Horse Chestnut with Pseudomanas syringae pv. Aesculi (Pae)
A remedy for Pae?
A horsechestnut suffering from Pseudomanas syringae pv. Aesculi (Pae) commonly referred to as bleeding canker of horse chestnut. I have chosen to use this tree as a case study as it produced some surprising results.
It is known Pae can result in mature trees becoming seriously disfigured, owing to the death of large sections of the crown, as a result of the disease in the supporting vascular tissues. Even very large trees can be killed in a few years, yet many mature specimens are able to stabilise the condition impacting only a localised area of bark.
There are currently no commercially available treatments, it is advised the trees are monitored to see if their condition stabilises. Improving soil conditions and thereby the vitality of the tree can help the recovery of infected trees. (Watson,56,2013)
The industry chooses to reduce risk through pruning therapies rather than other treatments and in this case a reduction of 30% had been carried out. As part of the assessment process, I choose here to mirco-drill to help me develop a prognosis.
Drilling reading
The drilling results did show evidence of heartwood decay but what was clear… effective CODIT walls in place. This is can be seen on the resistophgraph reading as an increase in resistance just before the decay explained by “ the lumen of the cells..plugged by suberines and phenols, leading to a significant higher density and building a special kind of barrier zone”(Rinn,2016,205)
Prognosis
The decay volume was much lower than expected and perhaps further reduction at this stage was not a necessary action. Other strategies for Pae management are limited.
Soil decompaction and mulching by air spading was considered. Vertical mulching is often perceived as a difficult, messy and expensive process, though in my experience, I would argue this is not the case and it an underused remedy. SEE APPENDIX
A difficulty of treating trees is the way in which they are programmed to defend. Both active and passive defence mechanisms are in place BEFORE attack and thus influencing during an attack has proven to be, sadly, less effective than before an attack. (Pervival et al 2014)
Research that compared both preventative and curative PDA (Plant Defence Activator) (Walters 2005) application concluded that the best control option to Pae was provided by preventative treatment. There was some reduction curatively but of a lower significance. (Percival & Banks 2014,17)
It would be interesting to carry out a similar research project comparing the impact of vertical mulching as a preventative and curative answer to Pae but we can only conclude that only preventative PDA application will be impactful.
I could still prescribe PDA application to the remaining horse chestnuts in the park, yet Pae is latent and little research has been done on timescales of latency so this recommendation would be without substance. This leaves me managing risk of failure through pruning as my only sensible and scientifically supported option (Londale, 1999, 210) therefore I will reluctantly recommend, considering financial and risk implications reducing the tree to a monolith of no more than 3m in height.
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