Diseases

• Diseases are the most common and likely culprit of brown spots on cactus plants. Brown spots at the base of a cacti are caused by either root rot or basal stem rot. No chemical will remedy these conditions. They are caused by excessive watering and/or poor circulation. Remediation is obtained by correcting these practices. Brown spots on older parts of the cactus are caused by corky scab. Remove infected tissue, and treat like rot infections by improving circulation and being mindful of watering practices.
  
  

Insects

• Cacti occasionally host insects whose appearance or damage can present as brown spots on the cactus. The outer shell of adult scale insects can be brown and appear as raised spots on a cactus. Scales are insects that feed by sucking out nutrients from cacti. Spider mites and aphids feed in a similar manner and are also pests of cactus. Lesions left from feeding wounds can appear as brown spots. Cacti can be treated with a horticultural oil directly applied on the insects with a cotton swab or small paint brush.
  
  

Other Causes

• Brown spots can sometimes be caused by the environment that an individual cactus is in. Excessive amounts of dry air can cause brown spots on some species of cacti. Remove the affected portions and mist frequently with distilled water. Some species can be sensitive to too much direct light. If brown spots appear on your cactus during times of the year when the sun is more intense, remove affected leaves or portions and move the cactus to a slightly less sunny sill or location.
  
  

Proper Care

• The best way to avoid any of the various afflictions of cacti is to observe proper cultural practices. Soil appropriate for cactus should be capable of retaining moisture yet sufficiently drain. Most species of cactus do best in bright light. A southern exposure is best. Water cacti when the top 1/2 inch of soil is dry. Allow cactus to dry out between waterings. Ideal temperatures for cactus are between 65 and 95 degrees F.