scab is prevented by lowering the pH. Potato yield is increased by
high phosphorous. So why not try to add rock phosphate next year,
which will do both?
and the way the plants have developed, it sounds like your garden (or
the way you fertilize) is very N-rich. So just the rock phosphate next
year, perhaps a bit of sulphur to further lower the pH, and nothing
else, and see how it goes.


Thanks for the advice...

pc