Scale insects suck sap from trees

This is the time of year to check closely for magnolia scale, a kind of sap-sucking insect that creates an overall thin condition when it infests these flowering trees.

Gardeners often notice a black, sooty mold on the magnolia leaves before noticing the actual insects. The mold grows on the clear, sticky honeydew that the insects exude after feeding on the plant. The mold itself is not a real problem.

The scale insects, which are immobile as adults, are about the size of a pencil eraser and are typically found on the underside of magnolia branches. On small trees, simply pick off the scale and squash them.

Other control measures can include spraying with summer-weight oil or applying an insecticide when the scale is in the active crawler stage of its life cycle, generally in early September. Spraying with insecticide at other times will not be effective, as the adult insects are protected by a waxy coating.

For trees with a history of magnolia scale, an application of dormant oil in late winter to early spring will provide good control. If your tree is affected, provide extra water during summer to minimize stress.

If your hybrid roses have been losing their lower leaves and remaining leaves have yellowish foliage with dark spots, it is likely you have black spot, a common fungal disease. Begin a spray program with an approved fungicide immediately. The fungicides need to be applied once every seven to 10 days. They work to prevent the spread of the disease but will not cure foliage that is already infected.

Be sure to clean up any leaves that have fallen from the plants so the fungus spores do not infect other plants.

Many landscape shrub roses are resistant to black spot so they do not need to be sprayed. Rugosa roses (Rosa rugosa) should not be sprayed for black spot, as the fungicides can damage their foliage.

This is a good time to cut some of your outdoor flowers for indoor display. It is best to cut the flowers during a cool part of the day and to put them in water as quickly as possible.  Remove any foliage that will be under the water in the vase. Adding a preservative to the water can lengthen the life of the flowers.

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