If pests and diseases cannot be successfully prevented or treated by using suitable cultivation methods and organic control, then it may be necessary to treat them with chemicals. Although we seem to be moving away from the idea of using pesticides and fungicides, a prudent combination of organic and chemical techniques may often provide the best solution to difficult and recurrent problems.

Pesticides and Fungicides

Pesticides are used to kill insects, mites and other pests, whilst fungicides are used to control diseases caused by fungi. Both types work either by contact action or systemically.

Contact pesticides kill pests when they are hit by the chemical (during spraying) or when they subsequently crawl over a treated plant. Systemic pesticides are drawn up through the roots or taken in through the leaf pores and then transported by the sap stream throughout the entire plant. This makes pesticides of this type particularly useful for sap-suckers, although less so for pests that chew the plant, such as earwigs, caterpillars and beetles, or for food crops.

Although contact fungicides kill germinating fungal spores and prevent further infection, they have little effect on established fungal growths. A systemic fungicide is needed to kill fungi in the plant tissues.

Ingredients and Formulations

Pesticides and fungicides can be bought in a range of different formulations, including concentrated liquids, dusts, powders, smoke and pre-diluted sprays. You should always avoid making up more concentrates than is absolutely necessary, as they can become ineffective within 24 hours. It is usually best to buy ready-to-use sprays for smaller areas, as these keep for just as long as undiluted concentrates (about 2 years) and do not need to be mixed.

A few of the useful active chemical ingredients that may be used to treat pests and diseases are shown below:

• All-in-one insecticide and fungicide: Pirimicarb, bupirimate and triforine mix (will not kill beneficial insects)
• Slugs: Metaldehyde or aluminium sulphate (organic)
• Insect pests: Permethrin or pyrethrum
• Fungal diseases: Thiophanate-methyl
• Organic treatments: Insecticidal soap, quassia, derris, pyrethrum and sulphur (fungicide)

Chemical Safety

When working with chemicals, it is vital to minimise any risk of harm to the user or to the environment. Always read the information on the label, carefully follow the instructions and never exceed the recommended dose. In addition, any pesticide that may be potentially harmful to bees and other beneficial insects should only be sprayed in the very late evening when they are no longer working the flowers.

You should also carefully follow the instructions for disposal of excess chemicals; they should never be poured down sinks, drains or watercourses. Our section on Safety in the Garden has a feature on Chemical Safety, with a range of useful tips for storing, working with and disposing of pesticides and other garden chemicals.

Plant Care

Plants that have been damaged by diseases or pests should be fed with a general fertilizer after an attack to restore them to health. Use a foliar feed if you suspect root damage, as this may have made the uptake of nutrients from the soil difficult.