Cactus
Fungi, bacteria and viruses attack cacti, the first two particularly when plants are over-watered. Fusarium rot can gain entry through a wound and cause rotting accompanied by red-violet mold. ""Helminosporium" rot" is caused by "Bipolaris cactivora" ( "Helminosporium cactivorum"); "Phytophthora" species also cause similar rotting in cacti. Fungicides may be of limited value in combating these diseases. Several viruses have been found in cacti, including cactus virus X. These appear to cause only limited visible symptoms, such as chlorotic (pale green) spots and mosaic effects (streaks and patches of paler color). However, in an "Agave" species, cactus virus X has been shown to reduce growth, particularly when the roots are dry. There are no treatments for virus diseases.
Opuntia ficus-indica
The cattle industry of the Southwest United States has begun to cultivate "O. ficus-indica" as a fresh source of feed for cattle. The cactus is grown both as a feed source and a boundary fence. Cattle avoid the sharp spines of the cactus and do not stray from an area enclosed by it. The cactus pads are low in dry matter and crude protein, but useful as a supplement in drought conditions. In addition to the food value, the moisture content adequately eliminates watering the cattle and the human effort in achieving that chore.