First limit pests and disease and then treat responsibly
It's bound to happen in the garden. The foliage is spotted, yellowing or curling. Or something is enjoying leaves as a snack. Our first reaction might be to reach for pesticides, but this may be unnecessary and, in fact, can be counterproductive.
First, pinpoint the problem. Environmental factors, like frost, drought or excess rainfall, can cause injury that looks like damage caused by insects or diseases. There are many reliable sources that can help identify the cause of the damage.
University of Illinois Extension has offices in each county with a Help Desk staffed with Master Gardener volunteers ready to help. Local independent garden centers have horticulturists on staff who can often identify problems. There are books written specifically about plant pests and diseases available at the local library, and the internet can be a great source. Just be sure the website is credible before following any recommendations.
Once the trouble has been identified, determine the best course of action. Some issues can be prevented, or at least lessened to an acceptable level, with proper plant selection, siting and maintenance.
Replacing a disease-prone plant with one that is resistant is an option. Moving the plant to its preferred growing location is another. Proper care is also a consideration. Water plants according to their cultural requirements. Most perennials do not require supplemental fertilizer and if given too much, may grow overly lush, attracting insects. Wet foliage, caused by overhead watering late in the day, encourages disease.
Keep the garden free of insect- and disease-hosting weeds. Remove garden debris throughout the season and rid the garden of foliage that shows any signs of damage from insects or disease during fall cleanup.
Encourage natural predators, like birds, frogs, toads and predator insects, by providing water and shelter. Our pond is host to a variety of natural predators, but a bird bath is fine to satisfy birds. Flat stones that collect rainwater provide water for beneficial insects. A brush pile offers shelter for toads. And remember, to invite predators there must be some prey to eat, so a low population of unwanted insects is necessary.
If a control is deemed necessary, start with the least toxic option. Often, a strong blast of water from the garden hose can greatly reduce the number of mites or aphids on a plant. Hand pick spittlebugs or caterpillars. Knock Japanese beetles into a bucket of soapy water. Prevent slugs from chewing on hosta leaves by sprinkling finely crushed eggshells around them.
There are a number of products made from plants, animals or mineral-bearing rocks that are preferred by organic gardeners. Other organic alternatives include horticultural oils, insecticidal soaps and diatomaceous earth.
Insecticidal soaps and horticultural oils are effective controls of many pests and are safer than insecticides, but they can also kill beneficial insects. The timing of application is important so be sure to follow the instructions on the label carefully.
Biological controls, like Bt (Bacillus thuringiensis) for caterpillars and Milky Spore (Bacilus popilliae) for grubs, are safer for the environment than insecticides because they are specific to those organisms. But, like insecticidal soaps and horticultural oils, the timing of application is critical. And Bt kills all caterpillars, not just the ones you don't want.
Products made from plants, like Pyrethrum or Neem oil, may be less toxic than traditional insecticides but they can also harm beneficial insects. Use then with caution and follow the directions carefully.
Many of the traditional, synthetic insecticides and fungicides are broad spectrum, meaning they kill a broad range of pests or diseases. Consider other options first and then use with great care.
Whether organic or synthetic controls are chosen, always follow the directions on the label completely. Just because a product is labeled organic does not mean it is gentle. Apply only at the directed rate — more is never better. Always wear protective clothing and wash your hands after using.
• Diana Stoll is a horticulturist, garden writer and speaker. She blogs at gardenwithdiana.com.