Stem Rot

• Stem rot can kill an opuntia in two to four days. Stem rot begins in soil that is too wet and quickly works its way upward. It frequently attacks young cactus seedlings. On an older plant, it infects through broken spines or puncture wounds resulting in top rot. At first, the lesions are small and yellow. As the infection grows, the color changes from yellow to dark green and progresses to dark brown. The infected area also appears water-soaked. The cactus starts shriveling and shrinking and is covered with brown spores. Before dying, the entire cactus collapses. The fungicide captan is used to control this fungus.
  
  

Scab

• Scab is common on the prickly pear. The fungus begins with small blisters that grow and swell. After the blisters open, rusty-colored scabs form. This condition is caused by overwatering and poor air circulation. To control this disease, reduce moisture and increase light.
  
  

Scorch or Sunburn

• This is a common and serious fungal disease of opuntia. It results from the plant being exposed to direct sunlight after being sprayed with a chemical. It begins as a small, localized spot. The spots gradually enlarge and spread until the entire cactus pad turns a reddish-brown color and dies. The center of the infected area may be grayish-brown and cracked. Cover the cactus or wash the chemical off with water before exposing the plant to direct sunlight.
  
  

Phyllosticta Pad Spot

• The phyllosticta fungus causes small, black, circular spots to develop on the cactus pads. This condition is caused by soil that is too wet. It is quickly spread to other plants by dripping water or wind-blown rain. Severely infected pads and plants should be removed to prevent spread of the disease.
  
  

Cactus Anthracnose

• This fungus results in moist, light brown rot with pink surface blisters. At the onset of infection, the spots are small. They enlarge and become covered with spore-producing blisters. If the infection is widespread, it may destroy the entire plant. Remove all diseased pads and severely infected plants. Spraying with a copper fungicide may help control the disease.