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Article posted: 7/10/2013 5:00 AM

Scale, an insect, can stress out magnolia trees


This is the time of year to check magnolia trees closely for scale, an insect that can cause the tree to thin out.

The flat scale insects typically are found on the undersides of branches and appear as spots about the size of a pencil eraser.

Often the first signs of magnolia scale that gardeners notice are black, sooty patches on the bark or beneath the tree. This is a fungus that grows on the clear, sticky honeydew the scale insects exude after feeding on the magnolia.

The mold itself is not a real problem, but the insects are. If your tree is affected, provide extra water during summer to minimize stress.

On small trees, simply pick off the scale insects and squash them. For larger infestations, other control measures can include spraying with summer-weight oil or applying an insecticide when the scale is in the active crawler stage, generally in early September. The active crawler stage, when the next generation of insects has just hatched, is the only time they are moving and vulnerable to insecticides.

For trees with a history of magnolia scale, an application of dormant oil in late winter to early spring will provide good control.

Weeds growing between cracks in brickwork or sidewalks are unsightly but easy to eliminate. Treat them with a nonselective herbicide when they are small to avoid having to pull them out by hand.

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 approved fungicides immediately to deter the disease from spreading in the plant or to other roses. The fungicides work to prevent the disease but do not cure what is already infected. They need to be applied every 7 to 10 days.

Be sure to clean up any leaves that have fallen from the plants to avoid spreading the fungus and keep it from overwintering in the leaf litter.

Many landscape shrub roses are resistant to black spot and do not need spraying.

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

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