The Horse Chestnut is being attacked by pests and diseases. But how serious is the threat to one of our favourite, and most recognisable, trees? JOHN SCRACE, Plant Pathologist, explains the symptoms, the causes and the remedies.
CONKERS Under threat!
WHILST it is not a native tree, the Horse Chestnut (Aesculus hippocastanum) has been present in Britain for nearly four hundred years. Instantly recognisable from its leaves, flowers or conkers, it is much loved (particularly by children!) and has been used widely, both in urban street planting schemes and parks. Until recently the tree has been free from serious pest and disease problems but, in the last two or three years, this has changed. You may have read articles in the national press about a two-pronged attack on the trees, by a pest called the horse chestnut leaf miner and a disease known as bleeding canker. The threats to the tree described in these articles have ranged from widespread death of trees to a lack of conkers for the World Conker Championships! Is this more scare mongering by the press, or do these new problems really pose serious risks?
Horse Chestnut Leaf Miner
Those of you in the south-east (particularly in the Greater London area) may have wondered why, in July and August, certain horse chestnut trees looked as if autumn had arrived early. I was working at the Hampton Court Flower Show, during the first week of July, and was shocked to see that the leaves on many of the horse chestnuts in the surrounding area were already brown and starting to fall. The reason for this was not drought (the really hot, dry weather was only just starting) but a severe attack by a moth called the horse chestnut leaf miner (Cameraria ohridella). The moth itself is very inconspicuous, just five millimetres in length and brown with white stripes. It does not feed directly on the trees; it is the larvae (caterpillars) that cause the damage.
Until 2002 the moth was unknown in the UK. It was first seen in the late 1970’s in Northern Greece but did not start to cause problems until 1989 when it began to spread rapidly through central and Eastern Europe. It is now spreading west, and was first found in the UK in Wimbledon in July 2002 (it will never be known whether or not it arrived accidentally with someone’s tennis kit!). Whilst the most damaging attacks are still in the London area and the south-east, the moth has now been recorded as far away as south Wales and north Norfolk, and is likely to continue spreading rapidly. Whilst the moths can be carried for long distances on the wind, such rapid and widespread dispersal is thought to be due to either moths or infested leaves being transported accidentally by cars and other vehicles.
Leaf Miner Symptoms and Life Cycle
Adult moths lay eggs on the top of the leaf - up to 700 eggs have been found on a single leaf. The eggs hatch in 2-3 weeks, and the larvae feed by burrowing between the upper and lower leaf surfaces to produce a brown blotch. The individual blotches are sometimes bordered by the main veins of the leaf but, if numerous mines are present, they will merge together. Severely affected leaves will turn brown, curl and fall prematurely. If
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100