Portions of my fall collard greens grew much smaller than normal, and they are wilting. When I pulled them up, the roots were swollen and knotted. Do you know what would cause this?

I suspect your greens have club root disease. Club root is a fungal disease that affects many plants in the crucifer family, which includes turnips, kale, cabbage, broccoli, brussels sprouts, radishes, rutabaga and cauliflower. Because infected plants are unable to obtain adequate moisture and nutrients, they wilt and become stunted.

Once the disease is in your soil, it can remain there for many years and is very difficult to control. There are very few cultivars of the affected crops that are resistant to the disease, and it is not possible to treat all of the infected soil with a fungicide.

You can help prevent the disease by rotating your crops so that vegetables in this family are not repeatedly planted in the same area.

Also, because this disease thrives in acidic soil, you may be able to reduce its severity by raising your soil pH. It is generally recommended that you raise the soil pH to around 7.0 with quicklime (calcium hydroxide).

We would like to plant several new shade trees this fall to replace trees we lost in a storm this summer. Can you recommend trees that might be somewhat unusual but would be available through local nurseries?

The katsura tree (Cercidi-phyllum japonicum) is a very nice plant that has bluish-green, heart-shaped leaves and grows to around 50 feet in height. It has a pyramidal shape when young but often broadens with age. Several cultivars are available, including one that is a weeping form.

The Chinese elm (Ulmus parvifolia) has relatively small, dark green leaves and, like the katsura tree, grows to about 50 feet. This tree is best known for its mottled, exfoliating bark and has year-round attractiveness.

If you want a larger, native tree, you might like the Kentucky coffee tree (Gymno-cladus dioicus). It has a coarse limb structure but has compound leaves that give it a finer texture in summer. It is an upright grower that reaches 60-75 feet or more in height.

Dennis Bishop is an urban horticulture educator for the Baltimore office of the Maryland Cooperative Extension Services. If you have a gardening or pest problem, you can call the Home and Garden Information Center hot line (Monday through Friday, 8 a.m. to 1 p.m.) at 800-342-2507. You can also e-mail questions, order publications and diagnose plant problems by visiting the Web site, www.hgic.umd.edu.

Checklist

1. Have you held off on fertilizing your lawn because of the drought? Given the recent rains, this would be a great time to fertilize your lawn.

2. With few exceptions, annual vegetable garden plants should be removed from the garden at this time. Diseased plants should be destroyed. All other plants can be added to the compost pile.

3. Avoid excessive watering of pansies. They are subject to several fungal diseases that develop in moist conditions.