When to Use Pesticides and Fungicides

Gardening by the Yard : Episode GBY-719 -- More Projects »
Do you reach for the sprays or dusts at the first sign of pests or disease? You may not need to, says master gardener Paul James.

"I'm convinced that at least half the products we use to combat pests and disease problems are a complete waste of money," says James. "And many of the products gardeners use pose a problem to children and pets."

So what is the best way to keep your garden safe from pesky critters and deadly diseases? The first step is to make sure your plants are growing in a healthy environment. That means providing plants with ideal conditions such as good soil, proper drainage, plenty of light, regular watering and weeding, routine applications of compost and mulch, and protection from drying wind. Establishing the best conditions for plant growth increases your chances of having healthy plants that are better equipped to withstand attacks from pests and diseases.

But what if you take measures to make growing conditions ideal, and the problems still persist? Resist jumping to conclusions, and review the facts. Assess the entire situation to determine the cause of the problem, and customize your plan of attack to treat the identified perpetrator.

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Figure A
For example, the flowers on this hibiscus are gorgeous but the foliage has been ravaged by a hungry pest (figure A). At this point, many gardeners would just grab the closest all-purpose spray or dust and treat the plant without really knowing exactly what did the damage in the first place. And very often, that pesticide, whether a malathion spray or garlic dust, kills indiscriminately--wiping out not only the possible culprit, but any nearby beneficial insects as well. And remember, even organic treatments can still include chemicals. Check whether the damage is harmful to the plant or just creates an unpleasant appearance.

In the case of the hibiscus, the chewed leaves are a bit unsightly, but they pose no real threat to the plant. And believe it or not, the fact that this plant has been chewed may be one of the reasons it's doing so well. Many plants release chemicals when their leaves are chewed. Some of these chemicals float into the air and are detected by predatory insects such as wasps, which then swoop down to attack the pests. Some plants release toxins when their leaves are chewed that naturally repel hungry pests. "So, in a curious sort of way, a little bit of damage can be a good thing," explains James.

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Figure B
This hydrangea has a fungal disease known as yellow spot (figure B). Gardeners might reach for the nearest fungicide, but fungicides aren't very effective for treating current fungal problems. Rather, they are most effective when used as a preventative control, which means they must be applied at regular intervals throughout the growing season. James suggests doing nothing. "Actually, I may get around to pruning some of the infected leaves if the disease appears to be spreading."
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Figure C
Another hydrangea appears to be healthy on one side, while the other half appears to be dead. The leaves on the sick side (figure C) have either fallen, or turned brown and curled up. James analyzes the problem first. Seeing no signs of pests, he determines that the cause must be some sort of disease and notes that the insides of the stems are hollow. This characteristic suggests a number of different fungal diseases that promote rot. James prunes back the dead stems to the green, new-growth areas. In between cuttings, James wipes the blades of the pruners with a bleach soaked rag.

"The lesson to be learned here is really quite simple," says James. "Don't overreact to plant problems. Spray or dust with chemicals only when absolutely necessary. And realize that in many--if not most--cases, the best course of action is no action at all."

Also in this Episode
Gardening by the Yard : Episode GBY-719
•When to Use Pesticides and Fungicides
•Colorful Window Boxes
•Peppers: Some Like 'Em Hot!