Does a spell caster need to hold onto their arcane focus when concentrating on a spell?

Question

The long-short of it is that I want to use the maneuver Disarming Attack to make a mage drop their arcane focus. While I know this will delay their spell casting (as it requires an action to pick up the focus), and it might even ruin a readied spell, I was wondering if losing the focus would break the concentration of a spell, or if it is only required to activate the spell in the first place?

Answer 1

No, losing your Arcane Focus doesn't break your concentration on a spell

Thomas has already provided an answer detailing all the effects that can break concentration on a spell and as stated there, "losing your Arcane Focus" is not on the list of them, and therefore doesn't break concentration. My answer attempts to look at the question from a different angle, as well as expand a bit on the rules regarding the use of Arcane Focuses, since it seems like you may have some misconceptions about how they work.

The spellcasting focus is mentioned only in the basic rules on Spellcasting in the context of material components, and the rule states:

Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in “Equipment”) in place of the components specified for a spell. (…)

Arcane Focus allows you to replace the material components needed to cast if the spell requires them. That is the only thing it does. The components' requirements only matter at the time the spell is cast:

A spell's components are the physical requirements you must meet in order to cast it.

They do not interact with the concentration needed to maintain the spell in any way. Concentrating on a spell isn't "casting" it; you have already cast it, now you are only sustaining it. You do not require any components at this point, therefore, you don't require the Arcane Focus either.

Similarly, dropping your Arcane Focus will not ruin a readied spell since the spell has already been cast, you are just waiting to release it:

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs.

Note that if the spell doesn't have any material requirements, you don't need to be be holding your Arcane Focus at all to cast it, so not having it doesn't prevent all spellcasting; the mage is still free to cast all his spells that don't have material components.

Answer 2

Arcane focuses are mentioned in the rules for casting a spell, and never in the rules for concentration.

The rules for concentration state:

The following factors can break concentration:

• Casting another spell that requires concentration. You lose concentration on a spell if you cast another spell that requires concentration. You can’t concentrate on two spells at once.
• Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon’s breath, you make a separate saving throw for each source of damage.
• Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die.

No mention here of needing to hold onto your focus, so it isn’t necessary for maintaining concentration