European servers

[saeedaisspecial]

"I am right and you're wrong" is all you're saying.

Bane/Fester right as the debuff icon is applied and the game may still fail to register it.

Also buy a dictionary and find the definition of instant.

As soon as the icon is visible on the mob, you can Bane/Fester for the effects. Yes, it says instant but you still have to wait for the animation to complete. Like all abilities in game.
Also, go ahead and read the descriptions of abilities such as Bio, and Miasma II. The cast time is stated to be instant, the effects are not (as they are subject to the time it takes for the animation to complete).
Please, go and do a little research before lashing out.