Why are my plums rotting?

There are still plenty, and I'm grateful, but so many are rotting away like in the photo. What can I do to stop this happening? What causes it?

Thanks!

I can use the rotten plums for an expensive wine? Don't tempt me WG! I'll go to wilkos, buy plastic demijohns etc if I can - seriously! I know loads of people who like sweet wine....


So, this botrytis. Is it fungal? viral? bacterial? How do I treat the tree?

My vote goes for brown rot. Pick damaged fruit as soon as it shows and bin them. Spores get in via damage caused by insects and birds.