Help! My Saguaro pup is spotting.

[Esoteric_Ocelot]

So I'm not sure if these spots were there when I bought him, but I have a feeling they weren't. Here's the story. I brought him to Virginia from the Phoenix airport in Arizona about 10 days ago and couple days later potted him in a mixture of 45% nutrient garden soil, 45% medium coarse sand, and 10% peat. I've given him only about 4 tablespoons total of distilled water and he was placed in a south facing window behind a layer of light vine foliage for some shade. I think I'll move him to a clay pot soon as I've heard it helps remove moisture from the soil better than the plastic pot he's in now. The rocks on top came with the pup and are very light so I assume it's perlite. He has a next door neighbor who is an Echinopsis and has been growing beautifully for about 7 years, but I believe the South American one may be more suited to acclimate to my environment than a desert cactus. Any help is appreciated! The black spot is expanding rapidly, about 1mm per day so I might slice the top off soon and hope for new clean pups to surface, but doubtful that will happen if his roots haven't taken in the new soil yet. I've read advice saying that sulfate fungicides might slow the growth but honestly I wouldn't mind a little cactus surgery if there's a chance new pups might sprout. Thanks in advance!

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[hegar]

I am sure, you wonder, why nobody has yet responded. Most likely, it is, because everybody who has looked at your cactus image is scratching her/his head and cannot give you a diagnosis. I also am at a loss and, whatever I say, I may be wrong, First, I do not like to hear, that the spots are increasing in size. That would lead me to the conclusion, that an infective agent may be involved, perhaps a bacterium or a fungus. On the image I only see one darkened area of medium size around one of the areoles, and another, reduced one, a little higher to the left. If a fungus was involved, except for a wilt or rot organism, by now you would most likely see some kind of growth on the largest of the lesions. It could be a whitish fibrous growth or some tiny bumps within the affected area, most likely in the center. If it were a rot, the tissue near the infected area would be softer than the rest.
A rot also spreads rather quickly and for most cacti it is a death sentence, especially if they are still small.
If you are lucky, then perhaps the discoloration is due to an injury the cactus suffered when being moved from Arizona.
How is the plant doing now? Has anything changed for the worse?

Harald

[Esoteric_Ocelot]

So I posted some new pictures, but even with my eyes it's difficult to see what's going on since the pup is so small (only 2.5 cm tall). The biggest spot did stop growing for a while just after my first post so I thought we were in the clear, but it just started expanding again a few days ago. The outer area of the spot is now a yellowish color. I took a closer look at the roots and noticed a fine white powder, but it didn't look like any type of fungus and could easily be dust from the perlite that it was packaged in. I poked the end of a root with a toothpick to check its health, and it basically crumbled without any effort. There's also a small white spot of dust on the side about halfway down that can be barely seen in the top picture. It appears to be on the surface and isn't increasing in size so I assume it's perlite dust from when he was repotted. Anyways, I took him out of his composty soil and cleaned off his roots with an organic fungicide. The largest root appears to still be somewhat healthy and is bendy and doesn't show any signs of fungus or rot. The white dust on the base washed off with ease. I couldn't find any signs of new root growth. So his new pot mix is basically vermiculite with a little peat moss to encourage root growth. I was planning on moving him to a soil mix with better aeration once the roots start to grow. I'm wondering if I should use any type of root hormone or water him at all. I've done hours of research on cactus illnesses, and I'm honestly stumped myself. The black spot is firm and not squishy at all, and I don't see any signs of fungus growth unless its very tiny and expanding incredibly slow. I keep my air conditioner running to dry the room out, but he's in a sunny window sill which hits a minimum of about 75 Fahrenheit during the middle of the day, and he was also in an air conditioned store when I got him so I'm not sure if its temperature shock. My last resort is just to cut him in half below the lowest infection site, and hope that some pups form on top.

[hegar]

I am sorry to see, that your baby Saguaro cactus looks worse now than a few days ago. It may have developed an internal rot. However, then the area around the discolored spot should be softer than the surrounding tissue.
With a plant, only 1 inch tall, it is very difficult to produce offsets, if the top is cut off. You could go ahead and try it, but do not expect any success.
The tiny cactus may already be too badly damaged to be saved. If you were to cut the discolored area off, you would see for sure, if the inside of the plant does look normal or if it is diseased and close to collapse. You can also go ahead and just cut off the areole with the major lesion. Then you could more successfully examine it for insect infestation or a pathogen.
I also realize, that a lot of times a digital image will show a more dramatic appearance of lesions present, than if you had the cactus right in front of you.
I just wish your plant were bigger. Then you would have more choices of what you could do.

Harald