Thank you for your question. African Milk Trees (Euphorbia trigona) are succulents that make great houseplants. From what you’ve described, your plant’s stress symptoms could be due to water, light, or pot size issues.

The yellowing and drying up of new growth could be the result of the plant getting too much or too little water. Succulents need only minimal water this time of year when the top inch of the soil is dry. Be sure to water the soil evenly and thoroughly so that all of the roots receive moisture. The pot must have adequate drainage holes to allow any excess water to flow out freely. Always let the soil dry out between waterings.

Succulents require a lot of light, ideally 14-16 hours a day. You can supplement their light deficiency during these winter months with the addition of artificial lights. Check out this nice Extension publication by Deborah L. Brown for more information about growing cacti and succulents as houseplants: https://www.extension.umn.edu/garden/yard-garden/houseplants/cacti-and-scculents/

The “scab” that you are referring to may be the natural maturing of the stem, especially if it appears at the base of the plant. It may also be something called scale or even just a section of the plant has died. As always, it’s best to know what the culprit is before control measures are taken.

A seven foot tall plant in a seven inch pot may benefit from being repotted into a larger pot. Depending on your specific plant’s growth rate, repotting every year or two will help ensure that the roots do not become pot bound. Removing and re-potting the plant will also allow you to examine the roots to see if they are very dry and dehydrated or soaked and rotting. Rotting roots are typically black, mushy, and often time have a swamp-like smell to them.

Repotting a plant this size is a big job and may require two people to avoid damage to the plant. Be sure to wear the proper attire when working with these plants to avoid damaging yourself. The plant’s spikes and the toxic milky white sap can cause injury. Leather gloves, eye protection, long sleeves, pants, and footwear should be worn. You may want to try taking a cutting from a healthy part of the plant to start a new plant. Here’s some tips for doing that: http://www.plantstogrow.com/P/93/euphorbiaceaeeuphorbiatrigona

Here are more helpful tips for repotting houseplants: https://extension.psu.edu/repotting-houseplants