Tree pests & diseases
Horse chestnut leaf miner
The horse chestnut leaf miner (Cameraria ohridella) is a moth whose larvae feed within horse chestnut leaves, causing brown patches of regular shape and giving an autumnal appearance. The pest was first reported in Wimbledon in 2002 and is now found in much of southern England. There is no evidence that infestations lead to a decline in tree health, possibly because the damage tends to occur late in the season. For more information:
Horse chestnut bleeding canker
Once thought to be due to two species of Phytophthora, this condition is now attributed to Pseudomonas syringae, a bacterium isolated from a related horse chestnut growing in India, Aesculus indica. The frequency of the condition has increased over the last five years, and it is no longer confined to southern England. It kills bark, resulting in a conspicuous black exudate, and is more or less progressive. Branches and trees, including young trees, can be completed girdled and killed, or there may be remission, resulting in an elongate bark wound that may resemble lightning damage. Very little is known about spread or treatment; some individual trees may be innately resistant. For more information:
Oak processionary moth
The larvae of this moth (Thaumetopoea processionea) defoliate oaks by feeding on the leaves. Their hairs can also cause skin irritation and asthma in humans. Repeated attacks will weaken trees but there is no evidence that trees have been killed. At present it is confined to London. For more information:
Oak decline was first recorded in the 1920s. Affected trees typically become stag-headed over many years, although it is thought that the condition is becoming more progressive and less chronic. It has complex causes: infection by root infecting fungi, recurrent drought and insect attack. Declining trees occasionally show dark, watery fluxes from cracks in the bark.
Sudden oak death
This fungus (Phytophthora ramorum) was first identified in California in 2000. It has about 40 host species, not just oaks, and depending on the host it may appear as a lethal stem canker (associated with bleeding from the bark) or be confined to the foliage and shoots. In England, the most susceptible tree species appear to be red oak and beech while our native oaks, Q. robur and Q. petraea, are not very susceptible.
Many Phytophthora species cause bleeding from the stem in English oaks, so this symptom does not signify Sudden oak death.