In fact, a terrible year for plums.
I have a client with several plum trees in their walled garden, and normally they supply me with plenty of fruit (it's all right, I have permission to pick!) but this year I have been woefully plum-free.
This has been the problem:
Plenty of fruit set, plenty of fruit growing, but then they all succumbed to the festering brown pustule rot. I believe it is properly called "Brown Rot", but festering brown pustule rot gives you a much better idea of the problem.
First they go brown and puffy, then the whitish pustules appear, then they shrivel up into mummified prunes. Having, of course, spread the disease all around themselves.
It seems to affect plums, apples and pears, and there is no treatment other than constant vigilance, and removing any infected fruit as soon as you see them. Oh, and don't compost them, whatever you do - otherwise you'll just spread the infection all round the garden. Burn or bin them.
Clearly, I didn't get to these ones in time - but then, I am only there once a week, and I have a large area of garden to patrol, not just the fruit walls. Unfortunately this is one of those jobs where the responsibility has to hang with the owner, who is able to check on a daily basis.
Personally, I think we can help the plants by keeping the branches uncongested, which I achieve by pruning them gently pretty much every year - I take out crossing branches, and any that are too close together, aiming to get a nice even framework. I am also scrupulous at removing any rotters as I find them, and in clearing up the mess underneath the fruit trees each week. Even ordinary fallen fruit gets cleared away - it just encourages the wasps etc, and no, I don't believe that wasps are sensible enough to target the sacrificial fallen fruits! I prefer to clear them all away as often as possible.
Finally, it's always worth a quick check in autumn, to gently remove any mummified fruits that were missed during the summer.
It's such a shame, I love plums... I assume it's been particularly bad this year due to the general coolness of the weather, and the amount of rain we have had, as wet weather helps the spores to spread, and helps the fungus to multiply once it has spread.
So, not much we can do about this year, other than to keep our fingers crossed for better weather next year!