Prevention
Identification
Crop rotation
Resistant cultivars
Planting and harvesting date
Clean seed and healthy plants
Soil preparation
Garden management and hygiene
Weed control
Fertilisers and irrigation
Biological control
Companion planting
Chemical control
 
 
 
Few people make a full-time living from their home garden, so it follows that most gardeners have other reasons for following this pastime. For some it is a hobby, many want to create a peaceful living environment while others want to save money and produce nutritious food. Despite these vastly differing objectives everyone wants healthy plants in their garden. This is where integrated garden management (IGM) comes in.

Ever since humans changed from being hunter-gatherers to actively farming plants and animals for food and other necessities, people have looked for ways to increase yields, improve quality or to reduce the effects of pests and diseases. Over the past several hundred years this has become an occupation for some people – the scientists who spend their lives studying certain plants or animals and looking at ways to get better returns from farming.

Towards the end of the 20th century, scientists in New Zealand began to pool their knowledge and develop packages for managing certain crops. This arose out of the need to decrease reliance on chemical control of pests and diseases and became known as integrated pest management (IPM). A technical definition of IPM is “the control of pests by employing all methods consistent with economic, ecological and toxicological requirements while giving priority to natural limiting factors and economic thresholds”.

The IPM packages developed by scientists for farmers can also be utilised by home gardeners. The concept behind IGM is to use a range of approaches to ensure that plants remain healthy and the effects of pests and diseases are limited. IGM principles are often even more relevant for home gardeners than commercial growers, since people are often very concerned about maintaining a healthy environment around their home and also have a greater tolerance to damage caused by pests and diseases than many of our export markets!

IGM involves a range of techniques, not all of which are relevant for every plant being grown in the home garden. In fact, some techniques beneficial to some plants and in some situations may be harmful to other plants and in other conditions. Like most things in life, it is all a question of balance! Some of the most common IGM techniques are discussed below and more information is available in the sections on specific plants.

Prevention

The first step in IGM is always to prevent a pest or disease from getting into the garden. Some of the techniques listed below will help with prevention, like disposing of diseased plants by burying or burning so that subsequent crops are not infected. Always buy healthy plants and certified seeds to ensure that new diseases or insects are not brought into the garden.

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Identification

Observe your plants closely. Watch for distorted growth, wilting or yellowing. These are all signs that the plant is not healthy. Is this caused by a disease, an insect pest or perhaps something wrong with the soil? Look up books, use the internet or ask an expert at a garden centre to help with the diagnosis. Often if a problem is identified early enough then a solution can be found.

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Crop rotation

Many pests and diseases are specific to one crop. By changing the position in the garden that the crop is grown in every year these diseases can be limited, as the new crop is not reinfected from the soil or previous crop residues. For example, nematodes and powdery scab that infect potatoes may survive in the soil or on tubers remaining from the previous crop. In the ornamental garden it is often advisable to plant a different species when replacing a plant that has died.

Improved soil fertility can be another benefit of crop rotation in the vegetable garden. Leguminous plants, such as peas and beans, have root nodules containing bacteria that fix nitrogen. The nitrogen fixation process makes atmospheric nitrogen available to the growing plant and may also increase the level of soil nitrogen available to subsequent crops.

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Resistant cultivars

One of the best methods of limiting the effects of pests and diseases is to grow resistant plants. In this situation the resistant plants are not infected or they can tolerate the pest or disease without major losses in yield or quality. Much of the effort in plant breeding programmes is directed towards selecting for pest and disease resistant plants. Where possible choose cultivars that are resistant to pests and diseases. For example, the sugar pea Snow Queen is resistant to powdery mildew, while the floribunda rose Iceberg is known for having disease-free foliage.

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Planting and harvesting date

In the vegetable garden planting and harvesting dates can be adjusted to avoid the times when pests and diseases are at their worst. For example, delaying planting of potatoes until soil temperatures have risen will help prevent powdery scab damage, while it is advisable to harvest carrots before the period of peak flight activity (April-early May) of carrot rust fly.

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Clean seed and healthy plants

It is a good idea to buy seed that has been certified free of diseases. Some home gardeners prefer to harvest their own seed but make sure that the parent plant is healthy. When buying plants for the garden look at them carefully to make sure you are not bringing new pests or diseases into your garden!

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Soil preparation

Before sowing seeds or a potted plant, make sure the ground is well prepared. This includes providing good drainage, supplying appropriate fertiliser (which may include adjusting the pH) and ensuring the soil is well aerated and not compacted. Research has demonstrated that peas growing in compacted soil are more likely to become infected by root rots, while camellias require neutral to acid soil.

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Garden management and hygiene

It is well known that healthy plants are more resistant to pests and diseases than plants that are growing poorly. The garden needs regular maintenance and application of the IGM principles. This need not take a lot of time – a nightly stroll round the garden can reveal an area that has not been watered or some tomatoes than fertiliser. Diseased leaves of roses can be picked off and disposed of by burial or burning.

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Weed control

Plants may be weakened and become more susceptible to pests and diseases if they have to compete with weeds for moisture and nutrients. In addition some weeds harbour pests or diseases that are transmitted to ornamental or crop plants. For example, hemlock can harbour leaf blights or spots that can infect carrots. So remember that weeding isn’t just to make the garden look good!

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Fertilisers and irrigation

Plants that don’t have sufficient fertiliser or water won’t produce good yields and they are more likely to be affected by pests and diseases than healthy plants. However, ensure that the correct fertiliser is being applied and don’t over-water. For example, Aphanomyces root rot is frequently found in peas growing in wet soils, while excessive nitrogen fertiliser on potatoes can lead to increased levels of bacterial soft rot.

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Biological control

Biological control is when an organism is used to minimise the effects of another organism that is a pest or disease to the crop or ornamental plant. It is important that the beneficial organism does not damage the target or surrounding plants. Insects, micro-organisms and plants can be used for biological control. A common example of biological control is ladybirds eating aphids. The beneficial organism can be artificially reared and released or the home gardener can create conditions in the garden that will encourage the presence of the beneficial organisms. This may include minimising the use of damaging sprays or companion planting, which is described below.

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Companion planting

A practice much loved by organic gardeners, the science behind companion planting is only just beginning to be revealed. The theory is that companion plants produce compounds that attract beneficial organisms or repel organisms that are pests or diseases. For example, nasturtiums produce a mustard oil that aphids dislike. Another good companion plant is buckwheat, which produces nectar to feed parasitic wasps. Although the wasps sound rather nasty, the are quite small and only attack insects such as leafroller caterpillars.

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Chemical control

If all else fails it may be necessary to use chemical sprays or dusts. Some chemicals are more selective than others, which means that only the target pests or diseases are affected. These chemicals should always be used rather than the “broad spectrum” chemicals that may affect beneficial organisms as well as the targets. Always read the directions carefully and follow them. Increasing the rate of chemical is not a substitute for correct application. If repeated applications are required, try and use a chemical from a different chemical group to avoid the build-up of resistance within the pest or disease population. Note that some chemical sprays and dusts, e.g. sulphur and Derris Dust®, are certified for organic use.


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