Is a Rusting Grasp touch attack a standard action and do you have to hold the charge for the duration of the spell?

The druid spell Rusting Grasp (PH 273) says

You may employ rusting grasp in combat with a successful melee touch attack.


The spell lasts for 1 round per level, and you can make one melee touch attack per round.

Most touch spells have one "charge", you cast them and then discharge them with a successful touch attack. There are also some spells which grant more than one charge (like Chill Touch (PH 209) which can be used "up to one time per level").

A touch spell that lasts for a fixed duration limiting the number of attacks to one per round seems to be very unusual.

My two questions are:

  • Can you make just a single melee touch attack per round – as a standard action? Or can you make more than one attack (if, for instance, the druid uses Wild Shape and has a couple of natural attacks) but only one of your attacks can deliver the spell?
  • Do you have to hold the charge* for the duration of the spell (and lose the spell if you cast another one) or does the spell simply last for one round/level giving you a "new charge" every round until it expires?

*Touch Spells and Holding the Charge: In most cases, if you don’t discharge a touch spell on the round you cast it, you can hold the charge (postpone the discharge of the spell) indefinitely. You can make touch attacks round after round. If you cast another spell, the touch spell dissipates.