freqmod

cacti are mostly water, like most plants. there might be more fibre in taller cacti displacing water volume. it would make sense because a lighter, more reinforced structure would be less likely to fall over. short cacti dont really need to resist wind.

if it wasnt soft underneath, it looks like poisoning. ive had the same thing from just using too much product. a lot of products cause plants to sunburn easily. the data sheets often say never to spray during the day and the company has no liability if all the leaves drop off your plants just before flowering time. a stray drop of sulphur concentrate has done that to one of my cacti. most "dual action" miticide + fungicide products are just weakly fungicidal(guess why mites live in humid conditions and what they eat). truth is, mites are really hard to kill. some mites just dont respond to anything. anything that kills them tends to be extremely toxic to humans so eradicating mites centres around eradicating their food source.

they can be grown in zone 9-10 outdoors. you may be experiencing the effects of blight fungus. it's endemic and releases a sort of anti-anti-freeze to make the cells burst at higher temperatures so it can feed on the plant. it was first identified sometime around the 50s. they noticed some crops would wilt with frost while the same crop in nearby fields wouldnt. they noticed that the ones that did wilt under the same conditions had leaves colonised with a type of fungus. the older portions of cacti form a layer of wax to protect themselves from it which is why the tips die. most of the cheap agricultural sprays are ineffective at controlling fungus because there's always THAT GUY using a low concentration and/or buying in bulk and not cycling through different fungicides through the seasons creating tolerant strains. which is why I tend to look for something that's banned/controlled access schemes/difficult to use due to allowed residue limits in commercial crops.

soft rot is a real threat. it plagues my dragonfruit. I lose large sections every year to it. but that is just surface fungus/bacteria. the dry air inside would have allowed the exudate to dry faster. part of cacti self defense is that the sap is saturated with calcium salt. before it can manage to form a callous the sap dries forming a layer of salt which reduces the water availability, preventing surface fungus from growing. i found out the hard way, grafting inside, that they need sunlight even just for an hour to trigger callous formation because my rootstock wouldnt heal and end up forming a deep cup without it, the scion dying eventually. although it pains me to do it, with deep burrowing damage like in those pictures, I often cut a shallow downwards V into mine to allow drainage and excise the surface to the point where it cant pull together. most of the time there's a bug still in there keeping the wound open. the spots like the large one in the third picture are caused by a burrowing larva. I have to use pesticides because I live near a national forest. the parks service releases cactoblastis regularly to control cacti infestation. we had a problem about 100 years ago when they tried to cultivate cochineal (unsuccessfully) in australia and the birds carried the prickly pear seeds. an area of farmland larger than the whole of the Netherlands was lost to dense sprawling opuntia. here's a picture of one of my beautiful baby girls, 2 years old.

this is an old post but i am answering it due to the ignorance of cactus care displayed. that cactus is perfectly healthy but over fertilised. trichocereus grows natively on rainy cold mountaintops. it is tolerant of heavy watering and cold. its why it's used as a rootstock. cacti dont split from over watering. they split from growing too fast. the green algae/moss growth on the soil indicates it is over fertilised. the particular pattern around which the crown has split indicates it is growing so aggressively that it's bursting out the top. give your cactus bursts of fetilizer in spring and summer. switch to a low nitrogen fertiliser before flowering and autumn. too much N keeps them soft and growing through winter. you dont want those delicious fresh soft tips during colder damp months for bugs to eat. those pictures do not show rot. that is normal surface fungus/bacterial growth that forms on any damaged cactus. I get it every time I graft. do not take a split cactus inside. direct sunlight triggers the formation of a callous. the spots are something nibbling it. birds and snails come to taste mine. the birds learn. the snails, slugs, grasshoppers die because I treat mine with a systemic pesticide, imidacloprid. the cactus soaks it up and anything that tries to eat it for the next six weeks cant navigate anymore so they starve. cacti fungus can be treated with zaleton.