The leaven of the Pharisees is hypocrisy. Jesus Christ calls the Pharisees pretenders when they sought a sign from Him, though He preached daily.
Traits associated with hypocrisy includes lying, guile, defamation, malice and envy.
But in vain they do worship me, teaching for doctrines the commandments of men. So we should be on guard for not just false ideas, but 'false' people. The Pharisees also with the Sadducees came, and tempting desired him that he would shew them a sign from heaven. And he left them, and departed. And when his disciples were come to the other side, they had forgotten to take bread.