Cacti are remarkably durable and low in maintenance. They need little more than sunlight, well-drained soil, and rare moisture. The pests and problems common to the plant group are minimal and usually easy to surmount. Cactus problems may range from sucking pests, like whitefly, to common rots from bacteria or fungal disease. One of the telltale signs of a problem is a soft, mushy cactus.
The arid gardener may ask, "Why is my cactus going soft?" Likely causes are disease, cultivation and improper site and ambient conditions.
Cacti generally have low moisture needs. They thrive in temperatures above 70 to 75 °F (21 to 24 °C) in sunny locations and require little supplemental nutrients. Potted plants need good drainage holes and a soil mix with plenty of grit. In-ground plants have similar requirements.
As with any plant, cacti can become diseased or damaged. A common problem is soft spots in the flesh of the plant. These may be discolored or corky around the spot and the center is mushy and wet. The reasons for such spots may be disease or simply mechanical injury to the pads and stems of the cacti. Cactus rot issues must be dealt with quickly to prevent spread to the rest of the plant and serious loss of vigor, which may become permanent.
Problems with Fungal and Bacterial Diseases
Bacteria and fungus are introduced to the plant from openings in the flesh. The open areas may be from insect or animal activity, damage from inanimate objects or heavy weather, such as hail. The action of injury isn't important, but the damage from fungal spores or bacteria is crucial.
Warm, moist conditions accelerate the production of fungi spores and increase bacterial production. Once the organism takes hold in your plant, you will see soft, mushy cactus. Symptoms to watch for include small sunken spots, discolored scabs, round soft areas surrounded by fruiting bodies, and black or other colored dots on the surface of the cacti skin. You may even notice some oozing of your cactus plants.
Cactus problems that have gotten into the root usually result in a slowly dying plant, while topical issues in the upper body can be treated easily. Most cacti respond well to excising the diseased tissue. Use a sharp sterile knife to dig out the damaged flesh and allow the hole to dry out. Don't water overhead as the wound closes.
If the damage has infected the roots, there is very little you can do. You can try to repot the plant, removing diseased soil and replacing it with sterile soil. You should wash the roots off well before replanting in fresh potting medium.
A soft, mushy cactus can also be saved by taking cuttings and letting them root for a fresh new plant. Allow the cutting to callus over for a few days before you insert it into the sand. Rooting the cutting may take several weeks. This method of propagation will produce a healthy cactus that is the same as the parent plant.
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