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By Scott Aker
Thursday, November 26, 2009

Q: I have an eight-year-old kousa dogwood, chosen for its purported disease resistance, that lost its leaves from the inside out over most of the summer. The leaves just yellowed and dropped, and by late August the tree had leaves only on its outermost tips. The same thing happened last summer to a lesser extent. I see no obvious sign of disease or infestation. I wondered if the ground-cover plants at its base were absorbing all the moisture, so I did a few deep waterings by letting the garden hose trickle there for up to eight hours. Any ideas about the problem or the remedy?

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A: Although kousa dogwood is much more disease-resistant than most flowering dogwoods, it is still subject to some disease damage from leaf spot and powdery mildew. However, the widespread leaf drop is more likely caused by a root issue. Carefully dig around the base of the tree to look for girdling roots. If they are present, cut them to relieve pressure on the lower portion of the trunk.

Watering should not have been necessary for most of the summer -- we had a wet growing season this year. It may even be that your dogwood was waterlogged at times, leading to death of some of the roots and then leaf loss. Dogwoods are rather unforgiving when given long periods of soil saturation. You may need to improve the drainage of the area if this is the case by putting in a perforated drain pipe to remove excess soil moisture.

The mulch around our two-year-old house has been the source of artillery fungus, which has marked the exterior walls and garden plants. We understand it is viable for 10 years, and we don't know what to do except replace the mulch with rock at considerable expense.

Time will take care of the problem. Within a year, most of the spore bodies will have fallen off or weathered away. If you avoid adding mulch to the area, the food source for the artillery fungus will be exhausted after a time, and you should have no further problems.

You can avoid the problem by your choice of mulch. The fungus grows in relatively fresh woody debris, so use composted mulch or pine bark instead. Gravel and stone mulch are another option, but the expense is significant and the stone will inevitably get into your planting space, making digging and garden maintenance more difficult.

Aker is a horticulturist at the U.S. National Arboretum. Have a question about gardening? Write Digging In, Local Living, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071; or send e-mail to localliving@washpost.com.

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