Controlling Tree and Shrub Pests and Diseases

Controls and Treatments

Prevention is better than cure — if you follow this principle when growing plants, you will save yourself time and expense. A good start with proper planting, followed by ensuring that food and water are supplied as needed, combined with careful pruning and safeguarding against extreme weather conditions, will result in strong plants able to withstand pest and disease infestation with little or no appreciable effect.

Occasionally you may have to resort to a specific method of control, in epidemic years especially; try to choose the most natural method first and use chemicals only as a last resort. If you have to use chemicals regularly, it is a sign that your plant management is poor or that you are trying to grow plants unsuited to the conditions of your garden.

Insect pests can be divided into two groups: the suckers and the biters. Aphids (greenfly, blackfly, mealy plum aphis, woolly aphis), leafhoppers, scale and mealy bug are some of the suckers, which live on the sap they withdraw from leaves, shoots and sometimes flowers. Derris, bioresmethrin and quassia are some of the safest chemicals to use against such pests.

The biters are mostly caterpillars, also slugs, maggots, wasps and such insects as the rose leaf-cutting bee, rose chafer beetles and vine weevils. Derris can be used for most of these also and is most efficient when at its freshest and there is a good quantity of rotenone present. If slugs appear, methiocarb pellets will be an effective control, or use grit on the soil round soft, vulnerable shoots.

apple tree, branch(es) with canker

Image via Wikipedia

Some of the fungus diseases can be very serious: these include honey fungus, cherry bacterial canker and fireblight, and control is more difficult. Others are persistent, needing regular spraying for several months, although the new systemic fungicide, benomyl, has proved to be very useful for these. Many fungicides are protective rather than eradicant and so need to be applied repeatedly.

Bacterial diseases are hardly troublesome on woody plants; virus diseases occur mostly in the tree fruits in such forms as ‘chat’ fruits and rubbery wood. However, nursery-men take great care to use clean stocks and scions and much of the fruit available in Britain has been cleared under the EMLA testing programme, run jointly by East Mailing and Long Ashton fruit research stations.

Nutrient deficiencies are mainly of iron or magnesium and appear on plants growing in strongly alkaline soils; they can be put right with sequestered compounds of these nutrients and by reducing the alkalinity with peat, acidic fertilizers and sulphur. Such remedies are long-term ones and need skill in use; it is better to test the soil before planting, so that you can put in plants appropriate to the soil acidity.

Harmless repellents can be used to ward off such small mammals as mice, rabbits and voles and larger ones, too — deer in particular; quassia, extremely bitter, is a good one. Aluminium ammonium sulphate, anthraquinone and thiram are others.

Remember that there is a whole army of ‘beneficial’ insects, which are predatory on the pests, working for you; the less you spray, the more they will thrive and so maintain a balance between the various insect species. Once you begin to spray, especially the tree fruits, you run a great risk of ensuring that one pest breeds unchecked; in a monotype planting of — say — apples, it then spreads rapidly.

Remember also that bees and other pollinating insects are harmed by most chemical sprays, so never spray when the plants are in flower, except possibly when most of the blossom has set or fallen, and then do it late in the evening. When you have finished spraying, clean thoroughly all the sprayers and apparatus used and keep all chemicals out of the reach of pets and children at all times. Also, make sure the containers remain clearly labelled.

Another group of chemicals you may wish to use are those contained in weedkillers. Some are undoubtedly very useful and so far their application has not appeared to harm the soil. Simazine will keep the ground clear of weeds for twelve months; paraquat and diquat will kill seedling, annual and small weeds by disrupting the mechanism which produces chlorophyll, and dichlobenil will keep the ground round certain shrubs, roses, and trees free of all weeds, including perennials, for at least the growing season.

The latest weed killing chemical, glyphosate, combines some of the virtues of paraquat and dichlobenil, since it kills the plants through the top growth but is inactivated when it reaches the soil and will effectively control perennials as well as annuals and small weeds. Even the most persistent of weeds, such as bindweed, ground-elder, oxalis and horsetail, succumb to it. It has no effect on the soil flora and fauna.

The following is a list of pests and diseases specific to certain shrubs, trees, fruit and roses; details are given of appearance, damage, life-history and control. Do not assume that they will all automatically ravage your plants; one or more may do so, in some years, but some you may never have to contend with.

Roses

Black spot: fungus disease; fringed black spots 0.6cm (1/4in) wide on leaves. May start from early spring and can defoliate bushes in bad attacks. Pick off, rake up and burn infected leaves; spray with benomyl or captan as makers direct.

Canker: fungus disease; bark on stems cracked and flaking off in patches, especially near to soil level. Worst in humid districts and badly drained soil. Pare off with knife to healthy wood or cut infected shoots off to healthy growth. Improve soil drainage and increase supplies of phosphorus, calcium and/or magnesium.

Chafer beetle: insect; large brown or black flying beetles, which eat holes in flowers, flower buds and leaves, present in late spring and early summer. Control difficult; HCH (BHC) sometimes helps.

Leaf-cutting bee: insect; adults similar to honey bees, remove semi-circular pieces from edge of leaf for making nests. Effect on plant is negligible and control is unnecessary.

Leaf-rolling sawfly: insect; maggots feed in rolled-up leaf margins and leaves wither. In bad attacks much defoliation occurs. Adult lays eggs on leaves in late spring and mid-summer. Spray HCH (BHC) at two-to three-week intervals to prevent adults laying eggs, otherwise hand-pick infested leaves as soon as seen and destroy.

Mildew: fungus disease; white powdery patches on young leaves and tips of shoots from early spring; flower buds and flowers can also be infected. Disease spreads rapidly in badly ventilated sites, at beginning of growing season and in late summer and early autumn. Cut the infected parts off as soon as seen; make sure plants have sufficient soil moisture and improve spacing of plants, branches or shoots. Spray benomyl, dinocap or a sulphur-containing fungicide.

Rust: fungus disease; raised brown-red spots, later turning black, on undersides of lower and older leaves, from late spring to late summer, mostly late in the summer. Collect and destroy infected leaves, as spores can overwinter on them; spray plants with a protective spray such as thiram or zineb, at two to three-week intervals.

Shrubs, trees and fruit

Apple and pear canker: fungus disease; bark cracks, swells and flakes off. If branch or shoot encircled, it dies above canker. Disease enters through injuries, is worst in humid conditions and wet soil. Pare off diseased area back to healthy wood and paint wound with sealing compound or grafting wax. If too large, cut off affected part to below infection and treat cut area as above.

Apple codling moth: insect; pinkish grub feeds on pips and centre of apple from middle of early summer to end of mid-summer. Attacked apples may turn reddish, fall prematurely; collect and destroy. Apply sacking or corrugated cardboard bands to tree trunks late in mid-summer. Spray derris in the middle of early summer and again three weeks later if a bad attack is suspected. Repeat in late summer for second generation.

Apple (and plum) sawfly: insect; caterpillar which is dirty white with brown head, feeds on the flesh of young apples in late spring and early summer. Long, ribbon-like, corky scars on apple skin are also .sawfly damage. Destroy infested fruits; spray g-HCH (BHC) at 80 per cent petal fall stage in bad infestations.

Apple and pear scab: fungus disease; infects leaves from early spring to mid-summer and is particularly bad in warm, rainy seasons. Black spots on leaves, brown markings down central vein of leaf and black spots on fruits, which later crack and rot. Infected young shoots have blistered bark, in which spores over-winter, for one year on apples and up to five years on pears. Cut off all such shoots when pruning in winter and destroy; collect and burn all fallen infected leaves and fruits during season and in autumn and spray with captan, benomyl, or sulphur-containing fungicide as in spray guide.

Armillaria mellea see Honey fungus.

Azalea gall: fungus disease: young leaves thickened and twisted, with grey-white bloom on upper surface, and plants can be killed. Evergreen azaleas only are attacked. Pick off leaves and destroy; spray remainder with zineb to maintain protection.

Cherry (plum, peach) bacterial canker: bacterial disease; leaves in late spring have small round brown spots, which drop out, leaving ‘shotholes’. Bark cracks and flakes off, may girdle shoot, branch or main trunk, all of which subsequently die. Leaves on such branches are pale green or yellow. Canker infection occurs in autumn through injuries. Obtain trees with resistant rootstocks; spray Bordeaux mixture at leaf-fall in autumn, again just before blossom opens and at petal fall. Remove unhealthy parts, treat wounds and do any pruning in spring or in summer immediately after picking.

Capsid bug: insect pest; distorts apple fruitlets badly as well as feeding on leaves. Control difficult without resorting to phosphorus insecticide such as dimethoatc, but this may lead to build-up in red spider mite. Rely on predators if possible.

Clematis wilt: fungus disease; shoots of young plants wilt and collapse rapidly and suddenly. Cut off, to healthy growth, back to soil level if necessary, paint all cuts with sealing compound and spray subsequent growth with copper fungicide.

Fire blight: bacterial disease infecting shrubs and trees belonging to the rose family; flowers and leaves at tips of shoots turn black in spring, oozing patches appear just below bark; later, leaves wither and shoots appear to be burnt. Disease spreads rapidly in wet seasons and trees can be killed. No effective chemical control; cut out unhealthy shoots as soon as seen and destroy.

Holly leaf miner: insect; leaves have pale tunnels and blisters on surface, eventually wither and fall in bad attacks. Maggots feed within leaf tissue. Pick off and destroy infested leaves, spray remainder with dimethoate.

Honey fungus: fungus disease; infects roots of woody plants. Toadstools with honey-coloured surface appear at base of affected plant, bark peels off to show white covering on wood beneath; spreads by means of black threads or ‘bootlaces’ in soil. Affected plants stop developing, leaves wilt and shoots and branches die. Burn dead plants, including roots; do not replant in same site. Treat roots of still living specimens and soil with creosote-based fungicide specific to honey fungus. Mildew see Roses.

Peach-leaf curl (apricot, nectarine): fungus disease; spores over-winter beneath scales on outside of buds, infect leaves as they unfold in late winter and cause thickened, yellowish, later pink-to-red, distorted patches on leaves. Grey bloom on surface of patches in summer is spore-bearing stage which further infects leaves. Defoliation occurs, shoot growth stops and trees can be killed. Spray as buds open with copper or sulphur fungicide, repeat two weeks later and again in autumn, just as leaves start to fall. Destroy affected leaves as soon as seen and cut shoots back to healthy wood.

Pear-leaf blister mite: mite; in spring leaves have small, yellowish blisters which turn reddish and then black, fall prematurely. Fruitlets affected similarly. Hand-removal of affected parts is usually sufficient, otherwise spray lime-sulphur at bud burst stage.

Pear midge: insect; fruitlets deformed and enlarged, with central black cavity, in which will be one or more white maggots. Such fruitlets crack and fill prematurely. Hand-removal is usually sufficient, but in bad infestations, spraying with fenitrothion can be tried, remembering the possibility of red spider mite build up in consequence. Maggots may hibernate in soil two winters in succession.

Peony blight: fungus disease; new young shoots and young flower buds wilt suddenly; buds lower down on the stem turn brown and brown patches, later coated with grey mould, may appear on older leaves. Spray thiram or captan as leaves appear in spring and repeat at two-week intervals until flowering. Cut off all affected parts to healthy growth and paint cut surfaces with sealing compound.

Red plum maggot: insect; red caterpillar feeds in centre of fruits from early summer to early autumn and then hibernates in suitable hiding place until following spring. Treat as for apple codling moth.

Rhododendron bud blast: fungus disease; buds turn brown or grey-brown from autumn onward and by winter have growth of black bristles on them. Buds are killed; frost-damaged buds do not have black bristles. Remove infected buds and a little stem and destroy. Control leaf-hoppers, by spraying in mid-summer with derris or malathion, as they indirectly help to ensure the spread of the disease.

Silver leaf: fungus disease; infects plum, cherry; apple, pear, peach, nectarine and apricot to a lesser extent, also shrubs, especially laburnum and laurel. Leaves silvered on one complete shoot or branch, wood stained brown internally. When tree or shrub killed, purple and yellowish brown, plate-like growths appear from the branches. In advanced attacks, infected branches of tree should be removed and burnt and wounds covered with sealing compound. Improved feeding and manuring should enable remainder to recover. Prune when infection unlikely, from early to late summer.

Vine and clay-coloured weevil: insect; adult beetles, small and black or brown, eat holes in margins of rhododendron, rose and vine leaves. White grubs feed on roots of various shrubs and vines. Fork HCH (BHC) dust into soil occasionally from mid-summer to late winter, to deal with the grubs, and dust leaves lightly from early spring to early autumn, for adult control.

Woolly aphis (American blight): insect; patches of white cotton-wool appear on bark of apple trees, especially at junctions of shoots and branches. Does little direct harm, but looks unsightly, makes picking messy and provides entry for disease through injury made by feeding. Brush away with stiff brush, paint patches with methylated spirits or spray forcibly with derris, from early summer, as necessary.

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30. August 2011 by admin
Categories: Pest and Disease Control, Trees and Shrubs | Tags: , , , | Comments Off on Controlling Tree and Shrub Pests and Diseases

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