Common Garden Pests and Diseases

One of the most disappointing things for the gardener is to see his plants destroyed or damaged by pests and diseases. This is, of course, to a certain extent inevitable. It happens in even the best-managed of gardens. Everything that lives is subject to ills and diseases, and garden plants are no exception. The stalwart English Oak is liable to some 600 diseases, yet there are still many healthy specimens standing today. One has to be philosophical about these things. Pests and diseases are themselves living creatures, part of the great food chain, and they have to feed, though of course it is to be preferred if they feed off someone else’s plants. Damage and losses are as inevitable a part of gardening as are flowers and fruits, but with proper garden management and an understanding of the control of pests and diseases losses can be kept to a minimum.

Pests include all those creatures which attack plants, usually eating their leaves or stems, but sometimes eating their roots. These are mainly small creatures, including most of the gardener’s traditional enemies such as slugs, weevils, mites, aphids and a variety of insects. There are one or two larger creatures, such as deer and rabbits, that are also pests in some areas. Not all small creatures however, are pests: many are actually beneficial in the garden, keeping the real pests under control. Among these are ladybirds, centipedes, spiders, many ground beetles, shrews, hedgehogs and a large number of birds. A considerable amount of care should be exercised in identifying small creatures before deciding whether or not they need to be exterminated.

Diseases, on the other hand, are of two quite different types. There are those that are caused by physiological factors such as bad drainage, infertile soil, or unsuitable soil for the particular plant: and there are those that are caused by microscopic organisms such as fungi, bacteria and viruses. These agents of disease are, in fact, themselves plants: they are among the smallest, simplest and most primitive of all forms of vegetable life.

In dealing with pests and diseases prevention is plainly better than curative measures. Garden hygiene and correct cultivation are far more important and helpful than spraying the plants with all sorts of prophylactic chemicals. There are certain conditions that tend to promote the incidence of pests and diseases. A plant that is growing unhappily is far more prone to disease than one that is growing happily: plants growing in heavy shade are more liable to suffer than plants growing in the open: overcrowded plants too, are more likely to suffer disease, especially if they are all of the same kind. 

Certain other conditions tend to lessen the likelihood of disease: good, regular cultivation of the soil exposes many pests to their natural predators; the removal of weeds helps too, as these frequently act as hosts for pests; good drainage is another factor, as it discourages slugs and millipedes, and prevents injurious soil fungi from becoming established. 

Diagram showing development of pesticide resis...

Image via Wikipedia

Garden hygiene is of the utmost importance. Weeds left drying on paths are likely to attract fungi and diseases which could spread to growing plants. Prunings should never be left lying on borders or among shrubs for the same reason.

In spite of all these precautions, pests and diseases do occur, even in the best managed gardens. The answer is not to reach at once for the nearest pesticide aerosol: it is to go quite calmly about the business of identifying the pest or disease. Only once this has been done should the decision be taken as to whether or not further steps should be taken, and if so, what steps.

The modern gardener has a whole arsenal of chemicals he can use against pests and diseases, but the most successful gardeners are those who exercise considerable restraint in the use of these chemicals. Many of these chemicals are what are known as ‘broad-spectrum’ pesticides or fungicides — that is, they will kill a very broad spectrum of pests and fungi. This may at first seem an excellent thing. In fact it has its drawbacks: the broad spectrum pesticides not only kill the pests, they also kill the beneficial creatures. Many broad spectrum fungicides also kill soil bacteria which are essential to healthy plant growth. For these reasons these chemicals should be used with considerable discretion. In many cases there are specific chemicals for specific pests and diseases, and wherever possible these should be used in preference to the broad spectrum chemicals.

Soil-borne pests and diseases are best controlled by sterilisation or fumigation of the soil. Formalin is the best chemical for this, being convenient to use, economic and highly efficient. The soil should be dug deeply and saturated with a solution of 1 gal. of 40% formaldehyde in 49 gal. of water applied to the soil at the rate of 5 gal. per square yard. The soil should then be covered with damp sacks for 48 hours to prevent the solution from evaporating before it has had time to do its work, and four to six weeks should elapse before the soil is used for seeding or planting. Sterilisation should not, of course, be carried out in the vicinity of growing plants. Where sterilised soil is required for seeding or potting, the same treatment may be used, piling the soil into heaps before applying the formaldehyde solution. Once sterilised the soil should be kept in sterilised airtight bins until used. Unless really large quantities are required it is usually more economic to buy John Innes composts.

Under glass pests and diseases are more effectively controlled by fumigation or dusting than by spraying. There is a wide choice of proprietory products available for this purpose, many of them in the form of smokes. Some are exceedingly poisonous, and the manufacturer’s instructions should be followed implicitly. Some of these chemicals are poisonous to specific plants, but the manufacturers always list such plants.

The best time for ‘smoking out’ a greenhouse is the early evening, when the plants are dry. The house should be made as airtight as possible, and the work done quickly, starting from the far end and moving towards the door which should be locked when the work is done. The house should be opened up quickly in the morning and aired for several hours before it is safe to stay in it. On no account should fumigation ever be carried out in a greenhouse or conservatory that leads into an occupied dwelling house. In such cases dusts or sprays must be used.

Considerable confusion exists in many people’s minds over the functions of pesticides and fungicides. They tend to use them indiscriminately. Regardless of whether it is a pest or fungus that is to be destroyed. With very few exceptions pesticides kill pests, and fungicides kill fungi.

Pesticides are of two basic kinds: there are those that kill by contact, either by falling onto the insect itself or by providing a layer for it to walk over, and there are those that poison when the pest eats some part of the plant to which the pesticide has been applied, or a prepared bait. In addition there are the new systemic insecticides which are absorbed into the sap of the plant, and which poison the pest when it eats the plant. Many modern pesticides combine more than one principle. All this means that the modern gardener does not need to know whether the pest he intends to destroy is one of the sucking or one of the eating types. It is always preferable, however, to identify the pest and use pesticide designed to destroy that particular creature whenever practicable.

Fungicides are designed not only to destroy the fungus, but also to provide the plant with a thin protective coating that will kill any fungi spores that drift on to it from neighbouring plants. The commonest ingredients of fungicides are copper and sulphur, but other ingredients, such as thiram are now being used. These effective, modern fungicides can now be bought in handy form from garden centres, and are just as efficient as the old Bordeaux Mixture often mentioned in the older gardening books and indeed still often used in larger gardens, but impracticable for smaller gardens.

Many of the modern sprays and dusts are not only highly efficient, they are also very expensive. It is therefore important that the equipment used for applying them should be equally efficient, and considerable advances in design and performance have been made over the past few years. The choice is mainly a personal one, but it will depend to some extent upon the size of the garden. There is now a variety of pneumatic sprayers, power attachments, knapsacks, bucket types and small one-hand devices for liquid preparations, while for the dusts there is a similarly wide variety. Whichever type of applicator is chosen the aim of dusting or spraying must always be the same — to cover the whole plant evenly and thoroughly, including the underside of the leaves. The work is best done on dry, windless days, and preferably early in the day or else early in the evening, when dampness on the leaves will help the dust or spray cling to the plant.

Identification of pests and diseases is not always an easy matter. This is particularly true of the largest group of pests, the insects. These have four stages of development, egg, larva or grub, pupa or chrysalis and adult stage. The grub stage is usually the most damaging to plants, for it is at this stage that the insect has its greatest need for food. The adult stage is usually the least damaging to plants, since at this stage the adult is mainly concerned with finding a place to lay her eggs. Many insects produce more than one generation in a season, and many can lie dormant in the ground through the winter. Different seasons favour different insects, slugs for example being more of a pest in a wet year, ants, for example, being more of a pest in a dry year. It is difficult to anticipate which pests will predominate in any year, and all that can be done is to identify the pests and diseases when they occur and deal with them then. The following are some of the commoner diseases and pests.

07. September 2011 by admin
Categories: Disease Control, Pest Control | Tags: , , , , , , | Comments Off on Common Garden Pests and Diseases


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