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A large and popular shade tree, the horse chestnut is vulnerable to a number of bacterial and fungal diseases. Besides being beautiful the trees have a useful history. Close relations, Ohio buckeye trees are also susceptible to horse chestnut diseases. Most problems are treated by pruning and attentive maintenance. Strict import regulations on plants can help prevent the introduction of pests and infections into local ecosystems.
Horse chestnut trees are native to southeast Europe, but are prevalent in Great Britain where they were imported as shade trees. The name comes from a belief that eating the tree would cure the illness of horses, although it is actually toxic to them. Conkers, or the inner seed of the tree's nut, were used in munitions manufacture during World War II and for whitening fabric and removing stains.
One of the most devastating horse chestnut diseases is bleeding canker, first noticed in 2002 in Britain. A tree with this condition exhibits large weeping sores, caused by the bacterium Pseudomonas syringae pathovar aesculi (Pae), which made its way from India to Britain over time. A serious canker infection can kill a tree in just a few seasons.
Leaf scorch produces a brown and curled appearance on horse chestnut leaves. Hot dry weather or poor drainage may induce this condition. Although unsightly, it is rarely fatal unless the horse chestnut is very young or weakened by pests or another condition. Watering trees in very dry weather, along with aeration of compacted soil to improve percolation, may help prevent scorch.
Guignardia aesculi causes leaf blotch that initially resembles scorch. Black spots indicate its presence. Another fungus, anthracnose, shows similar infectious signs but affects the bark too. Disposal of infected leaves on the ground won't completely prevent fungal infections from returning. Most horse chestnut diseases resulting from fungus can be controlled by pruning the tree severely, a process from which it usually recovers.
Besides horse chestnut diseases, the trees are sometimes plagued with insects. Scale and mealybugs cause damage to the bark and leaves of horse chestnut. Weather changes can usually control them. Defoliating insects like the whitemarked tussock moth, Japanese beetles, and Cameraria ohridella, a leaf-mining moth whose caterpillars leave distinctive winding scars across the leaves, can wreak havoc on foliage. Insecticides and biological controls like targeted bacteria can eradicate horse chestnut pests.
The Ohio buckeye, a close relative, is also prone to a number of horse chestnut diseases. It is found mostly in the US from western Pennsylvania southwest through Texas, and in Ontario, Canada, where it was imported. Leaf scorch and Guignardia aesculi are most prevalent, along with powdery mildew. Pests that attack buckeyes, including the walnut scale and buckeye lace bug, rarely do significant damage. Walnut scale is easily controlled by pesticides or natural predators.
Although plant importation is heavily regulated in Great Britain and the US, fungal and bacterial infections such as Pae have still invaded. Research into the pathogens' genetic codes will help scientists find a way to control these horse chestnut diseases. Customs regulations in Britain and the US severely curtail the import of plants that are considered invasive or carriers. Inspection and quarantine have helped halt the spread of infections and pests before they enter the country.