Rot without ANY watering??

[ltrabit]

Hi everyone,
I recently ordered three cacti from a mail order website located in California. I don't normally order plants online, but because the retailer where I bought my older ones stopped selling them, I figured it would be worth a try. So basically I received the three, they all looked perfectly healthy and I repotted them into larger pots.
I didn't water them because the soil was fairly moist, so for the about a month I let them be. I started to notice one was rising on one side and shrinking on the other side, to which I assumed was elongated growth from insufficient sunlight since they were greenhouse grown prior to their arrival in Rhode Island. Today I picked up the pot to check if something beneath the plant was causing it to get taller on one side and when I went to touch it, it was so soft I already knew it was rotted. It then split apart from me gently lifting it up.
I simply don't understand how a plant that I had not watered rotted. I purposely did not water it because I had two die from rot due to over-watering in my first group. What could've caused this?? Could humidity in the air make it rot? I live several streets away from the beach and it is always extremely humid here, especially during the summer. But if humidity could do this, how come all the rest have survived several summers and are fine? Could there be any other possible causes for this to happen?
I'm really disappointed that this beautiful plant died and I do even know what I did incorrectly.. Attached are the photos of the poor plant.

[NY-Desert]

Wow ltrabit, that's a shame. I wonder if it was damaged during shipment allowing bacteria to take hold?

[miss_taken_identity]

Sometimes when a plant is moved from an environment it isn't used to and is put into a new location and environment it can cause horrible effects - that's why normally suppliers will recommend you slowly acclimate your plants to an outdoor environment or "full sun environment" however if the greenhouse was kept with little humidity and then the plant was shocked with large amounts of humidity it wouldn't surprise me if that's what's caused the rot on all the new plants but not on the ones that's have been there for years and years.

[greenknight]

More likely the beginnings of the rot were already there when it was shipped, just not noticeable yet. You said you repotted them into larger pots - so they were not bare root? I think bare root is actually better - not only is the shipping generally cheaper, but you can more easily inspect the roots. You should thoroughly inspect new cacti you receive, cleaning the old potting mix off the roots if they're already potted. You'll likely do some damage in the process, so leave them bare root for a week or so for it to heal. Then pot them up, and wait a week or two more before watering.

In a very humid climate, you should use a very free-draining mix that's mostly mineral grit - it looks like the stuff you're using has too much peat in it. This article explains it - http://www.cactiguide.com/article/?article=article8.php" onclick="window.open(this.href);return false; . You don't have to use the exact mix described there, but it gives you an idea of the type of mineral grit I mean. Many growers use straight mineral mixes with excellent results, supplying all the nutrients as fertilizer.