The most important disease-causing problem for the Christmas cactus plant is caused by a fungus – Fusarium oxysporum.
Stem lesions are water-soaked initially and rapidly enlarge to reddish-brown areas which appear sunken or mushy.
The fungus spores love this environment, and it is easy to find the dusty yellow-brown covering of spores.
Splashing water moves these spores to uninfected areas of the plant or other plants in the propagation area resulting in a rapid spread of disease.
Christmas cactus growers monitor and watch for fungus problems as they can spread rapidly through stock bed areas.
The key to control of Fusarium fungus disease on Christmas cacti must begin clean stock. Keep cutting as dry as possible until they root.
What About Fungus Gnats?
Fungus gnats can at times also contribute to problems with Schlumbergera (Christmas cactus) or Peperomia.
This necrosis or rotting at the soil line is illustrated in the picture.