Vegetable Pest and Disease Control & Prevention

The aim of any organic gardener is to create a healthy garden by making the soil fertile and by increasing biodiversity.

The most important task is to prevent an occurrence of pests and diseases in the first place rather than having to react to them.  In fact, once a pest or disease establishes itself on your crops there is often very little you can do to control it effectively.   Protecting your plants from pests and diseases begins long before the crops are sown or planted in your garden.


What keeps your plants healthy? 

Soil fertility –

A healthy soil produces healthy plants.  The care of your soil is the most important duty of every gardener.  It is the most effective method of preventing a pest or disease outbreak.  The ideal soil is a loose, moist humus rich soil full of worm and other soil life with a balanced nutrient content, including all the trace elements.  It may take a good few years to achieve this, but even the poorest soil can be made very fertile using organic methods.


A garden with good biodiversity is a lot less prone to sudden  attacks of pests and diseases as there is a balance of pests and predators. To increase the biodiversity in your garden is probably one of the most rewarding pleasures in gardening as you have the opportunity to create habitats and homes for many living creatures that have been pushed to the edge either through the  destruction of their habitats or chemical poisoning.

Beneficial habitats include: – Pond  – Log or branch pile – Dry stone wall or stone pile – Native hedgerow and include fruiting shrubs  – Native trees – Clump of nettles in the corner of your garden – Wildflower patch

Apart from creating specialist habitats you can also increase the biodiversity within your vegetable patch through:

– Crop rotations – where pests and diseases are eliminated by prolonged periods without their hosts.

– Polycultures by growing different crops next to each other (inter-cropping) or   undersowing with a green manure crop (under-cropping).

– Variety mixtures- by growing different varieties of the same vegetable in a plot.  This   technique has been very   successful with potatoes and lettuce and there is a lot of  scope for further experiments.



Hygiene in and around the vegetable garden is very important for pest and disease control.  This includes weed control and the removal of damaged or diseased leaves or plants from the garden.  If your vegetable garden is messy it is much easier for pests and diseases to spread much faster. Your wildlife areas should be positioned a fair distance away from your plot as the beneficial creatures usually travel further and faster than the pests.

Good seeds

You should always start with good and clean seeds.  They should always be stored in a cool, dry place and not for too many years.  I usually keep seeds for only two years and then buy new ones.  New seeds are a lot more vigorous.

Healthy transplants

The same applies to transplants.  It is very rare that all transplants in a tray are of the same quality and you should only plant the best.  There is no point of starting a plant hospital.

Right plant, right place

Plants that are not suited to your climate and soil conditions will never thrive and therefore will be the first ones to be attacked by pests and diseases.  In Ireland you can’t grow good cucumbers or tomatoes outside.  Some may survive and you may even get a few ripe tomatoes on it if you grow them on a south-facing wall in a sheltered garden somewhere in the south or east of the country.   However, the same plant may produce a hundred fruits if grown in a polytunnel.

Resistant varieties

If you find that every year your parsnips get canker and your potatoes blight you should consider using a variety that is  resistant or tolerant to the relevant pest or disease. Examples: Potato: Bionica, Sarpo Mira, Orla and Setanta are very resistant to blight Pea: Hurst Greenshaft is very  resistant to mildew Parsnip: Javelin F1 have some resistance to canker.

Timing of sowing

You can sometimes avoid outbreaks of pests and diseases by adjusting your sowing or planting dates.  The best example is to sow your carrots in late May or early June.  This avoids the first generation of carrot rootfly in May.  Another example is to sow your peas only in mid April to avoid foot rot disease.

Breaking the cycle

You could have brassicas (cabbage family) growing in your garden all year round.  This makes it very easy for all pests and diseases to survive and re-infect new crops.  I always clear my cabbage patch in mid January and only plant the first brassicas again in early May.  I’ll never get nice spring cabbage but at least I hope to get fewer problems.

Adjusting the spacing

If plants are spaced too closely they are a lot more susceptible to fungal diseases such as grey mould or mildew.  If you want to lessen any potential problem you can always space your crops a little bit further apart.  This increases the airflow through the crop and reduces the incidence of fungal diseases that thrive in more humid conditions.

Proper sowing and planting

Good care should be taken when sowing seeds and planting vegetables.  The better they start off the more likely they will do well.

Managing pests and disease

Whilst many pest and disease problems can be prevented, there are various occasions where pest or disease numbers increase to such high numbers that they can cause serious damage to your crops. It is crucial to properly identify the culprits.  It happens often that an innocent bystander who happens to be at the scene is accused of the act.  It may have been the one who has just eaten the culprit.

  1. Mechanical control


Mechanical controls include barriers that keep pests away from your crops.  There are various types of netting available to protect your vegetables (fleece, crop covers, bionet, bird netting).  It’s important that you use the right type of netting for the relevant pests.  A net to keep the small carrot root fly out needs to be a lot finer than a net for cabbage white butterflies.


Collars are used around brassicas to prevent the cabbage root fly from laying its eggs near the cabbage stems.


Many pests can be lured into traps.  The beer trap for slugs is a popular example.  If you use this method ensure that the lip of the container is above the soil surface otherwise you may also catch some ground beetles which would have eaten many more slugs than you have caught.  Personally I can think of a much better use of beer!

You can also buy yellow sticky tape from garden centres to catch flying pests such as aphids.  They are very good as indicators to find out which pests you have but they will not control pests sufficiently.


Handpicking larger pests such as slugs, leatherjackets or caterpillars can be quite efficient especially in a small garden.  I know some  gardeners who get great  satisfaction from this.

  1. Biological control

Biological control includes attracting beneficial creatures that feed on pests as well as introducing predators for the job!  These predators can be purchased by mail order but are generally very expensive especially for a small area.

It is much more efficient (and a lot cheaper) to attract native beneficial creatures into your garden than to purchase foreign species.

Examples of natural predators include: Hoverflies – the larvae and adult hoverfly feed on aphids. Lacewing – feed on aphids Ladybirds – feed on aphids Beetles – feed on slugs and many other small pests Earwigs – most people believe they are pests but they also feed on aphids Frogs – feed on slugs 

  1. Chemical control

There are a number of so called ‘safe’ organic sprays available to the gardener.  They are safe in the way that they are fully biodegradable within a couple of days (with the exception of copper sulphate) but nearly all of them will also kill beneficial insects and thus disrupt the natural cycles. The garlic spray (Envirorepel) is one exception.  It only masks the smell of host plants so that pests find it less attractive or get confused.  Apparently it also strengthens the plants so that they become more resistant.

A milk/water spray (1 part milk and 7 parts water) was probably the best new discovery in organic disease control.  I have successfully used it to control diseases such as grey mould and mildew on a variety of vegetables.  You can spray it with a little plant spray directly onto affected plant parts ideally three days in a row and the disease stops spreading further.  Unfortunately it didn’t work for potato blight.

Other organic sprays include: Derris – to control aphids,  caterpillars, flea beetle Pyrethrum –  to control aphids, caterpillars, flea beetle Insecticidal soap – to control aphids, whitefly, spider mites Soft soap – to control aphids Quassia – to control aphids,   apparently safer than derris and pyrethrum Bluestone (Copper sulphate and washing soda) – to control potato blight, apple scab Sulphur – to control powdery  mildew, rose blackspot

Note: All these sprays can also damage beneficial insects or can be harmful to fish, livestock and worms (i.e. Bordeaux mix)

Home-made sprays: Home-made sprays are often prepared by hobby gardeners to control various pests and diseases.  They are illegal in the EU as they are not tested.  Some of them are extremely toxic (rhubarb spray) to all sorts of wildlife.  The most interesting spray is a compost tea or extract.  You soak one part of compost with ten parts of water for about a week and stir it daily. You then dilute it with another 10 parts of water before spraying it onto susceptible crops to prevent or halt fungal diseases.  Many scientists all over the world achieve tremendous results in controlling a large variety of diseases.  But remember you are not allowed to make it yourself.

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