How to control pests and diseases

Pest or disease control relies on the use of good gardening techniques and many attacks can be kept under control with due care to the following:

  • Good planting preparation
  • Good plant feeding
  • Correct pruning
  • General garden hygiene
  • Leaving an air space of at least 5cm behind supports for plants grown on walls or fences
  • Wherever possible keep the growing area open so that good air circulation is allowed to flow

By ensuring these points are followed, plants will be encouraged to grow well and produce natural enzymes in their leaves that naturally control many pests and disease attacks and the main aim is to break the life cycle of the pest or disease to prevent reinfestation.

How can I control pests?

Biological control

Many pest outbreaks can be controlled by introducing a live predator that will attack the pests without damaging the plants

Biological control works particularly well in the protected environment of a greenhouse or conservatory and can go a long way to reducing the severity of an attack. Suitable predators are usually supplied online or by mail order but some garden centres stock them.

Chemical and mechanical control

There are a wide range of these available for serious pests and disease infestations, sold as insecticides, fungicides and herbicides. They can give safe and effective results if used properly, but they are also harmful to the environment so should only be used as a last resort and must be stored safely.

Always read the instructions and apply in the manner stated. Wear gloves and always wash your hands after using chemicals and keep all chemicals out of reach of children and animals.

Types of pest and how to control them

Aphids

These feed on the leaves, stems and flowers of ornamental plants, vegetables and fruits. They are also known as greenfly or black fly but the insects can be pink, cream or mottled. Broad beans and elder bushes are prone to black fly attack.

Symptoms – sap-feeding insects infesting plants especially on shoot tips, flower buds and leaf undersides. The foliage may be sticky with honeydew that aphids excrete and a black sooty mould often develops on the honeydew.

Control – pick aphids off plants by hand and nip out the affected shoot tips as soon as aphids appear. Natural enemies of aphids are ladybirds, parasitic wasps and larvae of lacewing and hoverfly. Spray plants with pyrethrum, rotenone, fatty acids, plant and fish oils or plant extracts. For higher levels of control use bifenthrin or imidacloprid.

Red spider mite

These are mainly a problem in greenhouses or for house plants.

Symptoms – pale mottling on the leaves and in bad infestations, fine webs around the leaf and shoot tips.

Control – keep the air humid by damping down greenhouses, spraying plants with water and placing houseplants on damp pea gravel. Destroy affected leaves and shoots. Predatory mite Phytoseiulus persimillis feeds on the eggs or spray with bifenthrin.

Whitefly

Mainly affect greenhouse plants, especially fuchsias and tomatoes and brassicas outdoors in summer and autumn.

Symptoms – clouds of small, white-winged insects fly up from the leaf undersides when touched. Leaves become sticky and have black sooty mould.

Control – use biological control, sticky traps or spray with bifenthrin, plant extracts, plant oils or fatty acids.

Vine weevil

Grubs attack the roots of almost any young plant and are also fond of the tubers of begonia and cyclamen. Plants in pots are particularly at risk.

Symptoms – plants turn yellow and wilt, by which time it’s probably too late to save them.

Control – pick off and destroy any newly hatched adult vine weevils and look for notches bitten out of the sides of leaves. Sprinkle a thick layer of grit around plants at risk and don’t leave old compost lying around in pots and baskets. To kill the grubs in containers , water pathogenic nematode heterorhabditis megidis into the potting compost or apply a pesticide that contains imidacloprid. This is available as a slow-release formulation pre-mixed with a peat compost, sold as Levington Plant Protection Compost.

Slugs and snails

These will attack any soft, lush plant. Hostas are a favourite, although hairy plants are usually safe.

Symptoms – slugs tend to attack plants close to the ground and eat all parts of the leaves and even tubers underground. Snails climb so look out for trails and nibbled leaves even on plants in pots.

Control – place barriers such as prickly leaves or really sharp grit around plants or a ring of petroleum jelly around the top of pots. Collect slugs and snails by hand and dispose of them. A nematode, phasmarhabditis hermaphrodita works specifically against slugs and is simply watered into the ground. Use slug pellets sparingly. You can also try beer traps with a jam jar filled partly with beer sunk into the soil near vulnerable plants and empty it regularly.

Plant diseases

Blackspot

This is a fungal disease most prevalent in wet weather as it’s spread by water droplets. If left untreated, susceptible plants can lose their leaves.

Symptoms – black patches appear on rose leaves and stems and leaves fall prematurely.

Control – remove infected and fallen leaves promptly and regularly. Hard prune infected bushes in spring and burn the prunings. Spray with penconazole, flutriafol or myclobutanil, alternated with mancozeb to prevent the fungus from developing resistance to the fungicides.

Mildew

Caused by a range of closely related fungal species. Some mildew spreads to cultivated plants from closely related weed hosts so weed control is an important part in limiting disease. Many garden plants are affected, both woody and herbaceous, particularly apple trees, roses, sweet peas and those growing in containers. Vegetable foliage is also prone, including beetroot, parsnip and spinach.

Symptoms – a dry whitish powder coating leaves, shoots, tips and flowers is especially visible in summer. Other symptoms include stunted and distorted growth and reduced flowering.

Susceptibility to mildew can be reduced as follows:

  • Keep plants well watered so they are not dry at the roots
  • Mulch to preserve soil moisture
  • Improve air flow around plants to reduce humidity. Prune woody plants such as roses to establish an open branch structure. Avoid overcrowding smaller plants and thin vegetable crops to recommended spacing
  • Avoid high-nitrogen fertilisers as these encourage soft, sappy growth that fungi is more attracted to
  • Ensure plants are in their ideal position
  • Check online/catalogues and grow resistant varieties where possible
  • Prune out infected areas as soon as they are seen and collect and destroy all infected prunings
  • For roses and other ornamental plants, use fungicides myclobutanil, penconazole, flutriafol or sulphurF
  • For other plants, check the labels carefully before choosing, as pesticides can only be legally used on the range of plants specified on the label.

Greenhouses and conservatories

Pests and diseases can thrive in a greenhouse or conservatory but risks will be reduced if the following points are considered and practiced:

  • Keep vents and doors open wherever possible throughout the year
  • Maintain a humid atmosphere
  • Store pots, tools and other materials elsewhere
  • Carry out gardening operations such as potting up away from the greenhouse or conservatory
  • Wash all pots and trays used for a second or subsequent time
  • Only use potting composts once and never use garden soil as both can carry eggs and spores or pests and diseases
  • Each winter wash down the inside of the greenhouse with a household disinfectant
  • Keep an eye for the first signs of pests or disease attack and remove any affected plants
  • Remove and destroy any dead leaves and flowers once seen
  • Do not allow plants to dry out or overwater them particularly in the winter months
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