Moon Cactus Stem Turning Brown

Age

• Over time, the skin of night-blooming cereus stems naturally become corky and more bark-like, turning brown or gray and rough rather than smooth and green. Eventually, there isn't enough chlorophyll left in the rootstock to manufacture food for the scion, or for the rootstock itself. To save the moon cactus, you need to regraft the scion onto a new rootstock. In addition to night-blooming cereus for rootstock, you can also use seedlings of upright columnar cacti such as blue myrtle cactus (Myrtillocactus geometrizans) or types of Peruvian cereus (Cereus peruvianus). These are longer-lived and don't become corky.

Freeze Damage

• Because night-blooming cereus is a tropical plant hardy to U.S. Department of Agriculture plant hardiness zones 10 and 11, even though top portion of the graft is hardy to USDA zones 8 through 10, moon cacti cannot tolerate temperatures near freezing. The stem becomes mushy and brown if exposed to too much cold, and the only way to save the top portion of the graft is to regraft it onto a new rootstock. Quick action is needed to prevent the possible spread of bacterial rot from the dead stem to the scion. Cut the top part of the graft off with a clean, sharp knife, making sure no brown tissue is present in the cut area before grafting it to a new rootstock.

Root Diseases

• Like many succulent plants, moon cacti are susceptible to overwatering. If the soil remains wet for too long, the roots die and plant pathogens such as bacteria and fungi move into the plant tissue, causing rot. Pathogens move along the plant's vascular system, which transports liquids and nutrients throughout the plant. Infection moves into the rootstock, and it will begin to die and turn brown near the soil level. If you catch it in time, you can cut off the rootstock above the area of rot, allow the cut surface to heal over and reroot the rootstock. If the stem is too far gone but the top is still healthy, you can regraft the moon cactus. If the rot spreads to the scion, the moon cactus will die. Prevent root rot by using a well-draining potting mix, allowing the soil to dry out halfway down the pot before watering and not overpotting the cactus.

Other Factors

• Sucking insects such as mealybugs can infest both the top and bottom portions of the grafted cactus. A heavy infestation of these plant pests can cause scarring and browning of the rootstock, resulting in decreased chlorophyll for nourishing the plant. If scarring is severe, the moon cactus won't get enough food and will have to be regrafted. To prevent scarring, be observant and physically remove plant pests before they harm the plant. Sunburn can also cause stems to turn brown. Moon cacti are produced under greenhouse conditions, and can't tolerate exposure to full sun. Both scion and rootstock can turn white after being in direct sunlight too long, with the affected plant tissues turning brown as they die and dry up. If too much of the rootstock is harmed, food production is affected. If the scion is disfigured by sunburn, the plant probably isn't worth saving.