My hubby and I have just noticed these strange pods on a tree in our back garden. On closer inspection many of the tree leaves are small and withered, and the ends of each twig of leaves is covered in greenfly. The tree is nestled among beech and holly trees, and is approx. 16 feet tall. This is the first in 12 years in this property that we have noticed the pods. They seem to be early-ish stage, hollow pods. We've just tried identifying the tree, but so far with no luck. Please help, if you can!
You have a plum or gage which has pocket plum gall, Taphrina pruni
(I suspect following that link will answer most of your subsequent questions.)
The galls are not fruit.
But the fruits are forming - you can see some in your second photo (left-centre of the top-right quarter).
Fungal pathogens are often difficult to treat effectively because there will almost inevitably be a larger area that is infected. ...So if you treat your tree successfully, it will get re-infected quickly (depending on the life-cycle of the fungus). If you're willing to re-treat the tree annually or more frequently (and if you plan to eat the fruit, you'd have to be OK with knowing the tree had been chemically treated), you could do so.
But it seems the tree is going to have some tough competition in its growth (I notice a beech very close-by, for instance), so it might be at risk of dying from competition (think roots and shade, not just aerial space). So I would be tempted to ignore the disease (let nature take its course), or remove the tree (the sooner you do it, the sooner adjacent plants can fill the space).
At a pinch, you could consider replanting it elsewhere (with less competition), but this is not a great time of year to move a deciduous tree. You'd have to prune the top back a lot (partly depending on how much root you managed to save), stake it, water it carefully, etc. And there'd still be a fair chance it might not make it.
Great advice, thankyou very much. I have wondered for a long time what this tree was as during February/early March it is a favourite for Bullfinches, who devour the buds. This probably explains why we've never seen fruit, and I wonder if it is them who have passed the fungus to this particular tree, as there are very few fruit trees in this area. I think we will probably let nature take it's course, but cut the overhanging branches back, as watching the Bullfinches from our kitchen while washing the dishes makes a gloomy Feb/March much more interesting! Thanks again :)