My lily-of-the-valley ground cover has spots on the leaves and does not look good. What is this problem, and what can I do about it?
— Patrick Swanson, Evanston
From your description, it sounds as if your problem is anthracnose, which is a common disease caused by a fungus. On lily-of-the-valley, it causes lesions that are often circular to oval in shape and dark brown with purplish-red margins These spots can be 1/2-inch or more in diameter and may fall out, which will expose the veins in the leaves. Infected leaves can die prematurely.
This disease is not usually fatal but does weaken the plant. The number of flowers is often reduced the year after a plant is infected with anthracnose.
Anthracnose will attack other plants in the garden too, especially in wet conditions. On hostas that are in wet or overwatered areas, it can cause brown spots that merge to form irregular dead areas on the leaves. Coral bells will develop small brown spots on the leaves.
Peonies get both anthracnose and botrytis, which have similar symptoms; the only way to make a precise identification of the problem is to have a lab culture a specimen. Both diseases cause large irregular blotches on peony leaves at this time of year. Both can cause buds, flowers, leaves and stems to die in spring and summer. Symptoms will vary from plant to plant or even on the same plant depending on the pathogen, host, time of infection, weather and other factors.
Anthracnose disease overwinters on dead plant material, so it is important to remove and destroy diseased foliage in the fall when you clean up. Badly infected foliage should be pruned out now and discarded in the landscape waste, not put in the compost.
Anthracnose spores can blow for many miles. Since wet conditions help spread the disease, be careful not to overwater. If possible, prune plants or move them to promote air circulation. Fungicide application can be beneficial in some situations.
Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe (chicagobotanic.org). Send questions to: Gardening Q&A, Sunday, Chicago Tribune, 435 N. Michigan Ave., Chicago, IL 60611-4041; e-mail to firstname.lastname@example.org.