. Investigations have showed that a bacterium, Pseudomonas syringae pv. aesculi, was consistently associated with the disease and it is now considered that this bacterium is the main cause of this new disease. Considering the severity of the disease and its current spread in Europe, the EPPO Panel on Phytosanitary Regulations recommended adding Pseudomonas syringae pv. aesculi to the EPPO Alert List.

EPPO region: Belgium, France (Nord-Pas-de-Calais), Germany, Netherlands, and United Kingdom (England, Scotland, Wales).
Asia: India. This pathovar of P. syringae was first observed from Aesculus indica in India but no further information could be found on the extent or severity of the disease it may cause in India or in other Asian countries.
In France, the first diseased trees were observed in 2001 in the city of Roubaix, and then in other locations in the Nord-Pas-de-Calais region (Villeneuve-d’Ascq, Lille, Tourcoing, Hellemmes, Mons-en-Baroeul, Lesquin, Noordpeene). At the same time, similar symptoms were also reported from Belgium. In the Netherlands, surveys carried out in 2007/2008 revealed that 30% of all horse chestnut trees were affected to a greater or lesser extent by the disease. Initially all infections were located in the western part of the Netherlands, but they are now seen across the whole country. In the United Kingdom: previous episodes of bleeding cankers of horse chestnut trees had been attributed in the 1970s to a Phytophthora sp. but these bleeding cankers were considered to be uncommon and only seen in the South of England. However, since 2003 an upsurge of the disease has been observed in the United Kingdom. From 4 cases reported in 2001, 60 were seen in 2003, 90 in 2004, 75 in 2005 and more than 110 were reported in 2006, and from locations as far north as Lancashire (north-west of England), and Glasgow and Fife (Scotland). In Germany, the presence of P. syringae pv. aesculi was confirmed in 2008 in one tree in Hamburg but the disease has been observed from other trees (without further details on their location in Germany).

On which plants: Aesculus spp. (horse chestnuts). A. hippocastanum (both white and red cultivars) is the most affected species. In particular, A. hippocastanum cv. 'Baumanii' appears to be extremely susceptible the disease. Research is being carried out on the susceptibility of other species (e.g. A. x mutabilis, A. flava, A. parviflora and A. pavia), with the additional aim of finding possible sources of resistance. Trees of all ages can be affected but younger trees (10-30 years) can succumb to the disease in 3 to 5 years.

Damage: Symptoms usually start with bleeding lesions: scattered drops of rusty-red, yellow-brown or almost black, gummy liquid ooze from patches of dying bark on the stems, branches or trunks. These lesions can be observed at the base of the tree or at approximately 1 metre high on the trunk (then extending upwards). Bleeding from infected tissues can be quite copious and under dry conditions it can leave a dark, brittle crust near the point of exit. Under the bark, mottled and orange-brown discolorations can be observed. Over the years, the areas of dead phloem and cambium underneath the bleeding areas may coalesce and extend until they girdle the entire trunk or branch. Symptoms on the tree crown then become visible, typically consisting of leaf yellowing, premature leaf drop, and eventually tree death. For example in the United Kingdom, on the basis of a survey carried out in 2007, it was estimated that 35 000 to 50 000 trees were affected and probably a few thousand have already been felled as a result of the disease.
Pictures of symptoms can be viewed on the Internet:

Transmission: So far, the epidemiology of the disease remains unknown. P. syringae could be isolated from the surfaces of horse chestnut leaves and branches, as well as on flowers and various parts of the fruits. Bacteria were also detected in rainwater in the vicinity of diseased trees. However more studies are needed to determine the possible role of water, insects, or even human activities (e.g. pruning) in disease transmission.

Pathway: Plants for planting of Aesculus spp., plant parts (e.g. foliage, wood, seeds)? soil?

Possible risks: Horse chestnut trees (Aesculus spp.) are widely planted across the EPPO region, mainly as amenity trees in parks and gardens or along the roads but they can also be found in woodlands. As the epidemiology of the disease is largely unknown, few control measures can be recommended. However, prophylactic measures can probably be taken to prevent disease spread (e.g. avoid as much as possible pruning, pruning equipment should be disinfected, diseased plant material should be transported in closed containers, incinerated or carefully composted). Data is generally lacking on the geographical distribution, biology and epidemiology of the disease. Considering the significant tree mortality which has already been observed in north-western Europe, it is desirable to prevent any further spread of P. syringae pv. aesculi as this pathogen represents a major threat to amenity trees, woodlands and nurseries.

EPPO RS 2009/117
Panel review date: -
Entry date 2009-06


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INTERNET (last retrieved 2009-06)
JKI web site (DE). New disease of horse chesnut Pseudomonas syringae pv. aesculi (in German). http://www.jki.bund.de/nn_932586/DE/Aktuelles/aktschadorg/rosskastaniensterben/rosskastaniensterben__inhalt.html
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