F. oxysporum is one of the most frequently encountered Fungi amongst hobbyists, and almost all Cacti and succulents are susceptible. Unchecked, Fusarium will eventually cause death after clogging the plant's xylem (water-conducting tissue), and starving it of water and nutrients. It is often a secondary symptom of root knot, since nematodes can carry the fungus. Unlike other Fungi, soil moisture is not as much a factor in a Fusarium infection. High soil temperatures are, however. Fusarium most often infects roots and lower stems initially, and eventually progresses up the stem. Stems then take on a bleached appearance. Leaves may show spots and sometimes a reddish discolouration, but wilt alone is more common. Wilt may seem especially bad during the day, with temporary recovery evident at night. Stem and crown rot may or may not be present. Root rot also occurs, and roots may show a reddish-brown discolouration. Orange spores may also appear on infected tissue.
If Fusarium is suspected, isolate any infected plants and treat unaffected plants with a soil drench. Clean infected plants thoroughly, removing all affected tissue and roots. Be sure to remove ALL soil, and discard the container. Fusarium can survive in soil for years, so it is best to not take any chances on re-infection. After re-potting, treat with a fungicidal drench.
HENDERSONIA OPUNTIAE (scorch/sunscald)
As the name implies, H. opuntiae is most common in Opuntia. It is characterised by large reddish-brown spots, which increase in size and are a precursor to tissue death. Infected tissue may show cracking. If you have an Opuntia with these symptoms, but also others, another fungus may also be present.
MACROPHOMINA PHASEOLINA (charcoal rot)
Charcoal rot most often manifests in stems, and causes progressive rotting and tissue death. Infected tissue to appears shredded. Shredding may also occur with Pythium and Fusarium, but only M. phaseolina have visible black sclerotia. Found in between layers of shredded tissue, the sclerotia are what gives the fungus its charcoal appearance. Infected areas sometimes weep a gummy-like material. Yellowing and death of leaves occurs fairly rapidly, and roots may show a pinkinsh discolouration. Drought favours M. phaseolina, and HOT soil and low moisture just after flowering also encourages its introduction. My sources say there is no effective control for charcoal rot. My feeling is that if a valuable plant is involved, and it is identified early enough, it is worth attempting treatment before discarding the plant. Always be sure to quarantine any infected plants immediately, however.
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