The Cactus Is Starting to Rot

Causes

• You may be watering your cactus too often or it may be growing in high humidity -- both are invitations to fungal rot. If you add too much fertilizer, your cactus will grow so fast it doesn't have time to develop a thick skin and so will be susceptible to fungal infection. Too much sun can kill or damage cells on the surface of your cactus, an invitation to fungi. Many fungi like darkness, and if your cactus isn't getting enough light, it may develop rot.

  Rots on cacti are slimy, mushy and usually smell bad. By the time you notice the rot, it may be too late to do anything about it.

Rotting Stems

• Three types of fungi -- species of Fusarium, Lapsiodiplodia theobromae and Phialocephala virens -- are usually responsible for stem rot on cacti. The rot usually starts low on a cactus and works its way up, causing dark brown or black spots. Over-watering is the usual cause. Do not water your cactus too much. If you spot rot on the stem, cut it out with a knife that you clean with alcohol every other cut; remove enough healthy flesh to keep the rot from spreading. Apply anti-fungal sulfur powders that you can buy at garden supply centers.

Rotting Roots

• If the roots of your cactus are rotting, it is likely because you watered it too much. Your cactus might not be well drained; if your cactus is too large for the pot and the roots are cramped it might not drain well. You also might have damaged the roots while repotting.

  It is unlikely that you will spot root rot early. If you do, stop watering the cactus and let it dry out. You might repot the cactus to get better drainage. If you try to cut the rot out, do it in the same manner that you cut out stem rot.

Rots from Grafting

• When you graft your cactus with a dirty blade ,the scion may shrivel and turn red, then brown. Remove the infected scion with a clean blade and apply sulfur powder to the wound. If a cactus has naturally evolved to grow in shady conditions, too much sun can cause a grafted scion to rot. When you graft a cactus, do it in the shade and keep the humidity low.