Overwatering

• Stem rot is caused by gardeners overwatering their cactus plants in the winter when the plant has gone dormant, according to the Cactus Museum. Fungal and bacterial pathogens thrive in moist soil conditions. Gardeners can prevent stem rot by allowing their cactus to dry out in the fall and winter. During those months, give your cactus water only sparingly -- enough to slightly dampen the dry soil, suggests the University of Illinois Extension.
  
  

Stem Damage

• Stem injury creates a pathway for harmful pathogens to enter. You may see browning occur near the point of injury. Otherwise, cacti with stem rot exhibit symptoms near the base of the plant. As the disease progresses, areas turn brown or black going up the stem of the cactus plant. Other symptoms include a blackish watery substance exuding from diseased cactus tissue, according to the Cactus Museum. When stem rot isn't severe, you can use cultural methods for treating the disease .
  
  

Cultural Treatment

• Place your cactus in a dry place if it is planted in a container. If planted outdoors, stop watering the plant for a couple of weeks to dry out the diseased area and stop fungal spores from germinating. Spread a layer of gravel underneath the cactus. Gravel will help stop the spread and germination of bacteria. Plants in containers should be repotted with sterilized potting soil. Avoid using the soil in your yard, because it can contain harmful fungi.
  
  

Cutting Out the Disease

• Use a sharp knife to remove the diseased or discolored areas of of the cactus. Spray your knife with a mixture of 70 percent denatured alcohol and 30 percent water in between cuts to prevent spreading any pathogens. Remove the area that is bordering the infection, because it is probably harboring the pathogens even if symptoms haven't been exhibited yet. Dust the area with sulfur powder to stop the fungal spores from spreading. If the top of the cactus is healthy, cut the bottom off and replant the top in sandy soil, as recommended by the Cactus Museum.