That thou art blamed shall not be thy defect,
For slander's mark was ever yet the fair;
The ornament of beauty is suspect,
A crow that flies in heaven's sweetest air.
So thou be good, slander doth but approve
Thy worth the greater, being wooed of time;
For canker vice the sweetest buds doth love,
And thou present'st a pure unstained prime.
Thou hast passed by the ambush of young days
Either not assailed, or victor being charged;
Shakespeare turns the slander to which the youth has been subjected into a compliment: the youth’s “prime”, here expressed in the  metaphor  of a flower “bud,” is so exceptional (the “sweetest”) that it is especially vulnerable to the disease (“canker”) of “vice.” 
 The “canker” reference appears frequently in Shakespeare’s sonnets. Note  Sonnet 54 ,  Sonnet 70 ,  Sonnet 95 , and  Sonnet 99 . 
 Shakespeare’s most famous use of the bud imagery is in  Sonnet 18 (Shall I compare thee to a Summer’s day?) , 

Sonnet 70