SHRUBS AND TREES PESTS CONTROL

Many shrubs and trees are not much troubled by pests and diseases, although there are a few to watch out for. Roses have the most problems, especially the bush roses.

Four pests which make holes in leaves

These pests feed on the leaves, causing holes of various shapes and sizes. Caterpillars are the ones that usually cause most trouble.

• Caterpillars Caterpillars of various moths can cause a lot of damage to trees and shrubs — almost defoliating them in severe attacks. As soon as you see them, pick off by hand. In severe cases spray with fenitrothion, gamma-BHC or pirimiphosmethyl insecticides. The biological control Bacillus thuringiensis is also useful.
• Chafer beetle Cockchafer (3cm/1 ‘AM long) and garden chafer (I.25cm/ 1/2in long), both brown, take out large holes in edges of leaves of many trees and shrubs in early summer. Most chafers also feed on flowers and fruits. If damage is severe spray with gamma-BHC insecticide.
• Tortrix moth The green or brown caterpillars of this moth spin leaves together and feed within, usually during early summer. Spray with fenitrothion.
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• Vine weevil Adult is a long-snouted black beetle-like creature which eats notches out of edges of leaves of plants such as rhododendrons, azaleas and camellias. As soon as you notice damage, treat soil with the beneficial nematodes Heterorhabditis megidis or Steinernema carpocapsae.

Nine problems affecting leaves and shoots

These are all serious trouble-makers. They include the major rose diseases. Treat as soon as signs are seen and do not let any of them get out of hand.

• Aphids Greenfly, blackfly and other aphids attack many shrubs and trees. They cluster around shoot tips and their feeding causes stunted and distorted growth. As soon as you notice them, spray with pirimicarb, dimethoate, hepltenophos or pirimiphos-methyl insecticides.
• Chlorosis A physiological disorder. When lime or chalk in the soil prevents plants from absorbing iron, the leaves turn yellow. Chlorosis will occur if you plant lime-hating plants (eg rhododendrons and azaleas) in chalky or limy soil. It can also occur with roses or hydrangeas grown on very chalky soils; in which case, drench the soil around the plants with iron sequestrene. Grow lime-haters only in acid or lime-free soil.

• Leaf miner Small grubs feed inside leaves of plants such as holly, privet and lilac, making either silver channels or blister- like mines. Pick off affected leaves or spray with pirimiphos-methyl or pyrethrins.
• Leaf spot Collective term for several different fungal diseases which create brown or blackish spots on leaves. Many shrubs and trees are vulnerable. Spray with a copper fungicide as soon as you notice the problem.

• Powdery mildew Appears as a white powdery deposit on leaves and shoot tips of shrubs, including spindle bushes, mahonias and clematis. Severe attacks cause distortion. Spray with benomyl fungicide or sulphur.
• Red spider mites Several species of these microscopic spider-like creatures feed on leaves, causing a bronzy discoloration. Prevalent in hot dry weather. As soon as you see the symptoms, spray with malathion or dimethoate.
• Rose black spot The most common disease of roses. Circular dark brown or black spots, up to 1.25cm/1 thin in diameter, develop on the leaves. In severe cases defoliation can occur. Spray fortnightly during growing season with myclobutanil or benomyl.
• Rose mildew Fungal disease causing a white floury coating on leaves, shoots and buds. Keep plants well watered, and spray regularly with benomyl or myclobutanil.
• Rose rust Fungal disease appearing as orange powdery pustules on undersides of leaves. The leaves fall prematurely. Spray with myclobutanil fungicide.

Four problems affecting roots and bark

One of the following are serious and,if not dealt with carly,can kill even large specimens

• Canker Several different fungi and bacteria cause canker, which appears as sunken and cracked areas in the bark. Sometimes wood underneath is exposed. A branch encircled by a canker will die. Cut out cankers back to healthy wood and paint wound with a proprietary tree paint.
• Coral spot Fungus, appearing as bright pink pustules on dead wood and twigs. Can spread to healthy wood, so cut out any dead stuff and paint wounds with a proprietary tree paint.
• Honey fungus Lethal fungal disease which can kill large trees. Kills roots and works its way up trunk, under the bark. First signs of attack are when branches or stems start dying, or fail to come into leaf in spring. In the autumn honey-coloured toadstools may appear round base of trunk or stems. A huge range of trees and shrubs are susceptible. If you start treatment soon enough you may be able to save an ailing specimen. Drench soil around plant with a proprietary formulation based on phenols, such as Armillatox.
• Scale Scale insects are immobile scale-like creatures found on stems and branches. May be brown, greyish or whitish. If only a few are present remove them by hand; otherwise spray plants with dimethoate, fatty acids or pyreturins.

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