HelpfulGardener.com

Hi folks,

it did get close to zero at night here a few days ago - do any wise heads here know if that might be the cause of this? Or does this look like the effects of disease?

Any help gratefully received

Cactus is am amazing tough plant, it can survive 115 degrees full sun 16 hours very day with no rain for months and snow, ice, wind, cold, blizzard conditions for weeks. Some cactus can deal with cold weather better than overs and some cactus deal with hot dry weather than others. If your cactus has damage it will show up within a few days. Green parts turn brown, gets soft, then turns black. The black is like cancer it will spread if you do not cut it off with a knife. If you have to cut a cactus in half plant the pieces in soil they will grow a new cactus. You can plant cactus cuttings, right side up, upside down, horizontal they will all grow. Cactus will grow new roots any place soil toughs it. Too much water will likely kill your cactus before cold weather. Soil should be very well drained.

That's freeze damage. It shouldn't spread, but if it does, you must take action. The scar will never go away, but they should continue to grow normally.