Sick trees need homeowners' help

Garden: Even as leaves sprout and branches reach out, pests and disease go on the attack.

In The Garden

May 06, 2001|By Ann Egerton | Ann Egerton,Special to the Sun

As we watch trees blossom and grow in the spring, it doesn't occur to us that some are sick, even dying.

Years of drought, air pollution, pests, development and diseases have damaged many of the trees we love and need. Even last year's heavy rains added stress because the excessive water drowned the extended roots that had been reaching out for moisture during previous dry summers.

And we never know from year to year how bad the gypsy moth infestation will be in our oaks, lindens, sweetgums and willows; some control is achieved by spraying with bacillus thu-ringiensis (BT) in early May.

Gardeners can learn which trees are having problems and what can help them. There are effective treatments for many ailments, and some cultivars or cousins of a species are more disease-resistant than others. Occasionally, we must simply get another kind of tree.

The most commonly mentioned trees that are in trouble are the elm (Ulmus americana), flowering dogwood (Cornus florida), sycamore (Platanus occidentalis) and eastern hemlock (Tsuga canadensis). The American elm, which used to shade the streets and yards of every city and town in the eastern and midwestern United States, has been dying of a lethal fungus called Dutch elm disease. Fifty years ago, there were 38,000 elms in Washington, DC; there are now 8,000.

Patricia Pyle, a forester with the Baltimore Department of Recreation and Parks, recommends some disease-resistant elm cultivars such as 'Liberty,' 'Washington' and 'Princeton' and the Japanese Zelkova, a cousin of the elm. All come close to the majestic height of the American elm.

Mike Galvin, supervisor of urban and community forestry for the Maryland Department of Natural Resources and Forest Service, says that many eastern hemlocks have wooly adelgid, an invasion of insects with a white covering. If it hasn't gone too far, injections of the chemical Merrit into the tree often help. Also, Jim McDaniel, head of Gardens at Ladew Topiary Gardens in Monkton, reports that spraying with horticultural oil is often effective. The hemlock is the only shade-tolerant conifer, so it can't be replaced in the shade except with a very different non-conifer look. American holly (Ilex opaca) is often used, but it won't give you the soft, feathery effect of hemlock.

Both the sycamore and flowering dogwood are suffering from anthracnose, a nasty, disfiguring fungus that can eventually kill the tree. According to Frank Dudak, vice president and owner of Carroll Tree Service Inc. in Owings Mills and past president of the Maryland Arborist Association, air pollution has damaged the sycamore and made it susceptible to anthracnose, which makes the leaves shed in May.

The tree produces new leaves, but producing two sets of leaves a year is very stressful. However, the London plane tree (Platanus X acerifolia) is a hardy, anthracnose- resistant cousin of the sycamore.

The flowering dogwood, which bears white or pink blossoms in the spring and then green leaves which turn scarlet in the fall, has been getting anthracnose fungus for at least 25 years. The fungus is manifested by purply brown blotches on the leaves, which fall off, and after many seasons the tree can die. Dudak says that the dogwood can be preserved for a long time with fungicides.

While the dogwood is an understory tree, it is more resistant to anthracnose if planted in the sun, at least until the heat and humidity arrive. Dudak strongly advises that we have our dogwoods inspected and sprayed every year. Nothing has the lacy look of dogwood in the spring, but there are several attractive and disease-resistant replacements such as Kousa dogwood, which blooms after the leaves are out, eastern redbud or a cultivar of crab apple, such as 'Adirondack.'

The best news is that after many years of research, horticulturists at Rutgers University have produced a hybrid of Cornus florida that is resistant to the anthracnose. It is now available in some nurseries.

And, of course, consult a reputable arborist if you suspect a problem, water your trees deeply and regularly, and mulch around the base.

Sources

Carroll Tree Service Inc.

74 Gwynns Mill Court

Owings Mills, MD 21117

410-998-1100

Maryland Arborist Association

1-888-MD Trees

University of Maryland Home and Garden Information Center

800-342-2507

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