hardship vs worm in the apple

hardship

noun
  • A burden, a source of difficulty that could impose a barrier. 

  • Difficulty or trouble; hard times. 

verb
  • To treat (a person) badly; to subject to hardships. 

worm in the apple

noun
  • A serious flaw in something otherwise good; a problem that ruins everything else. 

How often have the words hardship and worm in the apple occurred in a corpus of books? (source: Google Ngram Viewer )