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.
Ferocactus
They are desert dwellers and can cope with some frost and intense heat. The typical habitat is hot and very arid, and the plants have adapted to exploit water movement to concentrate their biomass in areas where water is likely to be present. Like "Sclerocactus", "Ferocactus" typically grow in areas where water flows irregularly or depressions where water can accumulate for short periods of time. They are most often found growing along arroyos (washes) where their seeds have been subjected to scarification due to water movement, but they oddly also tend to grow along ridges in spots where depressions have formed and can hold water for some period of time.