Identifying Insect Culprits

Perennials

Class 1: Some Perennials Basics
Class 2: Garden Design
Class 3: Putting Pencil to Paper
Class 4: Soil Preparation and Planting
Class 5: Maintaining your Perennial Gardens
Class 6: Troubleshooting

Many different fungi, bacteria, and other microorganisms can infect plants. Inspect plants carefully, with a magnifying glass if necessary, looking for clues to help you determine if a disease is present. Once you’ve noted the signs, consult a reference to identify the culprit.

Here are some things to look for:

1. Some diseases show up as leaf spots. Note whether the spots are brown or black, round or irregularly shaped. Do they have concentric rings, or is the color uniform?

2. Other diseases cause cankers, or sunken areas, on twigs.

3. Rust diseases show up as rust-colored bumps on stems or leaves.

4. Gray molds cover leaves and flowers with a soft, furry coating.

5. Powdery mildew is a common disease of perennials, making leaves look as though they have been dusted with talcum powder. Often this disease is cosmetic and does little harm; however, a severe infestation can weaken some types of plants.

6. Viruses can be difficult to diagnose, since they can show up as distorted growth, an overall yellowing of foliage, or mottled leaves. There are no effective treatments for virus-infected plants. Since viruses are spread by insects, it’s best to remove and destroy infected plants as soon as possible to limit spreading.

As with insect pests, it’s important to identify the disease before taking any steps to control it. Timing can be critical in controlling the spread of disease, and some controls are effective on some diseases but not on others.

Many common fungicides are mineral-based, containing copper and/or sulfur. These must be applied carefully, according to label directions; improper application can damage foliage. There are some relatively new fungicidal soaps on the market that may be easier to apply than the traditional mineral fungicides.

Synthetic fungicides are rarely warranted. And they are some of the more toxic and more persistent chemicals available to home gardeners. If you do choose to use a synthetic fungicide, follow label directions to the letter, and use as sparingly as possible.

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Pruning off and destroying infected plant parts can go a long way to minimize the spread of disease. Be sure to sterilize pruning shears with a 10 percent bleach solution between cuts, or you might spread the disease yourself and do more damage than good! (To make a 10 percent bleach solution, mix 1 part bleach with 9 parts water.)

Powdery mildew leaves plants looking as though they've been dusted with talcum powder.

Some people have good luck with simple home remedies. One common homemade fungicide consists of 2 tablespoons of baking soda and one teaspoon of dish soap mixed into one quart of water. This solution is then sprayed on foliage to help control fungal diseases such as powdery mildew, black spot, and gray mold.