Apple scab causes leaves to fall, get spots

Some crabapple trees may be losing their leaves at this time, with the remaining leaves developing black and yellow spots. This is due to a fungal disease called apple scab. A fungicide would only be effective against this problem if applied in spring; spraying at this point will not help.

Give an affected tree extra care by providing supplemental water as needed and fertilizing in fall or next spring. Consider replacing a disease-prone tree with a new one of a variety resistant to apple scab.

Grubs can be a problem in some years, especially in lawns that are regularly watered, such as those that have an automatic sprinkler system. The grubs are larvae of various kinds of beetles, and in early summer the adult beetles prefer to lay their eggs in moist soil. If one lawn is irrigated and neighboring lawns are not, they will be attracted to the irrigated lawn.

However, in this wet year all lawns are being rained on equally, so there is less chance that any particular lawn will have a concentrated outbreak of grubs.

The grubs feed on the roots of the grass, causing the grass plants to turn brown later in the season when hot and dry weather increases stress on the lawn. Often the first sign of grubs is when raccoons and skunks dig for them.

The level of grub infestation naturally varies from year to year, and in any growing season you may or may not have grubs. Deciding not to apply an insecticide for grub control will not necessarily result in an infestation.

If you do decide to apply a product to control grubs, read the label carefully to make sure you are using the right product at the right time of year. Products designed to prevent grubs are generally applied late June to mid-July.

There are other insecticide products designed to quickly kill grubs. These products would be applied later in season if the number of grubs reaches a threshold that is causing visible damage, typically 8 to 12 grubs per square foot. If you have only a small number of grubs, the lawn can withstand the damage and there is no need to apply any control.

Watch for spider mite damage on your plants. Mite populations can build up quickly, especially during hot, dry weather. The mites feed on leaves, causing fine spotting on the foliage. Where large populations attack, injured and chlorotic areas can join together, causing large portions of the leaf or the entire leaf to turn yellow or bronze.

To detect the small spider mites, take a piece of white paper and strike some plant foliage against it. Look for tiny mites moving slowly on the paper. It is not uncommon to find hundreds of mites and eggs on a single leaf. White webbing may be visible, but the silk-producing habits of spider mites vary.

Use a jet of water to knock the spider mites off the plant instead of applying chemical controls that also would kill beneficial predator insects. If the mite population is large, you may choose to apply insecticidal soap, summer-weight horticultural oil, or a miticide spray. Be sure to read and follow label instructions to maximize effectiveness and to avoid damaging the plant.

To make sure you are taking the right action for grubs, mites or any other plant problem, consult the Chicago Botanic Garden's Plant Information Service (chicagobotanic.org/plantinfoservice).

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