When it looks like there’s something amiss with a garden or indoor plant, most gardeners reach for a cure in the form of a plant spray. But there are lots organisms that affect plant health so there is no one solution fits all.

To be able to select the right type chemical to apply to cure a plant’s ailments, it is important to work out whether the plant is affected by a pest or a disease.

What pest?

Pests include aphids, beetles, bugs, caterpillars, mites, slugs, snails, thrips, weevils and lots more. Some can be visible to the naked eye while others can only be seen with magnification such as under a hand lens. As well, some pests attack the root system or only come out at night. Often the damage they cause is obvious even if the pest can’t be located.

Products that kill unwanted insect pests and mites are broadly known as insecticides.

What disease?

Three main groups of organisms cause plant diseases: fungus, bacteria or virus. They may be spread from plant to plant by wind, rain, insects, through soil, water movement or contact.

The effects of some are easy to see such as damage to a plant’s stems, leaves, flowers or fruit, but others attack the roots or the inside of the plant. Sometimes the disease isn’t obvious but the plant may wilt, die back or fail to thrive because of a disease organism.

Diseases are often given common names that describe the damage such as rust, freckle, powdery mildew and wilt.

Diseases are treated with fungicides or bactericides (normally just called fungicides). As in human health, virus diseases in plants are difficult to treat with chemicals.

How pesticides work

Pesticides, both insecticides and fungicides, work in several different ways to kill a pest or cure the plant disease problem. Most are sprays, which are applied to the plant to cover the area where the damage is occurring. These are termed ‘contact’ sprays and, in the case of insecticides need to come in contact with the insect to kill it – much like a fly spray being used to kill a fly or mosquito in the house.

The pesticide needs to be applied where the pest is to be effective such as under the foliage if that’s where the insect is feeding or the disease is growing.

Some contact pesticides are also used in pest traps such as fruit fly traps and snail and slug baits.

The second group of insecticides gets inside the sap of the plant or inside the tissue of the plant to kill the organism as it feeds or grows. These are called systemic insecticides and may be sprayed over the plant or placed in ground as a tablet or drench. Some pesticides have both a contact and systemic action.

Applying pesticides

All the information needed to correctly apply a pesticide is written on the label. The label includes directions about how much to apply, how frequently to apply and what precautions need to be taken including safety equipment, how to avoid damage to other organisms and the environment and safe storage. It is important to read the label and follow its directions. Most labels also include a customer service contact point such as a free call phone number for further advice and information.

As a general rule, apply pesticides whether to control a pest or a disease, especially sprays, when the weather is cool and still and when beneficial insects (such as bees) are not active.