Pseudomonas syringae These bacteria are found in more than 50 pathogenic varieties in trees, and since the turn of the century 55 outbreaks in trees have been documented worldwide in 26 countries, almost half of which have been in Europe. In Germany, the host-specific pathovar aesculi was first detected in 2007 on a horse chestnut tree in the district of Altona.
Horse chestnuts play a prominent role in Hamburg’s cityscape and in the public awareness, which has been seen, for example, in concerned questions that the public has directed at relevant authorities and a continuous media interest. Since the 1990s, this much-loved tree species in Hamburg has been struggling aesthetically and biologically with a variety of diseases and pests. This complex disease is the most recent disease the horse chestnut trees have faced. A disease is considered a complex disease when primary damage is followed by further harmful organisms then settling in, with a serious effect on the vitality of the trees. In this case, infection with the bacterium Pseudomonas syringae pv. aesculi (Pae) is followed by infection with wood-rotting fungi.
In order to observe the overall development of the disease in the trees in the city, horse chestnuts are not only monitored regularly but are also analyzed based on macroscopic features and documented in a separate geodatabase. Each year, the databases of the 7 districts are merged and the database is filtered in the online tree registry according to a uniformly introduced documentation procedure. Based on this data, spatial analyses are carried out. During the 3rd round of analyses, 94 horse chestnut trees from this registry were sampled and examined in 2018 using molecular biology techniques. Along with the clear identification of the pathogens P. syrinage pv. Aesculi and Phytopthora spp. using PCR, verification of the visual assessment of the suspected phytopathogen and the associated quality assurance of the spatial analyses were also the subject of the investigation.
Of the approximately 1,400 horse chestnut trees infected with P. syringae pv. aesculi, about half of these have been cut down to date because public safety could no longer be guaranteed. The red horse chestnut (Aesculus carnea) is particularly affected in Hamburg. Since 2013, the red horse chestnut stands have been significantly depleted and around 450 trees of this kind have had to be cut down. That’s almost one-third of the total red horse chestnuts in Hamburg. Although significantly more white flowering horse chestnut trees have been suspected to have the disease since monitoring began, significantly fewer of these have had to be cut down. However, the total number of white flowering horse chestnut trees that have been cut down since 2019 has doubled—a significant increase compared to previous years. More research must be conducted on whether the dry weather over the past few years and the associated drought stress or delayed damage dynamics is the reason for this increased occurrence.
