If the rice harvest drops, there'll be serious trouble.
Literal
Rice's harvest [subject-が] dropped-[if-たら] serious-thing become [warning-ぞ].
大変なことになる means 'it'll become a serious situation.' ぞ is a masculine assertive particle, stronger than よ — adding urgency to the warning. The ~たら conditional sets up 'if the harvest drops.' A sentence with real stakes.