Old fruit trees

Some feeding advice please for some old neglected fruit trees.
I think they were planted around 30 years ago, been pruned regularly but probably only ever had a bag of manure round the trunks once in a blue moon.
The apple trees look reasonably healthy, but have had hardly any fruit on for at least the last 5 years.
The pears have sparse yellow leaves with brown patches (maybe scab?)
The plums have cropped really well for the last few years, but this year hardly anything on them.

Any advice appreciated on what to feed with, when and how often and whether the pear trees would ever be healthy enough to crop well after so many years of neglect.

Thanks in advance.

 

Cant help much I'm afraid, but I'm thinking the problems might be more of pruning regarding the apples and pears rather than feeding.
Pruning can be complicated and you need to know the variety.

I think plums can become biennial bearing if not treated correctly also.

I'm sure someone can put you on the right track.