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What can infect your Horse Chestnut Tree?

Nurtured in Norfolk’s gardening expert Martyn Davey answers all your horse chestnut tree questions.
If you would like any horticulture query answered please do e-mail our head grower at martyn@nurturedinnorfolk.co.uk to add your thoughts to our horticulture hacks.

Dear Martyn, 

I wonder if you could identify what is causing the brown patches on the leaves of the horse chestnut tree that is growing in our garden. I would not like to lose it as it was grown from a conker planted by my daughter about 17 years ago. Although I have noticed many trees in the area that are much larger are also suffering from the same problem I am hoping that you can advise me on how to prevent this getting any worse on our tree in this coming year.

R. Bucke, King’s Lynn

 

Dear Reader, 

The problem on your tree is Horse chestnut leaf-mining moth which has spread rapidly across the country since it was first spotted in 2002. The effect on the appearance of horse chestnut trees in late summer can be profound.

Horse chestnut leaf-miner is a small moth with caterpillars that feed inside horse chestnut leaves, causing brown or white blotch mines to develop between the leaf veins.  It is usually easy to spot trees affected by the leaf-mining moth, especially as the season progresses.  Horse chestnuts produce normal foliage and flowers in the spring and the first signs of leaf-mining usually appear during June.  Elongate blotches, at first white but later turning brown; develop on the foliage from June onwards. Caterpillars, or circular pupal cocoons, can be seen within the mined areas if the leaf is held up to the light. By August, most of the leaf area may be occupied by leaf mines, giving the impression that the tree is dying, although it will survive. Heavily infested trees will drop their leaves early; however it has been found that this has almost no effect on the growth rate or health of trees, although conkers may be slightly smaller.

The small brown (10 mm wingspan) and silver adult moths lay eggs on the foliage. After hatching, the caterpillars enter the leaves and eat the internal tissues.

There are usually three generations during summer, and, by August, the foliage may be extensively damaged, leading to early leaf fall.  This moth overwinters as pupae in the leaf mines.

Horse chestnut trees appear to tolerate the moth and so control is limited as the trees are usually quite large.  Collecting and burning fallen leaves in autumn may reduce the overwintering pupae. Alternatively, the leaves can be composted in sealed bags which should be kept closed until the following July, by which time any adult moths will have emerged and died.

A pheromone trap that attracts male moths is available from several suppliers including Harrod Horticultural. In some areas where the moth is uncommon this may reduce the mating success of the moth and therefore the level of infestation.  These measures can delay the build-up of damage during summer but is only worthwhile for isolated trees where most of the fallen leaves can be gathered up.

Some chestnuts, such as Aesculus indica, A. × neglecta and A. chinensis, are not attacked or suffer only slight damage. Aesculus indica is the closest in size and appearance if a replacement tree is required for A. hippocastanum.

 

Jobs for this week:

Cut back deciduous grasses left uncut over the winter, remove dead grass from evergreen grasses.

Check tree ties and stakes holding new trees in place and adjust as required after the storm.

Although it is still cold at night in sheltered spots it is possible to sow broad beans now, cover the soil with horticultural fleece to give some protection from frosts.

 

Martyn Davey – Head Grower 

EDP Gardening Expert Columnist 

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