My grandpa has a cactus that is turning light blue at the top. The effected area is wrinkly but cannot get fingers close enough to tell if it's mushy. It is one of four cacti in the pot that are all the same type, but it is the only one effected. The plant is in the patio window. We live in WI but despite winter he keeps the house at 70°. I dont believe there is a draft but they are right next to the heater vent and have been in the same spot for years. He waters them once a week, when I am there the soil seems very dry. The soil is old and we are planning on re-potting. I am wondering if there is something we should do differently when re-potting? Or if there is something we should be looking for, besides usual signs of rot? Is this the result of not having enough room to grow? Didn't think this made sense but I am not sure exactly what the problem is. It's my grandma's cactus and keeping it alive or salvaging what we can is very important so any advice is appreciated.
The top is dead. I would cut it off. It is difficult to say why it died. I have seen it happen to cacti with other problems (with the roots, for example). I have never seen a cactus sensitive to draft, so that is probably not the cause. Consider separating the plants. The big, fast-growing columnar cacti tend to outgrow their pots quickly.
