Horse Chestnut pests and diseases

Horse Chestnut leaf blotch

Caused by the fungus Guignardia aesculi, this disease was first reported in Britain in 1935.

It appears in June and symptoms develop within 2-3 weeks of infection.

Sometimes the whole leaf turns brown and shrivels. Leaf blotch is considered disfiguring rather than damaging, but there may be interactions with other problems.

Look out for:

  • red or brown blotches on leaves, often outlined in yellow
  • blotches are on the leaf tips and edges, and you may also see tiny black dots on the underside of the leaf
  • hold the leaf up to light – the blotches are not see-through.

Could be confused with:

  • Horse Chestnut Leaf-miner (see below), which often appears in combination with leaf blotch.

Horse Chestnut Leaf-miner

Horse Chestnut Leaf-miner, Cameraria ohridella, is a micro-moth which arrived in the UK from Europe in 2002. However, 10 years after it was first seen in Wimbledon, those trees remain in good health.

The moth caterpillars feed by tunnelling inside the leaves. First signs appear in June, severely damaged leaves shrivel and turn brown by late summer, and then leaves fall early.

Horse Chestnut Leaf-miner does not permanently damage the tree, but it is an introduced pest and its rate of spread is of interest to scientists.

Look out for:

  • brown blotches on the upper or lower surface of the leaf
  • hold the leaf up to light – the blotches are see-through and you may see larvae in the holes
  • the moths can be seen in May
  • leaf discolouration is found on lower branches initially and spreads up the tree.

Could be confused with:

  • Horse Chestnut leaf blotch (see above), which often appears in combination with Leaf-miner.

Bleeding canker of Horse Chestnut

Bleeding cankerCaused by the bacterium Pseudomonas syringae pathovar aesculi, this is an introduced disease which suddenly appeared in the early 2000s and has spread rapidly.

It attacks and kills the bark of infected trees, which often becomes cracked and disrupted. The problem may be so extreme that the damaged bark peels off the tree exposing the wood beneath.

Ultimately the tree can die if the infection is very severe.

Look out for:

  • rusty coloured liquid oozes from the bark on the trunk
  • the liquid dries to a black crust at the point of exit
  • severely affected trees have thinning crowns with dead branches.

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Horse Chestnut scale

Horse Chestnut ScaleHorse Chestnut scale is caused by the insect Pulvinaria regalis, first found in Britain in 1964.

In May or June, the adult female produces a white woolly ovisac (egg capsule) into which she lays hundreds of eggs. She dies after laying, but remains attached to the ovisac.

When the young emerge, they head up the trunk to feed on the leaves.

It disfigures the tree but it does not kill it, although it could be a problem if it occurs in combination with others pests and diseases.

Look out for:

  • circular white spots topped with brown or orange (the dead females)
  • found on the trunks or branches of many tree species and is visible in May and June.

Could be confused with:

  • the white ovisacs could be mistaken for bird droppings.

 

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