The roots darkening, particularly if they are also becoming slimy or soft in texture, is usually an indication that the soil is staying too wet. It is a common problem with these cacti. Tropical cacti, like your Rhipsalis,  grow in a pocket of moss or debris in the crook of a tree branch or a rock. Despite the humid, jungle conditions of their native habitats, these cacti are adapted to grow in a media that is prone to dryness. By contrast, we tend to keep the plants in hanging containers that have few or no drain holes. The result is soil that drains very slowly and which leads to rot. The roots deteriorate and the cells at the tips of the branches burst, leading to yellowing and branch drop.

Soil should also be replaced annually with appropriate, fast draining planting medium to maintain great texture and drainage. More complete information on caring for your plant can be found in our Guide to Mistletoe and Coral Cactus.

What to do now? You can unpot your plant and inspect the roots. If they are mostly dark and soft/ slimy/ mushy, the plant is unlikely to survive. If there is just a bit of damage, cut back the damaged roots into healthy root tissue with a sterilized cutting tool. Repot in a container with great drainage and a mixture of potting medium recommended it the guide above. Make any corrections in care to follow the routine recommended in the guide, particularly as regard the periods of the annual cycle when water reduction is important.

Courtesy of NYBG Plant Information Service