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Britain's Horse Chestnut Trees are under assault from a number of diseases today. The most serious of these is Horse Chestnut Canker which is specific to the Aesculus family. It is unmistakeable; open bleeding wounds on the bark of the tree which can grow, and if they girdle (go all the way round) the tree, will kill it. It used to be thought that Horse Chestnut Canker was caused by a Phytopthera, but recent research has identified a completely different pathogen, Pseudomonas syringae pv aesculi as being the culprit. The canker will attack trees of all ages and although it can kill old trees, it is most dangerous with trees in the age range 10-30 years as their trunks are not yet so big that girdling is difficult. It is also not a guaranteed killer - some Horse Chestnut Trees fight off the canker and recover.
Despite research, an enormous amount is NOT known about the disease. We know it is bacterial, but we are uncertain as to how it is transmitted (wind, water, insect borne are all possibilities). We know what it does, but we do not have a cure and there are no clear guidances on what to do about it except for these which come from research in Holland (where 1 in 3 Aesculus hippocastanum have the condition.)
In order to stop the canker spreading, try not to prune or do other cutting work on infected trees. If you have to cut, immediately paint the wound with a pruning sealant such as Medo or Prune 'n Seal. If pruning is necessary, the tools must be disinfected in between trees. Use 9 parts methylated spirits to 1 part disinfectant cleaning agent, such as Jeyes Fluid. Do not disturb the surrounding soil.
Diseased trees are best left alone and they do not need to be felled straight away unless they are a threat because they are dangerous. Infected wood should be taken to your local waste processing centre for incineration or composting. Burning chestnut trees as firewood can spread the infection and is therefore not recommended.