August 23, 2008

Sooty Mold is Caused By an Insect Infestation

By Sid Mullis

Today I want to discuss a couple of problems you may be experiencing in your landscape. Sooty mold can be bad this time of year. The black, sooty fungus can be on a variety of ornamental plants such as crape myrtles and gardenias.

This unsightly condition is caused by a fungus that does not actually attack the plant but damages it by covering the foliage and thus reducing its absorption of sunlight. Instead of obtaining food from a host plant, as do most funguses, sooty mold lives on the honeydew secretions of insects such as scale, white flies and aphids.

If your plants have a bad aphid problem, more than likely they also have sooty mold. Shrubbery and flowers underneath insect- infested shade trees are especially prone to sooty mold. Plants under pecan and hackberry trees commonly have sooty mold.

The most practical control measures are to limit the insect infestations. If this is done, there will be no honeydew for the sooty mold fungus. Acephate (Ortho Systemic Insect Spray) is good for controlling scale, white flies and aphids. For a hackberry tree, imidacloprid (Bayer Tree and Shrub Insect Control) is the only option. This must be applied in May to allow time for the insecticide to work its way up the tree. This insecticide cannot be used on pecans since the nuts will be eaten. \

Dogwood Leaf Scorch: Another problem you may be experiencing in the landscape is marginal leaf injury or scorch on dogwood trees. It causes browning of margins of the leaves and indicates that leaves are losing moisture faster than it can be obtained from the soil.

If the dogwood has an inadequate root system, that will restrict water intake. A poor root system often results from planting in a wet location, planting too deeply, planting in a heavy soil without soil preparation or planting in a full-sun site.

Dogwoods that have been established for only one or two years usually have not developed extensive roots for water intake. Heavy applications of fertilizer can also injure roots. Water uptake can also be reduced if the trunk is being attacked by dogwood borers that tunnel underneath the bark near ground level.

Dogwoods that exhibit marginal leaf symptoms should be mulched, and watered once or twice a week during dry periods. Total weekly watering should be around an inch and a half. The exception to the every other day watering would be if you have fairly sandy soil where it dries out quickly.

With proper watering practices and mulch, many trees recover and produce normal growth by late summer. Dogwoods should not be planted in open, sunny yards. They need some shade, particularly afternoon shade. In sunny sites, they cannot move enough water to the leaves, even though it is there in the soil.

Don't get me wrong, there are plenty of dogwoods doing fine in sunny areas, but as a rule dogwoods won't do well in those sites.

Lawn mower or weed eater damage is a common problem for dogwoods. The wounds interfere with the movement of water and nutrients in the plant. Also, dogwood borers can be attracted to the wounded area.

Don't grow grass right up to the trunk of a dogwood, and keep the area around the tree mulched.

Reach Sid Mullis, director of the University of Georgia Extension Service office for Richmond County, at (706) 821-2349 or [email protected]

Originally published by Sid Mullis Columnist.

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