Last year I told you about our horse chestnut trees in Chippenham and how leaf miner moth is bringing an early autumn to them. At the time there was some speculation that a severe winter could bring this pest to its knees. If you click on the above picture to enlarge it, you'll see the leaves on these trees are starting to go brown. This picture was taken in early August, about 6 weeks ago. Well before autumn starts round these parts.
Alas, our worst winter in 18 years hasn't killed off the moth and our trees are now looking worse than ever. The picture above shows where the moth larvae have burrowed between the upper and lower leaf surfaces and thus destroyed those parts in the process. The guidance now is to collect all the infected leaves in the autumn and burn them, so that the larvae don't emerge in the spring to start the cycle all over again.
I had pondered whether composting might do the trick, but my research shows this isn't advisable in garden situations unless you can provide very hot composting conditions to kill the moth pupae or can put a few inches of soil over the piles of leaves to stop the larvae from emerging in the spring. I could do the latter, but I'm severely limited in the amount of leaves I could collect and store. Chippenham has many stately horse chestnut trees, so re-infection from nearby would be swift to follow for any trees I did manage to clear of the pest. I suspect our local council won't have the resources to do a major leaf clearance for the hundreds of trees affected locally.
Then Threadspider told me about the state of some of the horse chestnut trees towards the top of our estate close to where she lives. I went to inspect them and the picture above shows what I found in addition to the usual leaf miner damage I've shown you already. A quick look on the internet later confirmed our suspicions: these trees have bleeding canker caused by the Pseudomonas syringae pv aesculi bacterium.
The Forest Research website has lots of information about bleeding canker - as well as leaf miner moth - and it looks like it may have more serious implications as the disease can kill if it girdles the whole tree, which could happen in a few years time. Horse chestnut leaf miner is not considered to be a killer, it just impacts on the tree's vigour as photosynthesis is affected from around July when severe infestation occurs. This seems to happen about 2 years after the first attack from my observations and to me it also looks like the numbers of conkers are noticeably less, thus affecting reproductive capability as well as disappointing many children wanting to have their traditional autumn conker fights.
I was feeling pretty depressed about the trees on our estate, but researching for this post has thrown up some potential cheer. The RHS recently announced that treatment at Buckingham Palace on 5 trees with both bleeding canker and leaf miner moth may lead to them being given a clean bill of health. It also appears there's a treatment based on garlic extracts on trial at the moment, which has been successful in combating bleeding canker and has the additional effect of being a moth deterrent. This treatment is expensive, but should be cheaper than the cost of cutting down and replacement with another kind of tree.
And that's not all: a wasp predator of the leaf miner moth has been found. Schoolchildren in Bristol are helping the university to research whether this might be a suitable future treatment for trees suffering from moth infestation alone.
Now I'm waiting for someone from our local council to come and see me so we can discuss what's going to happen to our horse chestnuts. I'll keep you posted on what happens.
Don't forget Out on the Streets is all about your contributions and there's been some brilliant ones already. Just write a post about the public planting in your neighbourhood some time in September, add your link here and we'll come and visit :)