Pests aren’t always to blame for problems

Summer is a great time to work in your garden, but don’t forget to be alert — a Chicago Botanic Garden staff member recently discovered a bald-faced hornet nest in a tree at head level. I generally see these nests higher up in trees. This wasp is black and white and aggressive if the paper nest is disturbed. It’s a beneficial insect so it’s best to simply leave the nest alone without spraying it if it can be avoided.

In my home garden, I’ve only sprayed once for insects — for viburnum leaf beetle, which had defoliated my arrowwood viburnums. It’s important to identify the insects on your plants before treating them with an insecticide to determine whether they really are a problem.

If you do have a problem, try to assess whether the damage being done warrants control. The presence of insects feeding on the plants should not warrant automatic treatment. Some insects are beneficial and help control other insects naturally. If an identified pest is causing significant damage, it’s important to use the correct control with proper timing.

If a variety of treatments are available, use the least toxic control possible. Pests may not be the problem. Some plants can start looking tired and have tattered leaves because of weather conditions later in the summer. A friend thought his lawn had grub damage, but actually, a section of bent grass had invaded the lawn and died out due to the weather conditions.

Leaves suffer from rainy days

This year’s rainy weather conditions created a great environment for foliar diseases to thrive. Many crabapples have lost a lot of leaves in gardens in the Chicago area because of a common disease called apple scab. Apple scab is most severe during spring and early summer when the humidity is high, and the temperature is moderate. The most obvious symptoms occur on leaves in the spring and summer. Look for small velvety brown to olive-green spots that enlarge and darken to become more or less circular on the leaves. When infections are more serious, young leaves can become curled and distorted. Severely infected leaves and fruit fall prematurely.

There isn’t much to do for this disease at this time other than to clean up and discard fallen leaves and fruit as the fungus overwinters on plant debris. A limb without leaves may not be dead, so check it closely before pruning it off. Supplement the trees with additional irrigation if conditions become dry to help reduce stress. Consider a preventive fungicide spray program next spring. Make the first application as the buds begin to open, with repeated sprays at 10- to-14-day intervals until the leaves develop. The number of applications will typically be two to three but will vary depending on weather conditions (rainfall), plant susceptibility, rate of plant development, fungicide used and the amount of pathogen present. Another option is to replace your tree with a more resistant variety.

Cicada tips

Damage to trees from cicadas is very visible. You’ll see broken branch tips hanging on trees and falling to the ground. For aesthetic reasons, you might want to prune off the dead tips that you can reach.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.