I have a greengage growing espalier fashion. Copious fruits were produced this year (the first year it has properly fruited). But after a few weeks most of the fruits have turned a reddish colour, stopped growing and are slightly shriveled. They are very easily knocked off the branches. How can I prevent this happening next year?
This article outlines some common plum diseases: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm
If it is a rot disease, you need to get it identified so you can treat it. Please get in touch with the nearest botanical garden and the Master Gardener program; they can help you.
Greengages plums are a very special fruit. In doing some research, I found this very interesting article: http://www.nytimes.com/2004/09/01/dining/01GREE.html?pagewanted=print&position=&_r=0