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.
Sudbury River
Water caltrop, more commonly known as water chestnut, species "Trapas natans", is an invasive waterplant from western Asia. The initial introductions in the U.S. were in the 1870s in Cambridge, MA, followed by deliberate introduction into ponds near the Concord and Sudbury Rivers. This is now an invasive, habitat-destroying plant across many of the eastern states. On the Sudbury River, OARS (Organization for the Assabet, Sudbury and Concord Rivers) organizes annual plant pulling events. Volunteers in canoes hand-pull the surface-floating rosettes of leaves and nuts before the nuts mature and fall to the bottom. The infestation on the Sudbury River is particularly bad between the Fenwick Street and Saxonville dams, where the water surface can be more than 80% covered.