Like most cacti, the barrel cactus is very resilient and can recover from most problems. Because of the structure of the plant and the very slow growth rate, old injuries tend to remain visible for years. Don’t try to treat an old injury that has calloused over. The following will provide a guide to help you identify and understand warning signs for cactus diseases.
Black Dry Areas on the Plant
The plant has been damaged by frost. The black scar is unsightly but can not be repaired.
The plant has been sunburned because of poor orientation after transplanting.
Impact Damage Scars
These scars might be unsightly, but they do not affect the health of the plant.
Soft Waterlogged Areas at the Crown
A cactus that shows soft, dark brown areas at the crown has a fungal infection caused by over watering. The disease can occur when a barrel cactus has been in the rain a long time or is constantly watered from above. There is no cure, so diseased plants should be removed and destroyed before the fungus forms fruiting heads and scatters spores.
Soft Parts to the Body
Plants that have been transplanted can develop an internal fungal rot if they are moved poorly. Barrel cacti should only have their roots in the soil. The barrel itself should be free of soil. Often, unintentional damage to the plant can make the problem worse. This internal rot can not be reversed and can only be prevented by proper management of transplantation.
Black Crusty Deposit
You might spot a dark brown or black deposit on the barrel cactus. If you look closely, you might see a hole in the cactus skin immediately above the encrustation. This damage is done by the larvae of the cactus longhorn beetle which, hatch from eggs injected into the plant. Although the larvae can burrow down to the roots of the plant and eventually kill it, the best way to protect the plant is to remove the beetles by hand to prevent egg laying. The beetle is most active in the early morning or late evenings after rain. Barrel cacti can recover from minor damage quite well and are protected to a great extent by the spines.
Yellow Scar Tissue
The agave plant bug, which is only half an inch long, feeds on the skin of several cacti. The scar is formed where the bug has fed and is a natural reaction of the plant. These bugs can reach plague proportions on individual plants in late summer or early fall. Although the scars and damage might appear minor, they can lead to the death of the plant, so treat them early with insecticidal soap.
Holes and Damage at the Base of the Plant
In times of severe water shortage you might see ragged holes at the base of a barrel cactus, often with some of the surrounding spines broken off. This damage is caused by small animals like rats and mice seeking water. The only effective way to protect the plant is to fence it off until the water shortage ends. As long as the damage is minor, the cactus will be scarred but continue to thrive. This damage can allow fungal spores into the cactus, so it needs to be monitored so early action can be taken if necessary.