A deadly and unnamed virus is attacking populations of horse chestnut trees.

Horticulturalists in Sussex have been warned to look out for the affliction, which has sparked chilling memories of Dutch elm disease.

Gardeners looking after woods in places such as Stanmer Park, Brighton, and Borde Hill Garden, Haywards Heath, have been advised to watch for tell-tale signs such as shedding bark and gummy fluid forming on the tree trunk.

The mysterious disease is thought to have affected 40,000 trees across the UK last year.

A spokesman for the Forestry Commission said:

"It's everywhere, from the south of Scotland to the bottom of England.

"It is a little bit soon to tell what the long-term effects are going to be."

At first scientists thought the disease was a form of bleeding canker, where a fungus known as phytophthora causes one of two types of affliction. But when samples were tested, the fungus was not found.

The spokesman said:

"There are a small number of horse chestnuts getting these diseases, two or three a year.

"It wasn't causing any concern.

The last few years there have been reports of higher numbers of horse chestnuts appearing to have the same diseases. Scientists went out and had a look and in 90 per cent of cases couldn't find phytophthora, so we think there is a new organism around causing this increase."

Scientists from Holland, which led investigations into Dutch elm disease in the early 20th Century, are undertaking strenuous research to find out what is killing the horse chestnuts and how it can be stopped.

The spokesman said: "It is possibly a bacterial disease. It might be more virulent because of a change in climate. Once we know what we are up against, we will have a better idea what the long-term outlook is and what we can recommend to tree owners."

Horse chestnuts were introduced to the UK from the Balkans in the late 16th Century.

Anyone who spots symptoms of the new condition should report it to the Forestry Commission or the Royal Horticultural Society.