Pictures please. This might just be really important.

@milevskistefan the photo below might be hard to interpret. Everything on the top is healthy looking for the most part, definitely not any rot. Everything below the exposed surface is black but when you press in against it there is a hard core.

Even though this is planted in gritty mix, water appears to be collecting along the side of the plant below the surface. It doesn't help that California was drenched with rain the last two weeks, so it is definitely overwatered at a time when temperatures are rising and it is coming out of hibernation.

Although that rain was welcomed here, it's the perfect condition for rot as temperature went down in the process. I even had some morning frost the past few days and lost a couple small succulents already. Anyhow, the outter layer of your plant is the softer tissue and will become mushy faster when rot starts. The core becomes woody as the plant matures so it's stays harder longer. Don't know how large is your plant but I would definitely try to check what is happening below before it makes its way up the stem. The combo of blackening and softening is not good.

@milevskistefan I understand your point and it doesn't look good. That said, I have had a number of Euphorbias tolerate that situation and go on to thrive. So I guess I need to understand the different types of root rot. Is there a good reference somewhere?

The root rot that has almost always proven fatal to my plants invades the inner core and makes it go soft. In some species, once that process gets into a substantial part of the base, it almost always consumes the entire plant. What I am experiencing in this plant is something fungal that is more active on the outside of the plant and has not yet started to consume the inner core. I really don't understand it very well and maybe a plant disease expert could shed some light on this for all of our benefit.

Finally, how does one deal with this on a long-term basis when raising a Euphorbia? I can't be hauling 150-pound containers loaded with Turface and stone back and forth to different levels of shade and protection from the rain. I guess I just cannot believe that a Euphorbia cactus is unable to tolerate weeks of rain, as long as the soil is a fast draining soil. Maybe this particular species "Euphorbia Lactea (White Ghost)" would just do better in a porch area and left out of the rain 100% of the time.

The other thing that this experience is making me realize is that there is not *one* kind of gritty mix. For cactus and some euphorbias that are highly intolerant of any level of pooled free water, you need to think about reducing the percentage of Turface and maybe increasing the percentage of stone, to increase the drainage even more and reduce the amount of pooled water even further.

Frankly, it is already quite a consumption of time and energy to just make the gritty mix in any form. To now have to think about making different types of the gritty mix based on the type of plant makes the whole activity just frustrating.

Id just place some kind of waterproof nylon, in a manner so that the water would drain away from the plant roots. Growing plants outdoors is a bigger challenge by itself, you must sometimes ask yourself : why doesnt this plant grow here naturally"

Sometimes its humidity, sometimes its the heat, the cold... etc. Im sure youre familiar with the term " invasive species", and in our case, invasive cacti and succulents, that sometimes CAN grow where they dont belong. Im not familiar with species' of euphorbia invasive atributes, but i do know that if the biome is matching the plants desired attributes, it will thrive. To my knowledge, there arent many xeric euphorbias native to your region(since the cactus-like ones come from africa and southern asia).

Unlike cacti, which get a seasonal rain or a mist to moisten them, most euphorbias live in much harsher conditions, slightly hotter and more arid than most cacti. Some even live in areas with light frost. So there is that.

Then there is rot, which is a gradual process , sometimes it takes years to take effect. Your plants are likely to have a much shorter lifespan should this continue

@milevskistefan does anyone make a waterproof nylon - or maybe netting - that is pre-cut to fit around the top of a circular pot? I would love to have some of those sitting around for cases like this. I would cut back the amount of water reaching the soil during Winter dramatically. It seems like that would be a no-brainer for many Euphorbias.

You know what i mean by water proof... nylon isnt water soluble, but could be bent into a manner for it make channels where the water could go where isnt supposed to go. Hope depot/ hardware store, they should "hard" or reinforced nylon or pvc sheets, like for usage in polytunels or stuff like that. As for the other part, im afraid youll have to do that yourself. Youll probably need some manner of binds, and something to tie around the pot, and the plant, so only that a small amount of water can get in. Id use the same for cacti, except most of mine are rain shielded, or indoors during winter.

Just uh make sure you use fungicide before you put up the cover.

Okay, the plant with the potential fungus issue below the soil surface has some signs of systemic infection. The two attached photos are of limbs that are succumbing to what I think is rot. Is the instruction to just sever the entire limb back to the main trunk of the euphorbia?

Many of you will notice brown splotches all over this plant. Please take my word for it that these are some kind of "sunburn" and are not a rot. They only show on the sun-facing side of the plant. The shadow side of the plant is pristine. These white ghosts appear to be like "albinos" of a kind, and they appear to have a great light sensitivity. Mine might be getting too much sun and I'll address that later.

I will also take this plant out of the soil and let you all inspect roots later, but I want that soil to dry out first before I traumatize the plant further.

Not surprised- cultivar it is euphorbia lactae white ghost, and its proneness to sunburn is somewhat famous. It goes more than simply being light sensitive That being said, fungal infections may form through sunburn scars.... and before you start chopping its branches off, make sure you make a backup plant out of a healthy one just in case....

@milevskistefan I am okay with losing this plant in the worst case. I can buy another if needed. This plant has a sibling I bought in the same container that appears to be doing much better. How would I make a backup plant from this?

In either case, I would be chopping off the limbs, so why not chop off the limbs first?

The simple reason is that, by chopping of the limbs you increase the chances of additional infection, though not by much, chances are it might not secrete its poisonous but fungicidal latex due to sunburn and rot. Then again..... dealing with sunburn rot is one thing- its very treatable. Root rot... no so much. People usually need to re-propagate the plants afterward.

@milevskistefan what is your recommendation for dealing with the sunburn? I guess I need to get this plant into shade for a while.

Its not my recommendation , no. Ive lost several neoraimondia(neocardesia) herzogiana , cereus, and espostoa due to sunburn. My recommendation is not to get sunburned in the first place. My advice is to be cautious if you already have sunburn due to mold. Ive seen it happen even with easy species, with dire consequences , thus the whole reason i advise caution.

@milevskistefan if you are saying sunburn can lead to rot, then I guess common sense dictates that you should at minimum stop further sunburn

Yep, that is the gist of it. I take no credit for "discovery" of the sunburn=rot process, its something ive simply experienced , and seen other people experience it, and which is why i try to raise awareness at least.

If you want a slightly more scientific explanation on how, sunburned surface tissue is corky and woody. The bigger the area and the severity of the sunburn increase the chances of it cracking, and through the cracks, the fungus goes right in , spreading into the healthy tissue below.

@milevskistefan Well, unfortunately, all my experience with this plant agrees with you both that this is a sunburn process and that some of those sunburn areas turned into rot.

I finally removed the cactus with the slimy base from its pot. It's very strange, but this particular pot with gritty mix has not been watered in many weeks and yet a skewer showed it had a lot of moisture down deep. I may have a drainage issue here.

The photos are not encouraging me. It looks like some kind of surface infection tore away a large surface area of the cactus, all below the soil line. The infection moved up and looks like it is about to consume a major limb on the euphorbia. This plant is trying to fight back but it is slowly succumbing.

Is there anything I could do here to try to encourage that surface area that rotted away to dry up and scab? Could I let the plant lay sideways on the soil and then put just the roots under the soil while it recovers? That would leave the rotted section exposed to the air where it at least has lower humidity to deal with. Whatever this infection is, it is very slow moving.

Please keep in mind that many of the ugly looking brown areas higher up on the plant appear to be a sunburn and are superficial. But as others here have pointed out, those sunburns can then become systemic infections. I moved the plant to a sheltered area so it is receiving indirect light only now.

Is there anything I can coat over the rotted area to fight the infection or promote healing?