Epiphyllums are hybrids of ephytic cacti from tropical regions of South and Central America. Unlike traditional cacti, they have no spines. They have no leaves either; instead they have thick, fleshy stems and branches that perform the same function as leaves. Because of their tropical origin, they are more lush and require significantly more water than arid- and desert-climate cacti. This moist environment can result in a number of infectious diseases.
Fungal Leaf Spot
Fungi of the Phyllosticta genus have been known to infect the stems of epiphyllums. They occur during spring rains and attack the stems. As the name suggests, the first symptom is the appearance of one or more brown spots or lesions with raised centers on the epiphyllum's stems. These lesions can bore all the way through to the opposite side of the stem in the worst cases, but are not fatal, and more often than not create scar patterns and raised ridges which will never heal. As fungicides are ineffective against most fungi of this genus, affected tissue must be cut away to prevent the infection from spreading.
Black Rot Disease
The bacterium Erwinia cacticida can take advantage of the excessively moist soil and delicate root system of the epiphyllum to create what is known as black rot disease. It acts in much the same way as any pathogen, infecting the cells of the cactus's roots to procreate, using it as food, and spreading until nothing remains of the root structure but a wet, black mushy mass. Symptoms above ground include a yellowing of the upper stems and lateral brown striations rising from the trunk, as the fungi sends tendrils up the epiphyllum's vascular system. If caught early enough, affected portions of the root system can be cut away. Otherwise, there is no treatment.
Corky scab is also known as edema and, while a disease, it is not infectious. Symptoms are limited to the appearance of bumps, blisters, warts or scabs with a distinct cork-like, woody texture. They can occur on any part of the epiphyllum. Possible causes include reaction to biting insects, injury from sand or dirt particles carried by the wind, and the accumulation of water outside the cactus' vascular system. To that end, moderating your watering schedule should help in treating corky scabs.