Pythium is a common affliction of all manner of plants, and it can affect almost all Cacti & succulents. It can be easily confused with Phytophthora, and is a technically a water mould. It is soil-borne, however, and its spores can remain active for at least five years. Damping-off is the most frequent symptom, but Pythium can also be the culprit in many other ailments, including root, crown, seed, and soft rots. Roots often appear honey-brown to dark, and are mushy or slimy, sparse, and unusually thin. Oospores (dormant spores) and sporangia are evident in infected tissue. In early stages, it can manifest simply as wilt or poor growth, with upper portions of the plant eventually collapsing. Depending on the plant and the season, fruit rot may also be exhibited. In these cases, water-soaked lesions appear, eventually becoming 'mossy' from secondary infections by white and grey moulds.
Over-watering, and poor soil-drainage are the most frequent factors in Pythium infestation. Highly soluble salts, in combination with saturated soil, are the most favourable conditions for the entry of Pythium. According to studies by the Alberta Agricultural Department, a Pythium infestation during the 1950s that caused major wheat losses, may have started with phosphate deficiency of the soil.
Pythium will kill seedlings, but older plants can recover if treated early enough. Treating Pythium infestation, however, can be a major ordeal. Any plants with a known or suspected infestation should be immediately isolated. All other plants should then be administered a systemic soil treatment. For those infected, clean the entire plant, and remove affected roots. Soil and containers should be discarded. The disease is spread by contact (hands, tools, benches, etc.), so everything should be cleaned with disinfectant soap. Drenches of an etridiazol (Tryban or Terrazole) and thiophanate methyl (Banrot) are effective. Treatment with products containing Prothiocarb-Previcur (0.15%) or Fenamisul-Bayer (0.03%) also produces results. Using Apron, a systemic, as a seed treatment has also shown some effect as a preventative.
STEVENSEA (DIPLOTHECA) WRIGHTII (charcoal spot)
This fungus is most common on Opuntia spp., and is characterised by small grey-to-black spots. Fruiting structures are visible around the circumference of the infected areas. If the infection is advanced, the entire plant should be removed and destroyed. If caught early, removing affected tissue may be successful.
ERWINIA CAROTOVORA/SPP. (soft rot)
Erwinia spp. are similar to many Fungi, but are actually bacteria that enter through wounds and natural openings. They produce a particularly vicious enzyme, which is encouraged by conditions of high humidity and warmth. These enzymes cause the plant to become mushy, affecting photosynthesis and respiration, and causing rapid deterioration. Black, wet areas of rot frequently appear at the base of the plant first, gradually rising upward, and the plant eventually collapses and dies. Erwinia infection is extremely difficult to treat, and plants with severe infection rarely recover. If these bacteria are suspected, specimens should be isolated immediately. Avoiding high humidity, if possible, is an important preventative, especially if a plant is wounded. Treating wounds with copper fungicide can also help avoid entry. Careful watering is also important, as splashing spreads the bacteria.
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