Sometimes called by its genus name, Convallaria, the lily of the valley (Convallaria majalis) is grown in U.S. Department of Agriculture plant hardiness zones 3 through 8. Beloved for its attractive, sweetly scented flowers, modern moms should beware that all parts of the plant are extremely toxic when ingested by humans and pets. Although the plant's foliage isn't the star attraction, brown leaves can ruin its appearance and may be a symptom of a deadly fungal disease known as southern blight, or crown rot. To protect other ornamental plants in the vicinity, act quickly as soon as you notice brown leaves on the lily of the valley.
Crown rot of lily-of-the-valley is caused by a fungal pathogen (Sclerotium rolfsii). The fungus emits mycelium -- hair-like structures that spread out across the surface of the soil. When it finds a host, such as the lily of the valley, it emits oxalic acid drops and other enzymes that destroy the plant’s cell walls. Once the cell walls begin to break down the lower leaves begin to change color -- first yellow, then brown -- and the plant wilts.
Crown rot strikes the lily of the valley after a long stretch of hot, humid weather. You will notice discoloring on the lower leaves initially -- they will turn yellow and then brown before wilting. If a gentle tug pulls the leaves from the crown, suspect crown rot. Check the soil at the base of the plant for tiny white, tan or brown balls that resemble mustard seeds. These are the fungal pathogen’s sclerotia -- bodies that contain food reserves to help the fungi through stressful times and then germinate when the stress is relieved.
The only solution for crown rot is to pull up and destroy the lily of the valley. Since the sclerotia will persist in the soil for up to four years, you’ll need to sterilize the planting area after removing the lily of the valley. Rake the soil smooth and water it to a depth of 24 inches. Cover the area with plastic sheeting that is 0.025 to 0.4 millimeters thick. Use bricks or rocks to hold the sheeting in place and to keep air out. Allow the sheeting to remain in place for four to eight weeks. Apply a fungicide drench containing thiophanate methyl to guard against infection of your healthy plants. Use 1 teaspoon of the fungicide in 1.5 gallons of water. Pour the fungicide over the soil around the plants at a rate of 1 to 3 pints – or 2 to 6 cups -- per square foot.
The sclerotia sticks to the soil on your shoes, clothing and garden tools so always change clothes after removing an infected plant and always sterilize your gardening tools before using them on healthy plants. Routinely remove plant debris from the beds. Inspect all new plants before placing them in the garden. Look for yellowing or wilting lower leaves and sclerotia on the soil near the base of the plant.