“He who plants a tree / Plants a hope” Lucy Larcom
Threats to the Horse Chestnut – Aesculus hippocastanum
Documented during 2020 the Covid -19 pandemic
For some years now I have been aware that many trees in the UK and Europe are being increasingly affected by disease. The Covid restrictions in 2020 presented me with an ideal opportunity to look in detail at one of the trees so familiar to us in the UK – The Horse Chestnut. I was aware the Horse Chestnut was being affected by disease as the premature browning of the leaves on these trees is a common sight in late summer. I decided to use my skills as a botanical artist to highlight the affect that pests and diseases are having on our Horse Chestnut trees.
Travel and movement between countries spreads disease, as we witnessed with the spread of Covid-19. This combined with the changing climate is allowing diseases that affect our natural environment and plant species to flourish in new locations.
The Horse Chestnut, native to the Balkan peninsula, was first introduced to the UK in the 16thcentury. It has become one of our most familiar trees and is a common sight in parks, gardens, and streets.
The Horse Chestnut tree I chose to study is situated on the edge of the village where I live and is one of a row of Horse Chestnut trees growing on the verge between the road and the village brook.
The weather was unseasonably warm at the beginning of March 2020. The cases of the Covid-19 virus were on the increase but with nature untouched by the unfolding Covid crisis Spring was well and truly underway. I started painting the resinous sticky buds on the tree.
Painting No. 1 – “Bud”
As I watched the tree, by the end of March the buds were expanding and the soft green of new leaves starting to become visible.
Painting No. 2 – “Expanding” No 1 in RHS Exhibit
By the start of April, the buds had begun to break, the newly emerging leaves were appearing covered in a soft hairy down.
Painting No 3 “Breaking “
The mild weather conditions continued and by the end of the first week in
April the buds were opening rapidly. the flower buds and leaves were unfolding and developing, and the stems and petioles were still covered in a soft hairy down. These hairs may create a protective layer reducing the drying effect of winds on the buds that break in early spring.
Painting No. 4 – “Developing” No 2 in RHS Exhibit
As the Covid-19 pandemic spread across the world, the warm Spring weather meant the tree was changing rapidly and I continued making studies. The leaves were unfurling as the petioles elongated becoming long and smooth. The flower inflorescences with their many buds were growing and expanding.
Painting No 5 – “Unfurling”
By the third week in April the warm spring meant the tree was changing quickly. The flower buds were fully formed and beginning to open, and the leaves completely unfurled.
Painting No 6 – “Formation”
By the last week in April only six weeks from when I began my study the flowers were fully out, and the tree was in its prime, looking stately and beautiful. It was now covered in the creamy candle like inflorescences which change from yellow to pink as the individual flowers are pollinated. The lovely mid green palmate leaves with leaflets ranging in size from 10cm up to over 25 cm were looking spectacular. I have a small stereo microscope and I dissected some flowers and made some studies of what I could see. I drew lateral and transverse cross sections of the flower and drew the stamens. The filaments were dorsally fixed, and I could see grains of pollen.
Painting No. 7 – “Prime” No 3 in RHS Exhibit
As the spread of the Covid-19 pandemic seemed to be slowing with the case numbers dropping, the tree’s development had also slowed. The tree remained in full bloom well into May.
Towards the end of May the flowers started to fade, and the fruits started to form. It was at this time that I noticed a change in the appearance of the tree. The leaves were not looking as healthy and were developing pale cream to brown markings. I suspected that these were signs of the leaf miner moth.
Painting No. 8 – “First Signs” No 4 in RHS Exhibit
By mid-June the fruits were reducing in numbers but those remaining were growing and developing their spiky green outer cases. The leaf damage was becoming much worse with leaves browning and curling. The Horse Chestnut leaf miner moth (Cameraria ohridella) was first discovered in Macedonia in the 1980s and since then has spread across Europe reaching the UK in 2002. The moth is small about 3 to 5 mm in length. The moth’s larvae mine into the leaves of the Horse Chestnut causing pale white to brown blotches to develop between the leaf veins. I found if the leaves were held up to the light, the caterpillars and cocoons could be seen quite clearly.
In order to study the moths in more detail I decided to hang a trap for the leaf miner moth in the tree. The trap contained a pheromone lure to attract the moths. I left the trap overnight and to my amazement the following morning the trap was filled with dozens of leaf miner moths.
I caught a few moths from the trap and got a friend to photograph these using an electron microscope enabling me to study a moth in close-up. I was then able to make some pencil studies and to paint it.
Painting No 9 – “Infestation” No 5 in RHS Exhibit
By mid-July, the moths were extremely active and the leaves beginning to brown and curl and some to fall. I continued to make studies of the tree’s development. Leaf miner moths do not appear in themselves to significantly impair the health of trees just their appearance. However, the defoliation can reduce the tree’s vigour and make it more susceptible to other diseases.
I noticed at the tips of some of the leaves there were yellow edged brown areas and I suspected this to be signs of Horse Chestnut leaf blotch which is caused by a fungal pathogen. (Guignardia aesculin) The fruits were growing and were now 5 to 6 cm in diameter although had significantly decreased in number.
Painting No 10 – “Discolouration”
By the middle of August, I noticed the trunk and some branches had developed lesions and cracks in the bark. These areas were weeping a sticky brown fluid causing rusty brown staining. Under the bark the wood was a blue black with sooty deposits. On investigation, I identified this to be Horse Chestnut Bleeding Canker, an infection caused by a bacterium Pseudomonas syringae pv aesculi. This disease was first recorded in the UK in the 1970s. It is found in all parts of Great Britain. Whereas the leaf miner moth causes unsightly damage to the tree but does not cause the tree’s death, bleeding canker can cause the crown of the tree to thin and to severely weaken the tree and sometimes cause its eventual death.
At this same time, I noticed the yellow edged patches of leaf blotch disease appearing on many more of the leaves. Like the leaf miner moth, leaf blotch whilst unsightly does not appear on its own to cause significant harm to the trees.
In this painting, I wanted to document both the bleeding canker and the leaf blotch. The canker was a challenge to paint as it involved sketching areas of the damaged tree trunk on site. However, to record the details, I had to combine these sketches with my own photographic reference.
Painting No 11 – “Bacterium and pathogens” No 6 in RHS Exhibit
By mid-September, the tree was well and truly brown with a lot of defoliation. By early September the fruits were splitting open and shedding their seeds commonly known as conkers. The conkers whilst rather small and few in number, appeared undamaged and by now many were on the ground.
Painting No 12 “Seeds”
Throughout October the leaves continued to fall. The canker was still evident with the trunk and branches still weeping in places. As the year drew to a close, the Covid pandemic was once again on the rise. However, despite all the disease and damage the tree had suffered new buds had already started to form, and the life cycle had begun once again.
I was very pleased to paint and highlight the affect that pests and diseases are having on our Horse Chestnut trees. Climate change and the movement of peoples around the world is accelerating the spread of pathogens and tree diseases. An estimated 10% of Horse Chestnut trees in Great Britain have been infected by bleeding canker. In some trees studied the conkers on damaged trees have withered and perished.
Under the tree in my study there were significant numbers of conkers on the ground. Whilst rather small these appeared to be in good condition. It would however require study over a period of years to see if the diseases affecting this tree are having any impact on the numbers of seeds produced.
The impact of the combination of pests and diseases affecting our Horse Chestnut trees combined with our warming climate means our Horse Chestnut trees are under pressure.
The International Union for the Conservation of Nature [ICUN] has classified the tree as vulnerable of extinction. Scientists and horticulturalists are studying the risks to the trees to try and save what is one of our most familiar and loved trees.
“For most of history, man has had to fight nature to survive; in this century he is beginning to realize that, in order to survive, he must protect it“
Jacques-Yves Cousteau
