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 may be 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.
If the climate becomes warmer and drier, there may be less beech scale around but the resulting drought stress for the trees could make them more susceptible to Nectria infection at lower populations of scale insect.
How to recognise it
Beech scale insects are approximately 1mm long and live in colonies of females, eggs and nymphs, which form areas of white, waxy, woolly powder on trunks and branches. Scale eggs are pale yellow and laid on the bark in strings of four to eight. The white, woolly wax is not necessarily an indication that the tree is infected with beech bark disease or even that scale insect numbers are high enough to lead to infection. Leaf yellowing may indicate infection but it may also be caused by lime-induced nutrient deficiency on chalky soils.
The disease is caused by the fungus Nectria coccinea var faginata, which infects the bark through small feeding wounds caused by the beech scale insect, Cryptococcus fagisuga, a member of the Coccidae (soft scales). The fungus spreads through the bark, killing patches and weakening the tree.
Infection can kill large areas of bark. Even when this does not directly kill the tree, it can make it susceptible to attack by a wide range of secondary pests and pathogens. This is likely to render unsafe specimens in public places, even before they are killed by the initial beech bark infection.
The beech bark fungus produces two kinds of spores. Perithecia (see symptoms) mature to release sexual ascospores in the autumn, while asexual spores are produced by white, cushion-like bodies, which appear on the bark before the perithecia.
The female scale insects lay eggs from June to October. The eggs begin hatching in late summer and continue through to early winter. Late eggs over-winter.
Some larvae remain under existing female scale and others migrate to cracks in the bark. Some are blown onto uninfected trees. Although the adult insect is only 1mm long, the larva feeds by inserting a 2mm-long stylet into the bark.
Colonies may persist on a tree for some years. Populations on individual trees in the UK are rarely high enough for there to be a significant risk of beech bark disease infection.
The first symptoms of beech bark disease are small areas of dead bark, a few centimetres across, beneath the scale infestation. These patches may exude sap and become sticky, black and tarry.
As the disease progresses, larger areas of bark are killed, the foliage may yellow and leaves may be reduced in number and size. Cutting away infected areas of outer bark can reveal orange staining of the inner bark.
Clusters of small, bright-red, lemon-shaped fruiting bodies (perithecia, producing ascospores) may develop on infected bark. Black fruiting bodies, produced by secondary parasites or saprophytes feeding on the decaying bark, may also be seen.
Dimples on the bark may be an indication of earlier infection that the tree has controlled successfully by compartmentalising with corky layers.
Treatment: biological control
Some ladybirds feed on scale insects and may exercise a natural control. A naturally occurring parasitic fungus, Nematogonum ferrugineum, infects the Nectria fungus, but has not yet been fully investigated as a means of control.
Treatment: Cultural control
Beech scale is favoured by still air and high humidity. Good spacing in plantations or woodland is an effective means of prevention, where other controls would be uneconomical.
Maintaining a good mixture of ages of trees in plantations and woodlands helps to keep scale populations below levels that are likely to lead to the disease.
Keep plants stress-free by making sure they have good drainage, nutrition and water levels. In the USA, vigorous trees that are free of disease have been found in heavily infected areas and offer an opportunity for resistance breeding.
Treatment: Chemical control
Active ingredient Deltamethrin
IRAC code 3
Formulation Various including Decis (Bayer)
Action(s) Contact, synthetic pyrethroid insecticide with residual activity. Incompatible with biological controls.
Active ingredient: Fatty acids
Formulation: Savona (Koppert)
Action(s): Contact-acting insecticide. Compatible with some biological controls.
Active ingredient: Nutrient-based plus additional natural products
Formulation: SB Plant Invigorator (Fargro)
Action(s): Contact insecticide with physical action. Compatible with some biological controls.
Active ingredient Petroleum oil
Formulation Spraying Oil (Certis)
Action(s) Works by physical action, blocking the respiratory system. For dormant season use. Compatible with some biological controls.
Active ingredients Pyrethrins
IRAC code 3
Formulation Spruzit (Certis)
Action(s) Short-term knockdown product based on natural pyrethrin extract. Compatible with biological controls 7 days after application.
Active ingredient Spirodiclofen
IRAC code 23
Formulation Envidor* (Bayer)
Action(s) Contact-acting acaricide which interferes with development. Not on Cordylines. Compatible with some biological controls.
Fully updated by Dove Associates.
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Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.