Garden Pests and Diseases of Trees and Shrubs

Garden Pests and Diseases of Trees and Shrubs

Garden Pests and Diseases of Trees and Shrubs Most gardeners who grow trees and shrubs find that these plants suffer from very few pests and diseases. In my experience, I have found that a large amount of spraying and dusting is unnecessary. However, I am always on the lookout for an attack, and, should one occur, I take the necessary control measures in the early stages before the pest or disease becomes established. In order to prevent attacks of the very common pests and diseases I spray the plants regularly throughout the growing season. With widespread troubles I always feel that prevention is better than cure.

I have listed the pests and diseases most likely to cause trouble and although the list may seem rather long, it by no means implies that you will encounter all of them. Some you may never see at all, but it is best to be aware of them.

Garden Pests affecting Trees and Shrubs

Aphids. Greenfly and Blackfly are the most common aphids and attack many plants. They are barely 1/16in. long and quickly colonise leaves and young shoots, which they seriously weaken by sucking the sap. Euonymus europaeus is a winter host of blackfly and maybe really smothered with the pest in a bad season. Woolly Aphids are easily identified as they are covered with a white cotton-wool-like substance. A large colony is very conspicuous on branches of beech, hawthorn, pyracantha, Cotoneaster horizontalis and numerous other host plants.

I control aphids by spraying regularly during spring and summer with gamma-BHC (lindane) or derris. Woolly Aphids can also be controlled with menazon, BHC, malathion or nicotine and soft soap. The spray must be applied forcibly so that it penetrates the insect’s protective covering.

Capsid Bugs. These small, yellowish-green bugs are extremely active and attack a wide range of plants during spring and summer. They suck the sap and cause young shoots and leaves to become distorted. Deciduous trees and shrubs can be sprayed with nut in petroleum oil just before bud burst, about mid-March. General spraying can be undertaken in spring and summer, using gamma-BHC or DDT, at the first sign of damage.

Caterpillars. The larvae of moths or butterflies may eat the leaves of numerous trees and shrubs and will therefore render them most unsightly. Tortrix Moth larvae of various species often do a great deal of damage to rhododendrons, roses and many other trees and shrubs and usually spin webs, which bind the leaves together. Spray the plants with derris or DDT, starting in April or May, as and when necessary. In the case of Tortrix, spray before the larvae bind the leaves.

Cockchafers. The large fat white grubs, which are the larvae of the Cockchafer Beetle, live in the soil and feed off the fibrous roots of plants. This may severely check a young tree or shrub. If a plant seems to be in trouble it will pay to water the ground around it thoroughly with DDT solution. Grubs noticed while the soil is being cultivated can be picked up and destroyed.

Froghoppers. The larvae of this insect are probably best known as Cuckoo Spit, as they are covered with a mass of protective froth. They are yellowish-green in colour and suck the sap of plants such as roses and lavender. They are seen around the young shoots only. The adults are small, yellowish and jump when disturbed. Spray the larvae forcibly with either gamma-BHC, nicotine or malathion.

Leaf Miners. These are the larvae of several kinds of fly and as the name suggests they tunnel inside the leaves, making irregular and Unsightly silvery channels. Holly and lilac are, in my experience, the most commonly attacked shrubs. Pick off and burn badly infested leaves and spray frequently with gamma-Bite or nicotine.

Red Spider Mites. These minute creatures can only just be seen with the naked eye, but in spite of their smallness a large colony of them can do a good deal of damage They are reddish in colour and suck the sap of plants; they are normally found on the undersides of leaves. The leaves gradually take on a yellowish, finely-mottled appearance and fall prematurely. Among the numerous plants attacked are ornamental peaches, plums and crab apples. Plants should be sprayed frequently during spring and summer with either derris, malathion or dimethoate. Ornamental peaches, plums and crab apples can be sprayed with DNC in late winter.

Scale. These small insects are oval in shape and attach themselves firmly to leaves and stems and suck the sap from the tissues. This seriously weakens the shoots. The adult scale insects are covered in a hard, protective shell. .There are numerous species, but the greyish Mussel Scale is possibly the most common and attacks many plants. The Brown Scale is also fairly common and is very often seen on yew. Plants may be sprayed at any time of the year with malathion or diazinon, while small numbers of scale insects can be removed with a knife. I also find white-oil emulsion an effective spray.

Weevils. The most troublesome is the Clay-coloured Weevil, a small, greyish-brown insect with a typical, elongated snout. The adult chews the leaves and shoots of many plants, including rhododendrons, roses, azaleas and clematis. The small, white larvae eat the roots of plants. The Vine Weevil does similar damage to that of the Clay-coloured Weevil, and the adults are dull black in colour. Spray plants with DDT or BHC and to kill the soil-borne larvae dust the ground around the plants with DDT or BHC and fork it well in.

Garden Diseases affecting Trees and Shrubs

Armillaria. Many trees and shrubs, particularly lilac, privet, hawthorns, cherries, pyrus and rhododendrons, may be attacked by Armillaria mellea, the Bootlace or Honey Fungus. The tree or shrub may die suddenly for no apparent reason, and inspection of the roots will show the black, bootlace-like threads of the fungus which grow through them. White fungal threads can also be seen beneath the bark at soil level and honey-coloured toadstools appear on the soil surface around the infected plants.

The infected plants, together with their roots, must be dug out and burnt. If ever my shrubs are attacked I dig out as much soil as possible in the infected area and replace it with fresh soil. I usually take the risk of planting another specimen in the fresh soil. I have known of a recurrence, but not often.

Bacterial Canker. Cherries, plums and peaches are prone to this disease. The bark is killed and usually complete branches wither and die. Gum will ooze from the affected bark. I cut the branches right back to healthy wood and seal the cuts with a bituminous tree paint. This disease will eventually kill a complete tree if it is not treated in time. You can also spray heavily with Bordeaux Mixture in late summer and early autumn.

Chlorosis. This is not a disease but a physiological disorder and occurs when plants are grown on very limy or chalky soils. In alkaline soils iron becomes unavailable to plants, which results in the leaves becoming pale yellow, or covered in yellow patches, and in some cases stunting of the growth occurs. Roses and peaches are very often affected.

To rectify this trouble avoid using lime or chalk and incorporate plenty of peat in the soil. Water the ground around the plants — and also spray the leaves — with iron sequestrol — this is a readily available form of iron and is not ‘locked up’ in the soil by the action of lime.

Gumming. Gum very often oozes from the branches and stems of cherries, plums, peaches and related species and may be a sign that the branch has been damaged in some way. This could be due to Bacterial Canker, therefore this disease must be controlled. The actual gumming is a physiological disorder and usually is no cause for alarm.

Leaf Curl. This fungal disease affects peaches and almonds and causes the young leaves to become thickened and distorted. They appear yellowish at first but rapidly turn bright red. Shoots may die back from the tips and young trees may even be killed by repeated attacks. Spray with Bordeaux Mixture or a copper fungicide, or captan in late February or early March, just before the buds begin to swell. Remove infected leaves and twigs. After serious infections, repeat the spray at leaf-fall.

Leaf Spot. There are various fungi which cause brown or black spots on the leaves of numerous trees and shrubs. Remove those worst affected and spray the plants with either Bordeaux Mixture, colloidal copper, colloidal sulphur or zineb. Mildew. There are a number of different sorts of mildew, some of which attack specific plants, for example, the Rose Mildew. All of them are seen as white powdery or mealy patches on leaves, stems and buds. They thrive in a damp atmosphere and so are most prevalent in a rainy season. Plants may be weakened in severe, prolonged attacks and shoots and buds crippled.

Plants must be sprayed frequently with colloidal sulphur, thiram or dinocap, or dusted with flowers of sulphur. Start spraying in the spring and spray whenever there are signs of attack, especially after damp spells.

Rust. The most serious rust disease as far as trees and shrubs are concerned is the one that attacks roses. It is seen as raised orange spots, which later turn black, on the undersides of leaves and also stems. An attack will cause stunting and distortion of growth. This fungus can be controlled by spraying at frequent intervals during spring and summer with colloidal copper, zineb or thiram.

All badly affected leaves and sterns should be cut off and burnt.

Silver Leaf. This fungal disease will attack ornamental peaches, plums, almonds, cherries, hawthorn, roses and Portugal Laurel (Prunus lusitanica). The leaves take on a silvery sheen after the wood has been infected by the disease. Branches and even complete trees can be killed in time and fungal outgrowths form on the dead wood. Sometimes these are flattish, and purplish-mauve in colour, or they may be of the bracket type, arranged in ridges one above the other. The bracket fungus is purplish beneath and brownish above. Infected branches also have internal dark brown staining of the wood.

Infected wood should be cut out in June, July or August. Infection is least likely between these times. Cut back to unstained wood and seal the cuts with white lead or bituminous tree paint. The disease will only enter through wounds, particularly pruning cuts, so paint these as soon after pruning as possible.

Burn all affected branches which have been cut out — never leave diseased wood on the ground.

02. December 2010 by Dave Pinkney
Categories: Garden Management, Pests and Diseases, Plants & Trees | Tags: , | Comments Off on Garden Pests and Diseases of Trees and Shrubs


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