A desert rose plant (Adenium obesum), with its unusual, grayish-green stems and branches that are thick and swollen, sometimes looks almost grotesque. Green leaves grow at its stem tips and surround flower buds. Although it isn't a difficult plant to grow, a few problems and pests can cause it to drop leaves, but you can usually prevent serious issues if you act quickly.
A Natural Cycle
Often called a semi-deciduous plant, the desert rose starts dropping its leaves when cool fall weather arrives, and will continue losing leaves until most or all are gone. This is normal for the plant and signals that it has entered dormancy. Keep it in a sunny but cool indoor spot and water sparingly, every week or two, until you see new leaves appear in spring. If you live where weather is warm year-round, a desert rose may grow non-stop and never become dormant.
Desert rose grows outdoors as a perennial in U.S. Department of Agriculture plant hardiness zones 10b through 12, but you can grow it as a houseplant everywhere. It thrives in full sun and flowers from spring into fall. If you grow it in a container and keep it outdoors in summer, bring the plant indoors when temperatures fall below 55 degrees Fahrenheit.
Too Much Moisture
If a desert rose starts dropping leaves while it's actively growing and flowering, this probably indicates a problem. It needs excellent drainage and quick-drying soil, and does best in a pot that's only slightly larger than its root mass, because too much soil can stay moist and might lead to root rot. This problem interferes with the roots' ability to take up water. If not corrected, leaves may start dropping and the plant could die.
Keep a potted plant in a well-draining commercial potting mixture intended for cacti and succulents, only water when its soil is dry, and never allow the pot to sit in a water-filled saucer. Always use a pot with drainage holes. If a plant's soil stays soggy and you detect soft roots that indicate rot, you might save the plant by re-potting it in new medium, first trimming away any damaged roots. Wipe your shears with rubbing alcohol between cuts to prevent spreading disease. The desert rose's sap can irritate the skin, so always wear gloves when handling the plant.
Several pests may infest a desert rose, causing loss of leaves. The oleander caterpillar, a large yellow caterpillar, eats leaves and might strip an entire plant. Control these by picking them off. The plant should develop a full set of new leaves in four weeks.
You might also see fluffy white mealybugs, or aphids, small, crawling pests. Microscopic spider mites could also cause webby coverings on leaves. All these pests eventually cause leaf drop. Control them by mixing insecticidal soap, diluted at a rate of 5 tablespoons in 1 gallon of water, pouring the solution into a sprayer and covering all plant surfaces thoroughly. Repeat every two weeks as needed. Scale insects, which look like raised dark spots, might also cause leaves to fall. Control them by spraying as needed with horticultural oil, diluted at a rate of 3 tablespoons per gallon of water. Wear gloves, long sleeves, long pants and protective eyewear when working with garden chemicals.
The desert rose is usually a tough, easy-to grow plant when grown under the right conditions, but it might develop a fungal disease called anthracnose when grown in a humid environment or in overly moist soil. The leaves gradually turn yellow, then brown or tan, eventually dropping from the plant. It appears in summer or early fall and, if it's not severe, the plant usually recovers. It's best controlled by watering only at the soil line to keep the leaves dry and clearing away fallen leaves often.