Chemical control is the term used to describe the action of destroying plant pests and diseases by applying synthetic compounds to plants or soil.

Although the emphasis on organic control is becoming stronger the responsible and sparing use of chemicals still has a valuable role to play in pest and disease control.

A sensible combination of both methods may often provide the best solution to difficult and often recurrent problems.

Most pesticides (which are used to kill insects, mites and other pests) and fungicides (which are used to control diseases caused by fungi) either work by being brought into contact with the pest or disease organisms, or they are systemic.

Contact pesticides kill pests when they crawl over a treated surface or are directly hit by the chemical, when sprayed for example. Contact fungicides may kill germinating fungal spores and prevent further infection, but they have little effect on established fungal growths.

Systemic chemicals are absorbed into the plant tissues and are then transported by the sap stream throughout the entire plant. Fungicides of this type, for example benomyl, thiophanate-methyl and carbendazim kill fungi in the plant tissues.

Systemic pesticides such as dimethoate and heptenophos are predominantly used against sap sucking pests and are less useful, unless mixed with a contact compound, against pests that have a chewing mouthpart, such as caterpillars, beetles and earwigs.

Thorough spraying of affected plants, especially on the undersides of leaves, is essential with all pesticides. Pesticide resistant strains will sometimes occur, particularly with persistent greenhouse pests such as whiteflies and red spider mites.

Fungi that are frequently treated with systemic fungicides may also develop resistant strains. This problem can sometimes be overcome by using a different type of compound. With greenhouse pests the use of biological control where possible is often a better alternative.

Pesticides and fungicides are available as concentrated liquids, dusts, powders, smokes, baits and ready to use diluted liquids.

They are formulated for maximum effectiveness and safety, for both gardeners and the environment.

If the instructions are followed precisely-that means-only in the way and for the purpose the manufacturer describes-chemicals should prove effective with little risk to the user or the environment.

You should always take the following precautions however:

  • Think before you spray-Is it really necessary?
  • Make sure the chemical you are using is the right one for the job
  • Apply the preparation at the rate and frequency stated on the label
  • Never mix chemicals unless it is stated on the label and recommended by the manufacturer
  • Always store chemicals in their original containers
  • Keep out of reach of children and pets
  • Keep pets and children away during use of the treatments
  • Never eat, drink or smoke when using chemicals
  • Avoid contact with skin and eyes
  • Do not spray when it is windy
  • Where protective clothing and gloves

Some plants are liable to suffer adverse reactions to pesticides and fungicides. This is known as phytotoxity. The manufacturer’s instructions often list those species that should not be treated. Such lists however cannot be complete since the reaction of many ornamental plants to chemicals is yet unknown. If in doubt whether a certain chemical is suitable first test the pesticide or fungicide on a small area of the plant to gauge the likely reaction before undertaking the whole plant.

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