By Homebird
United Kingdom 
Strange shaped pods on the Greengage plum tree. It's never produced these before. They are like seed pods with a small seed in - not a stone. The tree is fruiting well with normal plums. Does anyone know why it's doing this. The photo shows the seed pod and a normal plum with stone.
6 Jun, 2010
Answers
I think the two halves on the left are what are called "Pocket Plum" and are caused by one of the gall making fungi. It's specific to plums so it won't spread to other plants, only plums. It is related to other species such as Peach leaf curl fungus and also Azalea leaf gall fungus, each species attacking their respective specific hosts only.
6 Jun, 2010
Just looked for a suitable image and found this:
http://www.liveforgardening.com/questionsandanswers/weeds/138803/pocket_plum_problems.html
6 Jun, 2010
I dismissed a gall at first , wish I had looked now. :o) I find the biology of galls really ineteresting too.
glad you got a correct diagnosis.
7 Jun, 2010
it is a 'false' fruit. sometimes a flower is pollinated but then fails to fertilize and develop a true seed. the fleshy part forms from another part of the ovary that is fertilized by the pollen grain tube nucleus.
6 Jun, 2010