Forum:MST versus traps

Scenario:

Opponent plays a set Dust tornado.

Counter: MST?

I know you cant negate the activation of a spell/trap card with MST, but can you destroy it since it gets activated from a set position on the field?

On a side note, if you are going to negate teh activation of a spell card do you have to wait for the opponent to declare a target? Or just negate the activation of the spell before knowing which card its goign to affect?

thx