POISON SPORES Support Does Not Work for Caster Weapons

I've slotted Poison Spores Support on the new Entangle spell skill gem.

The support gem info says "trigger toxic pustule on hitting an enemy". The pustule detonates after a delay. The pustule explosion says "Attack Damage: 58%" (based upon level of Entangle skill gem).

In addition, the support states:

- detonates in 15 seconds
- detonates sooner when affected by poison
- gains poison damage per second automatically during overgrowth
- poisons enemy on detonation

This support does nothing when using a Wand or Staff -- which is crazy as it's a spell and you'd generally want to use a caster weapon to scale the damage.

I never see the mobs get poisons (their health bar never turns green).
I never see a graphical indication for pustule explosion.
I never see the mob health drop due to pustule explosion.