Beech bark disease

摘要

Beech bark disease is most likely to attack trees with trunks that are more than 20cm in diameter, although it can also be a problem on young plantations. The disease occurs when heavy infestations of beech scale insect (Cryptococcus fagisuga) make enough feeding wounds in the bark to enable infection by a bark-killing fungus (Nectria coccinea var faginata). Until the fungus was identified in 1914, it was believed that the readily visible scale infestations themselves were the direct cause of the bark death. It is not possible to control the fungus once it has infected the tree. Some pesticides are available for use in amenity situations that can be considered if necessary to control the scale insect to keep populations low. Most are active only against the nymph (crawler) stages, before the waxy scale develops. The cost of the repeated treatments that maybe necessary has to be balanced against the risk of the disease developing and the potential costs of removing an infected tree if it does become a hazard.