The tall and wide-spreading catalpa (Catalpa spp.) trees, with their lush foliage, provide welcome shade on summer afternoons. Catalpa trees prefer well-drained, moist soil and partial shade to full sun. They grow in U.S. Department of Agriculture plant hardiness zones 4 through 10, depending on the species. The trees display showy, creamy white flowers reminiscent of orchids, heart-shaped green leaves and 1-foot-long seedpods. Northern catalpa (Catalpa speciosa), hardy in USDA zones 4 through 8, grows up to 50 to 90 feet tall and 40 feet wide. Southern catalpa (Catalpa bignonioides), hardy in USDA zones 6 through 10, grows up to 25 to 50 feet tall and wide. Seeing such gracious trees wilt can be disheartening, but with proper care, some of the diseases and other factors that cause wilting can be controlled or alleviated.
If your catalpa tree is wilting on one side in the summer, Verticillium wilt may be the culprit. This fungus disease also causes purplish or brown streaks under the bark of infected branches. Infected trees may die within months or survive for several years. If your tree is infected with this fungus, remove and destroy infected branches to prevent the fungus from spreading to other plants. An infected tree may survive if you provide 1 inch of water every 10 to 14 days during dry spells. If applied soon after wilting begins, ammonium sulfate fertilizer can also help. Wear a dust mask, goggles and gloves before applying. Then spread on the soil around the tree at the rate of 2.9 pounds per 100 square feet, and water in the fertilizer. Do not plant a catalpa tree or other susceptible tree in the same spot in which Verticillium wilt killed a tree. Immune or resistant trees include larch (Larix spp.), which grow in USDA zones 1 through 8, and oak (Quercus spp.), which grow in USDA zones 4 through 9.
Southern blight is a soilborne fungus disease that will cause your catalpa trees to wilt during the summer. Even if the trees seem healthy during cooler weather, the lower leaves will turn yellow or wilt during hot weather. Near the soil surface, you may notice small yellowish structures about the size of a mustard seed. To prevent southern blight in future trees, ask for disease-free catalpa seedlings from a reputable dealer.
Fungicides that sometimes control southern blight are generally available only for commercial use and often are not practical, according to the American Phytopathological Society. The fungus typically lives in the top 8 to 12 inches of soil, so one way to discourage its growth is to hire a landscaper to break up the soil surface with an aerator. Or you can cover the infected soil with clear for one to two months every spring to heat the soil and kill the fungus. Remove infected plants and soil from 6 inches beyond the diseased soil area.
Poorly drained soil causes wilting when it prevents the roots from taking in oxygen and nutrients. Waterlogged roots cannot absorb water, so the wilting tree may appear to be suffering from drought stress. Soggy soil promotes root rot, which causes yellow, wilted leaves in catalpa trees. Well-drained soil, proper watering and a soil pH below 5.6 helps prevent black root rot, according to the Cornell University Department of Plant Pathology and Plant-Microbe Biology. Mixing 3 1/2 pounds of sulfur into the top 10 inches of 100 square feet of soil in the spring or fall should lower the pH from 7.0 to 5.5. To avoid introducing plants to the garden that have black root rot, do not buy plants that have dark roots or wilted leaves. Certified professionals can apply fungicides such as thiophanate-methyl at planting time, but sanitation and cultural controls should be the first defense.
Sulfur dust can cause eye and throat irritation, so be sure to don a dust mask and goggles before applying. If you are pregnant or nursing, have someone else apply it. If the tree is an area of the yard where hardscaping, such as a wall and side of house, prevent good ventilation, don't apply it. Keep sulfur, fungicides and ammonium sulfate out of the reach of children and pets.
Hire a landscape professional to prune the diseased branches that are out of reach. Keep sharp pruning tools out of reach of young children.