Pathogens and pests
A number of well known pathogens and pests are known to affect horse chestnut and are described in the book ‘Diagnosis of Ill-health in Trees' (see Strouts and Winter, 2000).
- Guignardia leaf blotch
One of the most common diseases is Guignardia leaf blotch (caused by the fungus Guignardia aesculi). The disease is recognisable by the reddish or dull brown, irregular blotches that are often concentrated at the tips and margins of infected leaflets. The blotches are often outlined by a conspicuous yellow band (see the photo). Occasionally, horse chestnuts are seen with leaflets that are browned around the edges but without the yellow margin. The agent that causes this condition, known as marginal leaf browning, is unknown but it has been suggested that ‘xylem limited’ bacteria may be involved.
- Wood rotting fungi
Wood rotting fungi, such as Ganoderma and Armillaria are commonly found causing decay on mature individuals of horse chestnut.
- Dead and drooping shoots
Scattered dead and drooping shoots on horse chestnut trees usually indicates damage by squirrels.
- Horse chestnut scale
Horse chestnut scale, apparent as circular white spots on trunks or large branches (and sometimes mistaken for pigeon droppings), is caused by the insect Pulvinaria regalis. This insect pest is occasionally found disfiguring trees in urban localities.
- Horse chestnut leaf miner
Another more damaging insect pest, Cameraria ohridella, is a leaf mining moth which attacks the leaves of horse chestnut. This pest arrived in the UK in 2002 from elsewhere in Europe. It is starting to establish in a number of areas and more information about this pest, its impact on horse chestnut and its current distribution in the UK is available.
See also our research programme: Impact of insects on tree growth
Other bacterial canker diseases of trees
Many of the more common diseases of trees are caused by fungi, but various types of bacterial diseases (some of which are also pathovars of Pseudomonas syringae) are recognised and considered to have a significant impact on trees in the UK.
- Bacterial canker of cherry
The causal agent is Pseudomonas syringae pv morsprunorum. Susceptible trees include Prunus avium (wild cherry) and its ornamental and fruiting varieties and other Prunus species including plums, peach and apricot. Symptoms usually consist of scattered shoots which fail to flush in the spring, but sometimes this can affect entire branches or even whole young trees. Close inspection will reveal large areas of dead and dying bark, usually girdling shoots or branches, and an amber coloured gum also exudes from affected areas.
During the summer the bacteria which cause the cankering can be found on the cherry leaves causing brown spots. Cracks develop around the brown spots, so the discoloured tissue falls out to leave the so-called ‘shot hole’ symptom. As well as infecting leaves, the bacterium also infects the bark via scars left after leaf fall and through any injuries. It is apparently inactive over winter, but in early spring can grow rapidly in the bark causing cankers.
- Bacterial canker of ash
The causal agent is Pseudomonas syringae ssp. savastonoi pv fraxini and it causes bark proliferation and killing on Fraxinus excelsior and its varieties. Symptoms consist of raised or sunken patches roughened cracked bark, and some trees show many separate infections with this appearance. It can be very disfiguring but trees are rarely killed by the disease.
The bacterium can apparently persist for many years in the thickened, infected bark, slowly spreading into surrounding healthy bark each growing season. In spring or early summer, bacteria ooze out of infected bark as a yellowish slime, but little is known about how infection occurs.