A blackened stem base (“black leg”) on plants such as geranium is often caused by Pythium.
Black or brown discoloration at the base of poinsettia stems may indicate Phytophthora drechsleri infection.
New Guinea impatiens show thin black streaks extending up from the root crown when they are infected by Pythium irregulare.
New Guinea impatiens sometimes show extensive black bands or rings in the stem when infected by INSV or TSWV.
Impatiens may also show black stem sections when infected with INSV or TSWV.
Stem lesions, in conjunction with white spots on petioles and leaves, are strong indicators for a diagnosis of poinsettia scab.
Stem lesions such as these on poinsettia are characteristic of Rhizoctonia stem canker.
A brown canker at the stem base is the first symptom of Rhizopus stem rot on vinca.
Poinsettias may develop black lines along the stem in response to root uptake of dilute chlorine bleach.