Ask Mr. Smarty Plants

Cylindropuntia imbricata (Tree cholla) is native to Texas but not, as this USDA Plant Profile map shows, native to Travis County. However, if you are willing to provide the proper environment, there is no reason why it should not flourish. Here is an article with numerous pictures from Southwest Colorado Wildflowers; no mention of fungus, however, so we will look a little further.

We think you'll find the comments on this plant in this Dave's Garden forum interesting, especially considering the weather conditions they can survive, but still no fungus cures. From the Sonoran Desert Digital Library, more information on the plant.

Finally, a website mentioning fungus: from Arizona Cooperative Extension Pests of Agave, Aloe, Cactus and Yucca. Scroll down to Page 6, where you will find "Fungal diseases of leaves and pads." On Page 7, we found a section on Insects which included this description of the Cactus Longhorn Beetle:

"This beetle attacks several species of cacti including prickly pear and cholla cactus (Cylindropuntia species), barrel cactus (Echinocactus and Ferocactus species), young saguaro cactus (Carnegiea gigantea), and others. The adult beetle is about 1 to 1¼ inches (2.5 - 3 cm) long, shiny black, and has distinctive white markings on the antennae. The antennae are often longer than the overall body length of the adult beetle. Damage to the plants is the result of feeding on the margins of prickly pear pads or terminal buds of other cacti (Fig. 18).

Cholla cacti are attacked by the beetle when the adults lay their eggs, hatch and the larvae burrow into the stems. Waste (frass) is pushed out the entry holes and forms a black crusty deposit on the canes. Larvae may burrow into plant roots and cause collapse and death of the plants.

Cactus longhorn beetle is controlled by hand picking the insects off infested plants. The beetles are most active and easier to detect and destroy in the early morning or late evening, especially after warm summer rains. Very spiny species are less likely to have damage from the beetle due to a natural defense by the spines. Chemical control is not recommended since the populations usually are not high and hand picking is effective."

This may or may not be the situation with your cholla, but the pictures throughout the article are excellent and you may well find your problem somewhere else in it.

From the Arizona Department of Agriculture, we found Bacterial Necrosis of Saguaro Cactus. This article states that the same condition has been noted in cholla cacti. It gives instructions on cutting out the rot and treatments for it. Again, since we are not plant pathologists and cannot see the cactus, you will have to be the judge of which treatment, if any, you wish to use.