We have a 50+ old plum tree in Auckland New Zealand, that fruits prolifically most years and we've just found major rot in one of the branches (see photo attached). The bark has fallen off and the wood underneath is soft and powdery. There are also signs of similar smaller areas on other branches. Can anybody offer any suggestions as to what this might be and the best way (if any) to treat it? Thanks Paul
Your tree is a very, very old lady - average productive life span for a plum tree is around 10-20 years, though it obviously varies between individuals and many go on for longer, getting more gnarly as they do, see here https://www.hunker.com/12272347/fruit-tree-life-expectancy But 50 years and still fruiting well is quite remarkable, although it is not uncommon for a fruit tree to produce a bumper crop or two when it's on its way out. I don't think there's anything you can do to improve the situation, because ultimately, all things must pass.
The biggest risk might actually be the danger from falling limbs, so just ensure it's safe - any large rotten branches are best removed, assuming you're not ready to take the whole tree out. You might want to push against the main trunk to see if it moves at all, as well as inspect nearer the base of the trunk for signs of problems. If it does move, or there's rot low down, it could pose a real risk to people or property, and it might be better to remove it immediately; having a tree surgeon or arborist inspect would be a good idea.