I know you will get some reports of cutting out the rotted sections. Well if you do, more will rot and continue to die. I had the same problem and let nature take it's course and it healed up and dried out on it's own. It does leave a scare and may not be what you like. Also cut that sucker growth out. It can be potted up on it's own. It grows into a tree. If left on, all energy goes to it and not the graft.
It may be too late, if so, cut the good secton out and let dry a few days and plant them up. Watch out for that white sap. Use caution and clean up hands with soap and water.

Stan has given great advice. To take things a bit further. Should you decide to remove the infected area, do be very careful of the latex sap as Stan mentioned. For some people it is very caustic and causes a nasty contact dermatitis, especially lips and other facial areas. Perform the surgery at a sink and wash the cut portion of the plant with lots of water. This will stop the "bleeding". Once stopped, use paper towel and pat dry the cut and apply cinnamon. This should help prevent a recurrence. Once a good callous forms, you should be good to go!

Thanks so much to both of you! I will try to remove the branch, though I was not sure if I should, Cut it at the base of the growth and replant in dry soil, correct?

If the infection does manage to run itself out, will the crest that remains regrow, or continue to grow out? It is okay if it doesn't, but I would like to know what to expect.

Cut the sucker at the base and allow to callous at least 10 days. Use water to wash away all the sap you're going to have. After callusing plant in gritty mix.

As for the crest: If the infection continues to ravage your plant, you will be left with no alternative but to surgically remove it back to viable tissue. Coat the cleaned cut edge with cinnamon. In any event, there will be a scar left that will never go away and I seriously doubt will ever have new growth as this crest grows primarily from the top leading edge. The new growth will be red in color sometime this spring/summer with lots of SUN.

Thanks Howard, you finish off all what I left out. Good advise. If just some of your crest should survive, Like Howard said, it will grow back but from the edges. In time it will fill out all over the place. Some times it just rots and nothing can save it. Hope yours makes it.

That growth coming off your plant will grow into a small tree in time. Here's mine I cut from a section much smaller than yours at the time. As long as you leave it on, it will rob from the graft.
Stush

I assume that gorgeous crest is yours, Stan. I know I'm going to be waiting a very long time for mine to mature!

Speaking of trimming pieces off -- should this sprout be removed from my crest? It's about 2 inches long and is 3-sided - reverting to the E. lactea plant? And can I pot it up for a new plant - not cristata, I presume?

Thanks!

Yes and yes. Again cut as close to crest as possible and careful with all the latex!

Thanks! This will be fun - and I'll be careful.

All right, great! I have never replanted something like the sucker branch, so when you say to let it callous for ten days, that means to leave it out of soil, correct? And the little tree does not flower, according to research?

To let it callous means to allow it to dry out in the open and then plant.

Planto

Is it mushy? Unfortunately, you are going to have to slice the rotten part off:(. Just take a clean knife and do one straight cut. The rot will just spread and kill the rest of your plant. Sucks because the shape will be gone, but you will save your plant;)! Plus your Euphorbia will grow and take on a new shape.

Now the question is, how do you think the rot started?

I have no idea how it started. But it is mushy :(. Iso there anything I need to put on it after I cut it off? Or will it health itself with no help?

Some people use cinnamon, but I've never used it. As long as you keep the cut Euphorbia in a dry area you should be fine. I forgot to add when you cut your plant a toxic white substance will ooze out. Make sure to keep this away from eyes and skin. Cut till the flesh is fresh with no dry dark spots. With Euphorbias the area that's rotted will not have that white subcstance, it will be brown, dry, and mushy. Cut until you see nice light flesh that oozes that substance.

To stop that ooze you can run under warm water. Then just take a paper towel and soak up the water. Keep very very dry. Goodluck:).

Make sure to wash very well with a good soap many times. It seems to stay on skin even after washing. I have used cinnamon with no good results. Best to leave heal by itself as Kara pointed out. Mine have showed some minor rot from time to time and lucky it never spread or consumed mine. I have three big ones and very many cuttings started and gave away. I had one yellowish one once and lost it and never saw another one like it. Like White Ghost but with the yellow highlights.

BTW, Cold is the biggest killer this time of year. Keep very dry during cold spells.

The crest of my Euphorbia is beginning wrinkle and sag. It also has some kind of rot? I have had it about 3 months and have tried to avoid over watering. The soil is very dry and gritty right through. Could it be under or overwatered? What could I do to save my plant?

Catie, we need a picture, please. Have you read the previous accounts here and found them to not be applicable?

Difficult to cut away from where it is. Can't gather from other comments whether wrinkling sagging is due to under or over watering

Well join the club. I lost a few grafted on that stock. The ones grafted on triangle stock did well but not on the round ones. (Euphorbia Neriifolia round and Euphorbia lactea triangle).

There are some who grows it on it's own with out the graft and does well. Only thing I can suggest is to keep warm dry and lots of bright light. I don't think you got anything to cut and try to start on it's own.

I just lost one of my large old ones this past summer due to all the rains we had. By the time I brought it in to dry out, it was too late. Other two did ok.

Catie, that's an odd-looking injury, as it just the top few layers have been removed (what kind of critter would be gnawing on Euphorbias is beyond me; there are bees which make honey from Euphorb flowers), and not really a source of rot, especially if it's healed (it looks like it is) and not spreading, as well as being kept dry. Unless it worsens, I'd not worry about it as a source of rot, as it looks bad but not dangerous to the health of the plant.