Garden Pests and Diseases

Like people and animals, plants are affected by pests and diseases. Garden plants aren’t peculiar in this respect; wild plants too are attacked although the impact on them is seldom noticed. Wild plants have had hundreds if not thousands of years to adapt to their environment. Natural selection among populations has meant that neither extremely vulnerable nor extremely resistant plants remain. Very vulnerable ones would be wiped out in a short time; extremely resistant ones would result in the pest or disease itself being eliminated.

Garden plants have had inadequate time to adapt to their surroundings; most are exotic species and most are artificially raised or selected varieties. A plant from South Africa or China may have little or no resistance to the diseases or pests that it will encounter in a European garden. The processes of plant breeding and selection themselves can play a part, for in selecting some horticulturally desirable feature such as large flowers or early maturity, plant breeders may inadvertently have bred out what natural resistance to diseases and pests the plant did originally possess. If we compare plant diseases with human diseases, one big difference becomes apparent. In humans, bacteria and viruses are the most important causes. Sore throats, septicaemia, measles, chicken pox – all have a viral or bacterial origin. Diseases in humans caused by fungi are less common; ringworm, athletes foot and thrush are among the few likely to be encountered. With plants, the situation is reversed. Fungi, usually microscopic moulds, account for the bulk of common garden diseases, like mildew, rust, canker, blight, Dutch elm disease. Bacterial diseases are rarely as important – fireblight is the disease which most gardeners will have heard of. Viruses and other sub-microscopic organisms are important but generally less obvious for, although some do produce distinctive symptoms such as mosaic patterns on leaves, their effect is generally more insidious, bringing about an overall decline in vigour or crop yield.

All pests are animals; most major groups within the animal kingdom include a few species that can cause problems in gardens. At one extreme are mammals and birds; sparrows, bullfinches, rabbits and deer can, in their way, be extremely troublesome. At the other end of the size spectrum are eelworms, microscopic soil-inhabiting creatures barely 2mm in length. In between come creatures such as mites, myriapods, molluscs and crustaceans. Outweighing all others, however, are insects. Aphids, beetles, bugs, scales, sawflies whiteflies, and the larvae of butterflies and moths are just some of the most important.

Unlike diseases, some pest activity is indirect but still troublesome. Ants can undermine plants through their tunnelling activities in soil, for instance, but most pests cause harm by feeding on plants. And there is a broad but useful subdivision into those that are sap suckers, like aphids and whiteflies, and those that are chewers and eat pieces of plant tissue wholesale, like caterpillars and rabbits.

botrytis grey mouldSome fungi and bacteria feed solely on dead matter. They are called saprobes and are very important in the breakdown of organic matter in the soil and in a compost bin. Those that feed on living matter are called parasites. When a parasite causes a disease, it is called a pathogen; the plant attacked is called its host.

The division into saprobe and parasite isn’t, however, a rigid one. Some pathogens can live in innocuous fashion on dead organic matter and then turn their attention to living tissue. Botrytis grey mould is a very important example of this condition and is as much at home on piles of dead vegetation as it is on living plants. By contrast, there are some fungi and bacteria that simply can’t survive in the absence of a living plant and very familiar examples of this group include rusts and mildews.

Fungi can be seen en masse in the form of mould or sometimes as much larger, distinct individuals, like toadstools. Even bacteria, which individually are microscopic, can often be seen as coloured droplets, each containing millions of separate cells. Viruses, however, which can exist only within the living cells of another organism can be seen only with powerful electron microscopes. Fungi are very susceptible to damage caused by drying out and, by and large, they are unable to grow and feed at low temperatures, a feature of temperate-climate winters. This is a problem compounded for parasites by the fact that their host plant has very probably itself died down to survive only as a rootstock, corm or seed.

powdery mildew is probably the most common type of fungal disease in gardensDuring the winter, therefore, fungi are in a vulnerable condition and unable to disperse rapidly. The converse is that, during the summer, they are able to multiply and disperse very rapidly indeed. They do this by virtue of their method of reproduction which produces, not seeds, but spores; much smaller, relatively fragile bodies that are easily blown by the wind, germinate and grow into new organisms very quickly. Much of plant disease control and avoidance is directed towards preventing the formation and dispersal of these spores.

Pests too can be vulnerable in the winter. Being cold blooded, their activity tends to be related directly to temperature and they pass the cold months either as adults in a torpid, hibernating state, or as eggs, larvae or other immature forms. And, like fungi, they too are disadvantaged when their host plants die down. Lets then see how this knowledge of pest and disease behaviour enables us to control them most effectively.

Healthy plants

One common way for pests and diseases to be introduced into a garden is on planting material. When establishing a new planting, therefore, it is essential to obtain your plants, seeds, bulbs and all other planting material from a reputable supplier.

Newly purchased plants in the form of transplant sized individuals, crowns, rootstocks or young trees or shrubs should be inspected carefully and any obviously damaged or diseased parts cut away.

Seeds don’t usually show visible signs of any diseases that they may be carrying and, in any event, it is highly improbable that you will find particles of soil or plant debris with the seed in a packet. Nonetheless, many gardeners save their own seed; I encourage them to do it as it is highly satisfying but you should always try to emulate commercial quality standards and, above all, only save seed from plants that are themselves vigorous and healthy.

By contrast, it s still possible to buy poor quality ornamental bulbs. If you examine them, you will see many that are undersized compared with those from a reputable source; many will have small surface lesions, indicative of a disease problem and may also have scarring or erosion of the surface as evidence of a pest attack. When lifting bulbs yourself, throw away any showing signs of disease.

Where seeds or bulbs, corms and tubers have been collected or lifted from the garden, there’s a further factor to be considered; while they may be perfectly healthy at the time of lifting, they can deteriorate during storage. Cool, dry, well ventilated conditions are needed for bulbs and these are best supplemented by a dusting with one of the combined fungicide and insecticide dressings now available.

Virus contamination is much less obvious, and because it is present throughout the plant’s tissues, it is passed from parent to offspring when plants are propagated vegetatively. This is why seed potato tubers are best bought afresh each season and why plants such as dahlias, carnations and chrysanthemums should be examined carefully before they are used for new plantings. With longer term crops such as fruit bushes and trees, ensuring that healthy stock is obtained is even more important. Always buy only certified virus-free plants from reputable suppliers.

Resistance to pests and diseases

You will sometimes hear of plants that have some inbuilt resistance to pests and diseases and must wonder why these aren’t more widely available. The explanation is that plant breeders can only make use of such resistance as occurs naturally in wild plants. Where these are significantly different from the cultivated forms, it may be impossible to breed the resistance into the cultivated plants without losing some other desirable features. And because the process will, in any event, be long and costly, it is only really worthwhile with major commercial crops such as cereals.

Garden hygiene

A neat and tidy garden is much more likely to be healthy than one that is cluttered with rubbish, debris and the remains of old plants. There are two main reasons for this: rubbish creates hiding places for pests such as woodlice and snails, and as some fungi can live equally satisfactorily on dead and living plant material, they can perpetuate on debris and spread from there to attack living plants.

Traps and barriers

Although you can’t trap plant diseases, you can catch some pests. At their simplest, these traps can be sticky cards to enmesh whitefly, while at their most advanced, they can be pheromone-baited devices to lure and ensnare male fruit moths. Barriers can range from fencing to keep out deer or rabbits to netting over or around soft fruit to keep out birds, lightweight fleece over vegetables to protect them from egg-laying flies or caterpillars, to prickly twigs around soft plants to deter slugs.

Biological control

The use of natural parasites and predators to control pests certainly isn’t new. Farmers in China were doing it in the 13th century by putting ants into their litchi and citrus trees with the objective of protecting them from pest attack. Recently, the use of parasites and predators has been very valuable in the control of pests in greenhouses where the restricted environment is particularly favourable for their use. The greenhouse being enclosed, the predators are unlikely to fly away and, being warm, the environment offers them no incentive to venture outside. Only over the past few years, however, have production, marketing and packaging methods been developed to allow some biological control methods to become available to amateur gardeners. One or two more become available each season.

Encouraging natural biological control

Because a number of biological control methods are now available to gardeners, there’s a tendency to forget that comparable phenomena do already occur and operate in gardens. You can do a good deal towards helping pest control by encouraging these natural systems. Among many groups of beneficial garden insects are various species of ground beetle, hoverflies, lacewings, ladybirds, and many groups of flies, among which the ichneumonids that parasitise caterpillars are the best known. Try to keep garden pesticide use to a minimum so as not to harm these creatures, and particularly avoid chemicals that persist in the environment for some time. Quite often, and understandably, gardeners tend to use pesticides most when they see the most pests. In reality, the bulk of the damage to plants may by then already have been done, and the pest populations could be about to collapse as predators build up. Thus, by using pesticides then, you may be doing more harm than good.

The advantages and disadvantages of biological control


• No chemicals are involved, an attraction (or gardeners who prefer to avoid chemical use.

• The methods are natural in that they make use of a preference that a creature displays in the wild; although not necessarily in the same country.

• Treatments are often fairly specific, in that a particular control agent will only affect one type of pest or group of pests. This is rarely the case with chemicals.

• There is no possibility of damage being caused to the plants; some plant species, for instance, may be susceptible to chemical scorching.


• The methods are relatively expensive compared with most chemicals.

• Few methods are suitable for use outdoors.

• No chemicals or general traps (such as sticky yellow cards) may be used to control other pests affecting the same plants, as these will also kill the predators.

• In small greenhouses, such as those in gardens, the predator or parasite may rapidly eliminate the pest and then itself die out.

Repeat applications (and therefore repeat purchases) may be necessary. In commercial greenhouses, there will always be sufficient pests remaining for the predator to continue.

• It may be necessary to predict the occurrence of a pest problem some time in advance because few control agents are available off the shell; they must be ordered from the suppliers.

• There are no biological control methods available for gardeners to use against plant diseases.

Notes on biological controls

• The ways in which the different methods act vary. The bacterial spray for caterpillars and also the nematode-based controls all depend on bacteria to attack and degrade the target pest, so bringing about its death. The parasitic wasps and gall midges act by laying their eggs into the pest. These then hatch and the resulting larvae feed on the pest. The ladybird beetles, in larval and/or adult form feed wholesale on the pests.

• The recommended minimum temperatures vary slightly between the companies marketing the controls but, within defined limits, all work better as temperature rises.

• Always check the directions carefully with regard to the method of application and for details of how long and under what conditions the organisms will remain effective after you receive them.

• And finally, it should be added that none of these biological control organisms is in anyway harmful to humans.

Chemical remedies

Use of chemicals

Chemicals on sale through garden centres and shops, must, by law, have been cleared for use by the Ministry of Agriculture. The Ministry will have been satisfied that thorough evidence has demonstrated both the effectiveness and the safety of a product, used for the purpose described and in the manner directed. You must, however, follow the manufacturers’ guidelines as given on the product labels.

It is most important only to use chemicals sold specifically for garden use. You must not use household chemicals in the garden and nor should you turn for your supply to commercial plant-growers and farmers. They use far more of a far greater range of products than any gardener and quite different criteria apply to commercial situations.

Insecticides and fungicides

sap-sucking aphids achieve their debilitating effects through force of numbersInsecticides are chemicals to kill insects, whereas fungicides are chemicals used to kill fungi. But the spectrum of pest and disease-causing species extends, of course, beyond insects and fungi, so what is to be done about bacteria and viruses, invertebrates other than insects, and birds and mammals? Most insecticides will have some effect on other living creatures, which is precisely why we have to take precautions when handling them. Several insecticides will have some controlling effect on pests such as woodlice and millipedes but almost none will have any impact on mites or eelworms or slugs and snails. The latter are, however, effectively combated with specific molluscicides, of which two are available to gardeners. Some fungicides will effect some control of bacteria but not very efficiently. No chemical has any direct effect on viruses but the impact of viruses can be lessened by control of the aphids or other creatures which introduce them into plant tissue.

Choice of chemicals

If you walk into a garden centre or garden shop, you will see a very large display of both insecticides and fungicides on sale. How do you make a choice from the many that are available? First, you should appreciate that the number of chemical ingredients is only a small fraction of this total and the same substances are sold under several different brand names. Most advisory literature and radio and television guidance refers to the materials by their chemical, not their brand names. I’ve followed the same maxim here, on this site and you should therefore look for the chemical name or ‘active ingredient’ on the product label. It will generally be in smaller-sized print than the brand name but it must, by law, be there.

It is important to understand that not all fungicides or insecticides control all types of disease or pest with equal efficiency. Unless you have a very serious problem with one specific or unusual problem and must therefore use a very specific chemical, try to buy those products that will treat a wide range of problems. Many products contain a blend of chemicals specifically to enhance this range; many proprietary rose treatments for instance include both a fungicide to control mildew, black spot and rust, while also containing an insecticide to combat aphids. You must not, however, yourself mix together two different products unless the manufacturers state specifically that this is permissible. You may reduce the overall effectiveness and/or damage your plants. And do check also that your chosen product may be used on your particular plant. Some plants (ferns and fuchsias are common examples) may be harmed by products even such as those based on natural soaps that are otherwise perfectly sale.

Systemic and non-systemic compounds

You will sometimes see the word systemic on packaging of garden chemicals. A systemic chemical is one that is absorbed by the plant and moved in the sap from one part of the plant to another. By contrast, a non-systemic or contact product remains on the surface of the plant and kills pathogen or pest as the two come into direct contact there. There are advantages and disadvantages with both. The systemic product is required in smaller quantities, can be sprayed with much less accuracy, is not liable to be washed off by rain and have its effectiveness diminished, and is able to penetrate and eradicate pests or pathogens that are concealed and protected from more direct action. There are. Nonetheless, certain disadvantages with edible produce, for the fact that systemic chemicals are taken up into the plants tissues and have a long-lasting effect means also that the safe interval between time of application and the time that the produce may be eaten is correspondingly longer. In the kitchen garden, therefore, a contact chemical is often the best choice.

Safe use of garden chemicals

• Bead the label carefully and use the product only in the way and for the purpose described.

• Don’t use any chemicals that have lost their labels and don’t decant chemicals from a large pack into a smaller one. Garden chemicals must only be kept in their original packaging.

• Don’t mix or prepare garden chemicals in the kitchen, and keep sprayers, watering cans or other equipment specifically for pest and disease control. Don’t use the same equipment for fertilizers or weedkillers.

• Wash out equipment thoroughly after use and pour excess diluted product on to an area of waste ground. Waste concentrated products should be disposed of according to the advice offered by your local authority.

• Store all chemicals out of reach of children and pets, preferably in a locked cupboard and away from extremes of temperature.

• Don’t spray plants in strong wind, in bright sunlight or when flowers are fully open. The best time is the early morning or late evening.

Identifying pests and diseases

Correct identification is the prelude to effective control. By using the key, you should be able to place your particular problem in its group.

Chemicals available for garden pest and disease control

The following lists indicate the principal chemicals available for gardeners to use. But it is important to realise that these change annually as chemical ingredients or products are withdrawn, the permitted uses after formulations are varied or, very occasionally, an entirely new chemical is introduced. You must check the label recommendations carefully therefore before you buy and before you use any particular product for a particular purpose.

fungicides - chemicals to control diseases

insecticides - chemicals to control pests, especially insects

molluscides - chemicals to control slugs and snails

31. July 2013 by Dave Pinkney
Categories: Garden Management, Pests and Diseases | Tags: , , | Comments Off on Garden Pests and Diseases


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