We have a Victoria plum tree that we inherited on our allotment about 4 years ago. It was only small then so we don't know exactly how old it is. We have had the usual good year bad year as far as volume of fruit goes, but never this problem. A lot of the fruits (and there are loads this year) are turning purple/black and shrivelling up before they ripen. Is it because there are too many on the tree for it to cope with?