I have several Natal Plum shrubs, but only one is having issues. The stem has round, tan-like appendages. Some of the leaves are turning brown and curling. The plant gets plenty of water. I don't know whether these indicate a pest or a disease. Any suggestions would be greatly appreciated! Thank you.
These are a type of gall. Galls can be caused by pest or disease.
On this page about Carissa under pest and disease it lists the following galls,
"...stem gall from Macrophoma sp., Nectria sp., Phoma sp., Phomopsis sp., and both galls and cankers from Sphaeropsis tumefaciens; ..."
Of those mentioned, Sphaeropsis tumefaciens, a fungus, looks more like your picture.
Sphaeropsis Knot. http://www.freshfromflorida.com/content/download/11113/142969/pp106.pdf
Prune off well into healthy wood. Disinfect all tools. Keep an eye on it. I don't see any mention of a particular effective chemical treatment but there may be somewhere.
