If you’re growing a Desert Rose (Adenium obesum), you probably know that it has an odd look, with a thick, succulent stem called a caudex that can become a bit grotesque as it enlarges. But its name comes from its desert-like native habitat and its colorful, showy flowers that resemble small roses. Usually easy to grow when given the right conditions, leaves on a Desert Rose might turn yellow and start to fall. This can stem from normal changes as it cycles through the year, or it might signal a problem that needs to be addressed.
A Natural Cycle
A Desert Rose is sensitive to frost but can grow outdoors year-round in USDA hardiness zones 10a through 12. In colder regions, it also does well as a potted plant, either kept indoors as a year-round houseplant or grown outdoors in summer and overwintered indoors.
Whether an outdoor plant in a warm-winter area or a houseplant, Desert Rose usually continues to grow throughout the year, so its leaves should stay green if it’s healthy and problem-free. But in cooler areas where night-time temperatures fall between 39 and 46 degrees Fahrenheit (4 and 8 degrees Celsius) and become 60 to 64 degrees Fahrenheit (15.5 to 18 degrees Celsius) during the day, a Desert Rose behaves like a deciduous plant, with its leaves gradually turning yellow and dropping. You can prevent a potted plant from losing leaves by moving it indoors before the air cools in the fall. If you grow the plant outdoors year-round and it becomes dormant in fall, it should put out new growth as soon as the air warms in the spring.
Possible Pests
A Desert Rose might attract one of several pests that can affect its leaves, causing them to yellow and eventually dry up. These include mealybugs, fluffy white insects that feed on the foliage. If you see these insects, destroy each one by touching it with a cotton swab dipped in rubbing alcohol.
Other possible pests include aphids — small greenish insects — and spider mites, which aren’t readily visible but make web-like coverings on leaves and buds. Both pests feed on plant juices and can cause leaves to yellow, wilt and fall off the plant. For a mild problem, use a strong jet of water to dislodge the pests. If the infestation is severe, spray the plant with insecticidal soap, diluted at a rate of 5 tablespoons per gallon of water. Repeat the spray every two weeks as needed.
Fungal Problems
If a Desert Rose is grown in overly wet conditions, it might develop fungal stem rot, which usually starts at a stem tip and could also affect leaves on that stem, causing them to turn yellow, then brown. Eventually, the stem darkens or turns yellow and becomes soft, with the problem moving down the stem toward the soil. To save the plant, prune the affected part of the stem back, making the cut behind the soft area and into healthy, firm tissue. Disinfect the knife or shears by wiping the blade with rubbing alcohol between each cut to prevent spreading disease.
Source: ehow.com
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