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.
Riparian-zone restoration
Changing flood regimes can be especially problematic when exotic species are favored by altered conditions. For example, dam regulation changes floodplain hydrology in the southwest US by impeding annual flooding cycles. This modification has been implicated in the dominance of saltcedar (Tamarix chinensis) over the native cottonwood (Populus deltoids). Cottonwoods were found to be competitively superior to saltcedar when flooding allowed seeds of both species to cogerminate. However, the lack of flooding caused by altered hydrology creates more favorable conditions for the germination of saltcedar over cottonwoods.