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.
Cactus
The leafless, spiny stem is the characteristic feature of the majority of cacti (and all of those belonging to the largest subfamily, the Cactoideae). The stem is typically succulent, meaning it is adapted to store water. The surface of the stem may be smooth (as in some species of "Opuntia") or covered with protuberances of various kinds, which are usually called tubercles. These vary from small "bumps" to prominent, nipple-like shapes in the genus "Mammillaria" and outgrowths almost like leaves in "Ariocarpus" species. The stem may also be ribbed or fluted in shape. The prominence of these ribs depends on how much water the stem is storing: when full (up to 90% of the mass of a cactus may be water), the ribs may be almost invisible on the swollen stem, whereas when the cactus is short of water and the stems shrink, the ribs may be very visible.