Soft, mushy tissue on a cactus is rot, and water is almost always the cause. Cactus root rot starts below the soil line. Roots that sit in a wet, slow-draining mix cannot take up oxygen, so they die and break down. Press your plant where the stem meets the soil. If the base gives like a ripe peach, the rot has already gone a long way, and cutting back on your watering will not bring that tissue back.
A golden barrel on my own kitchen windowsill went yellow-brown at the soil line one November, and the plant tipped sideways when I picked up the pot. I pulled the root ball loose and the roots came away dark and stringy. The mix was heavy stuff straight out of a bag, still damp two weeks after the last drink, and a sack of coarse grit sat under the potting bench the whole time.
Roots need air in the soil as much as they need water. Every pore that stays full of water holds no oxygen, so root tissue starves and dies, and rot organisms move into the dead cells. The damage then travels up the plant, which is why you find the mush at the base first. Your cactus gives you no early warning either. A thirsty cactus does not wilt the way a fern or a coleus does. A soft, discolored stem is a late signal rather than a first one.
Four things cause most cases. Clemson Extension pins root and stem rot on a slow-draining soil mix or watering too often, and you usually get both in the same pot. A plant left standing in a full saucer rots faster, because the bottom inch (2.5 cm) of soil never dries out. Over-potting is the cause people miss. A pot much bigger than your cactus holds a ring of wet soil that small roots cannot use. The first overwatering cactus symptoms often show up a month or two after you move a plant up a size.
Not every soft spot means the same thing. Cactus stem rot that starts at the base and climbs is the serious version, because the cause still sits in the pot around your plant. A soft patch further up the stem, with firm tissue below and around it, points to damage in that one spot instead of trouble at the roots. So press in four or five places before you decide anything. A soft base is cactus root rot showing itself above the soil. A firm base sends you looking somewhere else.
Put the watering can away and keep the pot dry while you work out how far the rot has gone.
Press the stem right at the soil line. Firm tissue there is a good sign, and soft tissue means the roots are involved.
Use equal volumes of peat-based potting soil and very coarse sand or grit, in a pot with drainage holes that suits the size of your plant.
While the plant is out of its pot, check the stem and the root crown for mealybugs and spider mites.
That last step matters more than you would think. Cottony white spots tucked into the ribs mean you have two problems at once. Aphids, spider mites and mealybugs are the main indoor pests on cacti, and scale is harder to get rid of than any of them. Clear those off before your plant goes back on the shelf, because a cactus that is losing roots has nothing spare to feed a colony of insects.
Then set your plant up so cactus root rot never gets a second try. Choose a pot that the root ball nearly fills, and skip the two-sizes-up upgrade that feels generous but holds a week of wet soil. Empty the saucer within an hour of watering, since a cactus must never be left standing in water. Water only after the mix has dried the whole way through, and test that with a finger pushed deep into the pot rather than trusting the dry crust on top.
Read the full article: Cacti Are Not Hard to Kill, Just Hard to Read
