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

Question

The druid spell Rusting Grasp (PH 273) says

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

and

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?

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*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.

Answer 1

The functional difference between the 1st-level Sor/Wiz spell chill touch [necro] (Player's Handbook 209) and the 4th-level druid spell rusting grasp [trans] (PH 273) is that the former allows 1 touch per caster level but the latter only 1 touch per round.

Touch attacks are made normally as part of an attack routine. Thus, after the first attack that's usually made when either spell is cast, touch attacks with the spell's charge are just otherwise normal attacks that can be made whenever the caster can make an attack (see this question). During a full attack, for instance, each attack that hits will expend a chill touch charge until all the chill touch charges are expended, but only the first attack that hits will expend a rusting grasp charge.

Further, both spells remain touch spells, so casting another spell ends either. (See Touch Spells and Holding the Charge (PH 176).)

What the rusting grasp spell doesn't explain is how, when, or if a rusting grasp spell charge renews. For playability, this DM would rule that a rusting grasp spell's charge is expended after a successful attack, but I'd also rule that, while the spell's duration continues, at the beginning of the caster's turn the caster gets a new rusting grasp spell charge, replacing the old, if any. I imagine managing the spell any other way would lead to excessive bookkeeping or even more complicated house rules.