Insects and Diseases
When you see diseases or damaged leaves, the only treatment generally necessary is to remove them. Occasionally, though your entire plant may have some type of bug or fungus, and you may hesitate to cut the plant down to the ground. This section will go over a few insects and funguses that attack perennials. However, you should find that if you are careful about removing damaged foliage your perennials will reward you by causing you little consternation.
Insects
Aphids- are white soft-lice that cause a sticky residue to be left on the plants. These insects will cluster on one branch at a time until the whole plant is covered. To bring these pests to their knees, simply spray with strong blasts of cold water from your hose and spray with organic insecticide. Japanese Beetles-these harmful insects will gladly devour your entire garden. We avoid using traps when attempting to control them because they only seem to attract more of the beetles from around the neighborhood to the garden under siege. If you are inclined to try to treat them organically, use a milky spore treatment or plant garlic bulbs among your plants. If you aren’t, spray them thoroughly (with a Seven spray (available through Ortho® or through a cheap imitator at your local garden center, or K-Mart®).
Spider-Mites- Spider-mites will give the foliage a speckled yellow look and are only seen with careful, close observation. The look like tiny black or white spiders, and tend to cluster near the veins on the underside of the leaves. Spray them with your organic insecticide or with an insecticide (once again available through Ortho® at your local garden center or superstore).
A word about beneficial insects.
Not all insects are bad, there are many good insects, such as ladybugs, so do not go crazy and spray your garden with all kinds of high-powered insecticide after the first glimpse of any insect.
Fungus
There are essentially two diseases you, as the perennial gardener, will have to deal with.
Powdery Mildew will cover your plants with a white film. It especially likes to destroy tall garden phlox. To combat this, try to keep your foliage dry when watering and never water in the evening. Another thing you can do is try to thin your plants to improve the air circulation around them. You also would want to consider spraying the infected plants with a general-purpose fungicide (available through Ortho® at your local garden center or superstore).
Root Rot is caused from the plants being planted in soil that does not drain well and receiving more water than they can handle. The leaves will wilt and drop for no apparent reason and the soil around it will be wet. Unfortunately, you have no option other than to destroy the plant and improve the soil drainage. You have to improve the soil since it is what caused the rot.
Generally if you have taken the previously outlined steps, you should have no problems with insects and fungus.