Many infected crabapple leaves are falling prematurely

Many crabapple trees have dropped a lot of leaves in yards throughout 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 rainy season thus far has helped create good conditions for this disease to develop.

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 prematurely falling fruit are now noticeable in many crabapple trees.

There is not 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. Be sure that a limb without leaves is really dead before pruning it off.

Supplement the trees with additional irrigation as needed for the rest of the year to help reduce stress. Consider a preventive fungicide spray program next spring with the first application as the buds begin to open and 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.

• It is important to identify the insects on your plants before treating them with an insecticide to determine whether or not the insect really is a problem. If it is, then 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 is important to use the correct control with proper timing. If a variety of treatments are available, use the least toxic control possible. Some plants can start looking tired and have tattered leaves because of weather conditions later in summer. One example is a homeowner who thought his lawn had grub damage that turned out to be a section of bent grass that had invaded the lawn and died out due to the weather conditions.

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