The leaves of the tree have not just turned brown, they have turned to a crisp. Furthermore, although we are now in November, this has been happening for months. Let’s have a closer look at the leaves of another Horse Chestnut that I found further into the wood:
Horse Chestnut trees are under siege from several directions. The first is via a fungus called Guignardia aesculi, which was first reported in 1935. It first appears in June, and causes blotches which are often outlined in yellow, as in the leaves above. However, the trees have largely learned to live with this fungus. The real problem is a more recent invader.
Horse Chestnut Leaf-miner, Cameraria ohridella, first arrived in the UK in 2002. It is a micro-moth, and its caterpillars burrow between the layers of the leaf, usually between the veins – in the photo above, you can see that the leaves have become transparent, with a little black blob indicating a pupa. The new moths emerge in May. The infestation starts at the bottom of the tree and spreads up, causing the leaves to shrivel and turn brown. This leads to early leaf fall. Although this is not fatal, it makes the trees unsightly, and greatly shortens their period of leaf growth. As if this was not enough, the size of the nuts is also reduced, disappointing any children old-fashioned enough to like a game of conkers alongside their online activities.So, what can be done? One thing that many local councils are doing is to gather up the fallen leaves and burn them – this will reduce the number of moth pupae that hatch and produce new moths. Birds, especially tits, have been seen eating the caterpillars, and may be responsible for munching up between two and four percent of the insects. However, the final answer may come from a much smaller predator. There are hopes that parasitoid wasps may find the Horse Chestnut Leaf Miner to their liking, and that they will start to increase their numbers as they discover how much tasty protein is lurking between the layers of the leaves. There is also evidence that some trees have immunity to the Leaf Miner, and suffer to a much lesser extent.
The Forestry Commission emphasise that there is no reason to destroy a tree with a Horse Chestnut Leaf Miner infestation – although unsightly, the hope is that gradually a balance between predator, prey and natural immunity will develop. There is some evidence that the degree of parasitism is increasing in areas like London, where the Leaf Miner has been present for the longest time. In the meantime, we will have to wait, and hope that our Horse Chestnut trees are soon healthy and magnificent again, as they were in the days when Auntie Mary was alive.