Trees and shrubs, like all living things are susceptible to disease. They need a good supply of light, water, carbon dioxide, and nutrients. If they lack any of these trees or shrubs could come under stress and then not have the reserves of energy for their defensive systems, and hence, become vulnerable to disease.
LEAF MOSAIC VIRUS
- Symptoms: The leaves display cream or yellow spots like a mosaic. Serious infection can cause the foliage to deform and the plant to wither.
- Shrubs: affected: Roses and other ornamental species.
- Treatment: None available. lf the attack is not serious, it is enough to prune the affected parts. Otherwise, dig up and burn the shrub.
- Symptoms: Leaves at the tips of shoots are pierced with minuscule holes; eggs can be seen along the principal vein. These hatch into larvae that hollow a gallery in the leaf, leaving a brownish track. The adult insect sometimes eats the edge of the leaf.
- Shrubs and trees affected: Beech, buddleja, hazelnut, pines and others.
- Treatment: Spray with bifenthrin if the weevils are sufficient to cause problems.
OAK PROCESSIONARY CATERPILLAR
- Symptoms: The leaves are devoured. Large cocoons can be seen on the tips of branches.
- Tree affected: Oak.
- Treatment: Prune out the cocoons before the caterpillars emerge. If they become active, spray with Bacillus thuringiensis or an appropriate approved insecticide.
PEACH LEAF CURL
- Typically, peach leaf curl attacks peach trees as well as ornamental plants, such as camellias and poplars.
- Psylloidea are sap-sucking insects found on ornamental species, most commonly apple and pear trees.
Roses and dahlias are prone to this disease, which is characterised by a white coating on the leaves.
RED SPIDER MITE
- Symptoms: Leaves become dull, turn yellow with blotches, then fall. The undersides are covered in a fine web and eggs.
Shrubs and trees affected: Most ornamental species.
- Treatment: In a greenhouse, introduce the natural control predator Phytotseiulus persimilis as soon as the pest is noticed. In other situations, spray with an approved fatty acid or oil-based treatment.
- Symptoms: The leaves turn a yellow mottled colour. The transparent larvae of the insect can also be seen.
- Shrub affected: Rhododendron.
- Treatment: Avoid planting in warm, sunny places. Spray with a bifenthrin-based insecticide.
- Symptoms: Orange-yellow patches develop on the upper leaf surfaces, with pustules on the undersides causing the leaves to fall prematurely.
Shrub affected: Rose.
- Treatment: At the first sign of attack, spray the leaves with an approved fungicide.
ROSE SLUG SAWFLY
- Symptoms: The leaves are perforated, almost skeletonised, leaving only the principal vein. The remaining foliage withers.
- Shrub affected: Rose.
- Treatment: Spray immediately with an insecticide containing pyrethrin or derris. Repeat several times.
- Symptoms: Numerous black or brown spots are visible on the underside of leaves and on the stalks. If pressed with a fingernail, a hard covering can be felt. Waxy or cottony
discharge can also be seen. A significant presence of sooty mould is a characteristic sign.
- Shrubs and trees affected: Many species.
- Treatment: Due to the hard protective coat, regular spraying with a fatty acid or oil-based insecticide may be necessary to control this pest.
- Symptoms: The leaves take on a silvery tinge. As the infection spreads, the tree withers and dies back.
- Shrubs and trees affected: Members of the Rosaceae family.
- Treatment: There is not a cure for this disease. Prune back affected growth to healthy white tissue (infected wood has a dark stain) in the summer only. Treat the pruning wounds by applying an approved fungicide.
- Symptoms: This pest leaves whitish specks on leaves, which form a silvery marbling.
- Shrubs and trees affected: Privet, palm, gladiolus, ornamental fruiting species.
- Treatment: In your greenhouse, introduce the predatory mite Amblyseius, or hang up sticky boards to trap the thrips. Plant Nicotiana sylvestris, whose sticky leaves act as a trap.
This fungal disease attacks catalpa and Robinia pseudoacacia, causing the foliage to yellow and wither away.
- Symptoms: Irregular nicks occur around the edges of leaves and excreta can be seen on the leaf. Roots are sometimes devoured by white larvae. leaving deep gashes.
- Shrubs and trees affected: Many plants can be affected, including euonymus, hydrangea, rhododendron, yew, skimmia.
- Treatment: Drench infested soil with the natural control nematode Heterorhabditis megidis or an approved insecticide. Ring planters with grease bands to trap adults before they lay eggs in the compost.
- Symptoms: Variously coloured caterpillars devour the leaves and buds of affected plants and form woven, hanging, silky nests.
- Trees affected: Several plants including hawthorn, euonymus. cotoneaster, juniper.
- Treatment: Carefully prune out and burn the nests containing the caterpillars. If the caterpillars become active, spray with Bacillus thuringiensis or an approved insecticide.
- Symptoms: Pale green caterpillars devour the leaves, buds and fruitlets. They leave a trail of silky threads.
- Shrubs: and trees affected: Carpinus, rose.
- Treatment: Place grease bands around the trunks of affected plants and spray the caterpillars with Bacillus thuringiensis or another approved insecticide.
- Symptoms: The needles of affected conifers turn yellow. Those of the pine are covered with white, waxy filaments. On spruce, galls appear at the tips of branches. Fluffy balls hang from the branches of Douglas firs. On larch, the insect is covered with a white wool.
- Trees affected: Conifers.
- Treatment: Adelgids are not easy to control, but conifers are able to support populations without any ill effects.
Powdery mildew is host specific, so different powdery mildew infect different plants. The powdery mildew on your lilacs will not spread to your peaches or your roses, even though all powdery mildews favour the same conditions.
Powdery mildew overwinters in plant debris and begin producing spores in the spring. These spores are carried to your plants by wind, insects and splashing water.
To prevent Powdery Mildew
Dig in organic matter in autumn and apply a mulch during the spring to keep the roots moist.
Minimise humidity by thinning out your plants to avoid overcrowding and so give good air circulation.
If you have plants that are susceptible to powdery mildew, do not feed too much with nitrogen rich fertiliser. This tends to encourage sappy growth which is more susceptible to fungi.