Pests and diseases of shrubs and trees.
Controlling diseases on large trees and shrubs may sometimes require drastic measures. However, many species can support pest colonies without detriment.
- Symptoms: This bacterial disease enters the shrub through a wound and produces whitish swellings (galls), on top of the roots or at the base of the trunk. These turn black and block sap circulation, leading to death.
- Shrubs and trees affected: Cypress, euonymus (spindle), Ribes sanguineum, rhododendron, rose.
- Treatment: There is no effective remedy for crown gall. As soon as the symptoms are recognised, dig up and burn affecplants. Replant new stock in another place. Prevent the disease by improving soil drainage around the plant and feeding with a high phosphate and potash fertiliser. Avoid root damage when transplanting.
FOMES ROOT AND BUTT ROT
- Symptoms: Dark red-brown fungus grows on the trunk, or butt, of the tree at ground level. The lower surface is covered in minute pores. Fine White filaments appear under the bark, penetrating to the centre of the trunk. The needles on conifers turn yellow and the tree gradually dies.
- Trees affected: Spruce, pine, elder, birch, oak, beech, larch.
- Treatment: Dig up and burn the affected tree. Replace the soil before replanting.
- Symptoms: The foliage discolours, Wilts and will progressively die back. The whole plant can die rapidly or hang on for years. Infected plants produce a white fungal-smelling growth beneath the bark at the base of the trunk. Black root-like ‘bootlaces’ grow on the outside of roots, which is how the fungus spreads to other plants. Clumps of honey coloured toadstools may also grow around infected plants.
- Shrubs and trees affected: Most woody plants, climbers and some bulbs. Box, buddleja, caryopteris, chestnut, fir, holly, monkey puzzle.
- Treatment: There is no fungicide to control this disease. Dig out and destroy infected plants immediately, removing as much root and infected soil as possible. Replant with diseaseóresistant annuals or perennials.
PHYTOPHTHORA [ROOT ROT)
A blackish liquid oozes from wounds at the base of the trunk. On digging the soil around the trunk, the roots show signs of fungal rot.
- Symptoms: Infected branches shrivel. Cankers appear at the base of shoots and on the leaves along the veins, making the leaves appear burnt.
- Shrubs and trees affected: Many shrubs and trees, such as cornus, plane, salix, walnut.
- Treatment: Pick up all infected leaves, prune out seriously damaged branches and burn it all. Spray the whole plant with a copper-based fungicide and repeat according to manufacturer’s instructions if the disease continues to affect the tree.
- Symptoms: The bark flakes off to reveal galleries that radiate from a central point where eggs were laid.
- Trees affected: Most species, especially elm.
- Treatment: Since the bark protects the larvae, treatment is ineffectual. Cut off affected branches or cut down and burn infected trees. As a preventive measure, ensure that your trees are healthy, and therefore less vulnerable.
- Symptoms: Fungi develop in clumps on the trunk and branches. The tree weakens and its branches become brittle.
- Trees affected: Ash, beech, larch, oak, pine, plane, Robinia pseudoacacia and yew.
- Treatment: Scrape off minor infestations with a knife. When established, the fungi will have penetrated the tissue, and felling is the only option.
- Symptoms: Swollen-edged cracks form on the bark. The bark tissue is exposed and oozes a whitish substance, or resin in the case of conifers. The tree then withers and dies.
- Shrubs and trees affected: Ash, beech, chestnut, conifers, hawthorn, laurel, lime, plane, poplar, rose , sorbus, willow.
- Treatment: Cut the diseased tissue back to healthy wood with a pruning knife, then burn the parings.
- Symptoms: Branches are covered with little orange-red pustules that rapidly multiply, eventually killing the plant.
- Shrubs and trees affected: Most ornamental species.
- Treatment: Prune out the affected growth to healthy wood, burn the pruning’s and sterilise the tools after use. Clean up infected dead material around the garden, as this can be a source of infection.
Symptoms: The affected stern thickens, enlarges and flattens, forming an undulating ribbon, sometimes with curling edges. The plant continues to grow and flower as normal. This is an accident of growth and not a disease.
- Shrubs affected: Daphne, forsythia.
- Treatment: There is no remedy. If too unsightly, simply cut off the affected shoot.
- Symptoms: Entries to galleries that leak sap can be seen close to the soil. Infected mature trees gradually wither; younger trees become brittle from ground level upwards.
- Trees affected: Birch, ash, poplar, willow.
- Treatment: Seriously infested trees should be felled. Keep down grass around the trunks as it makes a good place for the moth to lay eggs.
- Symptoms: Sawdust at the base of trees beneath galleries bored out of the trunk.
- Trees affected: Poplar, willow.
- Treatment: Destroy the larvae by scraping the galleries with a steel wire. If infestation is too far advanced, fell and burn the tree.
- Symptoms: Conifer needles turn yellow and then brown. Fungi gradually invade the
Whole branch and affected parts are black-spotted.
- Trees affected: Conifers.
- Treatment: Pick up and burn infected needles. Use a foliar feed throughout the summer to aid recovery.
PHYTOPHTHORA (ROOT ROT)
- Symptoms: A blackish flux appears at the base of the trunk. The leaves turn yellow, then brown, and the shoot tips wither. Finally the shrub dies.
- Shrubs and trees affected: Aucuba, catalpa, cornus, holly, elm, skimmia, and rhododendron.
- Treatment: There is no cure. Remove and destroy the plant and infected soil. Improve the drainage and avoid damage to roots when transplanting.
- Symptoms: Small clusters of dense and stunted shoots appear at the tips of branches, causing the foliage to yellow. If shaken, a white dust drops off.
- Trees affected: Birch, carpinus.
- Treatment: This disease is more unsightly than dangerous. Cut off ‘brooms’ and burn the branches to prevent spores spreading.
Symptoms: Dark brown spots appear on the foliage.The spots grow larger and finally join together. At the same time, the leaves turn yellow and fall, leaving the stalks entirely bare.
- Shrub affected: Rose.
- Treatment: Collect up infected leaves, prune out heavily diseased wood and burn. In the spring, spray the whole rose plant and the surrounding soil with an approved fungicide and repeat as directed.
Symptoms: Grey caterpillars with red marks form colonies in woven silky nests that hang in trees and hedges.
- Shrubs and trees affected: Many types of trees and shrubs including apple, cherry, hawthorn, lime, plane.
- Treatment: Prune out the overwintering nests of caterpillars (wear gloves and take great care to prevent Contact with caterpillar hairs, which are highly irritant).
When the caterpillars are active in spring, spray with Bacillus thuringiensis or an approved insecticide.
- Symptoms: Branches suddenly look bare as the leaves have all been eaten.
- Shrubs and trees affected: All deciduous species.
- Treatment: When the caterpillars become active, spray with Bacillus thuringiensis or an approved insecticide containing derris or pyrethrins.
DUTCH ELM DISEASE
- Symptoms: This disease caused the disappearance of most elms in southern England
Wales and the Midlands and has spread into Scotland. The leaves change colour, turn yellowish and dry up. First the branches, then the tree withers and dies. The bark is covered in brown blotches and peels off. A cut branch will reveal a brown ring at the centre.
- Tree affected: Elm.
- Treatment: There is no remedy. Only cutting down and burning affected trees will limit the spread of the disease.
ELM LEAF BLIGHT
- Symptoms: In spring and summer, leaves are pierced with longish holes. Some leaves are covered with mould.
- Tree affected: Elm.
- Treatment: There is no cure. To prevent spreading, collect and burn infected leaves.
- Symptoms: Leaves crumble and look burnt. The bark splits and exudes a whitish ooze, with the inside tissue turning red. Contamination of neighbouring trees is swift.
- Shrubs and trees affected: All members of the Rosaceae family, such as hawthorn, cotoneaster, and pyracantha.
- Treatment : Prune out infected growth to healthy wood and spray with a copper-based fungicide. Disinfect tools after use.