Q Why did good plums go bad?

AThis is brown rot, a common problem with plums and other fruits. Birds and wasps peck holes in the skin allowing the fungus to get in, whence it spreads rapidly from fruit to fruit. This is why it’s a good idea to thin out the fruit if there’s a heavy crop. Thinning also ensures the fruits are a good size and that the weight of the crop doesn’t damage the tree and weaken it to the point where next year’s crop is poor. Remove infected fruit as soon as you spot them in autumn because the ‘mummified’ plums will make the branches die back, leading to more infection next year.