Hi Jeffrey,

Thank you for contacting the helpline. Common problems with cactuses or cacti can derive from improper sunlight and water management. Too much water or humidity can cause browning as well as too much direct sunlight (scorch). When you stick your finger into the soil about an inch is is wet and mushy? If yes, it is overwatered or has poor drainage. If the brown stems are soft or have a smell it could be rot. Sometimes a plant can rot due to excessive watering, but then dry out and become hard.

If you don't have or have had excessively wet soil, check to see if you can scratch some of those browning dots or patches off with your fingernail? If yes, it's most likely scale. 

Also, an older cactus can develop corking, which is a normal and natural process where the base of the plant changes from green to a rough, brown, bark-like texture to provide structural support as it ages. It is the plants natural instinct to protect itself from breakage or other environmental damage.

Try to see if you can further identify the issue using these tips and let us know what you find. I am also going to refer you to a colleague that has more expertise on house plants to better diagnose your issue. 

Hello Jeffrey:  We consulted with our Houseplant specialist, and following is the information she provided. Thank you for reaching out to us with your question!  Lisa

The “pencil” Euphorbia (Euphorbia tirucalli) prefers to grow
in full sun, meaning at least six hours of sunlight on most days. However, it can tolerate a bit of shade and might even appreciate some protection from the hot afternoon sun. Indoors, grow it by your brightest window. It can also tolerate bright, indirect light.

Soil
- Container plants do well with a succulent or cactus potting mix that doesn’t retain moisture. Make sure to choose a container with good drainage.

Water - It only needs water every two to three weeks in the spring and summer. Reduce watering to monthly in the fall
and winter. It's preferable to let the soil dry out completely in
between waterings to avoid overwatering. This plant is highly tolerant to drought, and too much water can rot its roots.

Temperature and Humidity - The Euphorbia thrives in warm temperatures ranging from 65 to 75 degrees Fahrenheit. Temperatures around the plant should not drop below 50 degrees Fahrenheit. Indoors, be sure to protect your plant
from cool drafts, including those from an air conditioner. The plant also thrives in low humidity. 

Fertilizer - This plant isn’t a heavy feeder. Feed your Euphorbia with a balanced liquid houseplant fertilizer in the spring, and it should be fine for the rest of the year.

Pruning - Pruning a Euphorbia should be done in the early spring and is fairly easy. The white sap from the Euphorbia is toxic, causing skin and eye irritation, and it will stain your clothing,
so always wear protective clothing and gloves when handling this plant. Use a pair of pruning shears and first remove any dead or damaged branches. Then clip off any branches to get the desired shape and height you want, making sure to cut down to the base of the branch. Take care not to take too much off the bottom part of the plant, as this plant does get top-heavy. Your pruning shears will be sticky afterward, so wipe them down with some rubbing alcohol when you're done.

Corking (normal) - Old wood on a Euphorbia tirucalli is a natural, healthy process known as corking. It is an essential part of the plant's growth and is not a disease. As the plant ages and grows
taller, the green skin on the main stems and branches develops a
tougher, brown or greyish, bark-like texture. You will note healthy
green growth above the corked area.

Rot (not normal/bad) – Where the texture of the plant becomes soft and potentially slimy. Spreads quickly and tends to start at the base of the plant and can spread quickly. Dark brown or even black and will smell bad. The plant will have yellow or drooped or shriveled stems.

I hope this information is helpful and your plant recovers nicely!