Does a Spores druid's extra damage from Symbiotic Entity apply to a creature hit by the secondary damage of Green-Flame Blade?

Question

For a Circle of Spores druid, they are able to add 1d6 Poison Damage to their melee weapon attacks while their Symbiotic Entity feature is active.

At 2nd level, you gain the ability to channel magic into your spores. [...] While this feature is active, you gain the following benefits:

• [...]
• Your melee weapon attacks deal an extra 1d6 poison damage to any target they hit.

These benefits last for 10 minutes, until you lose all these temporary hit points, or until you use your Wild Shape again.

—Circle of Spores, Guildmaster's Guide to Ravnica, pg. 27

When a [melee weapon] attack from Green-Flame Blade hits a target, the spell causes an adjacent creature to also take damage:

[...] On a hit, the target suffers the attack's normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.

—Green-Flame Blade, Sword Coast Adventurer's Guide, pg. 143

For the purposes of Symbiotic Entity (or any other feature that adds damage to "any targets hit by melee weapon attacks"), does the extra damage (on the second target) associated with Green-Flame Blade count as being hit by that damage, making them eligible for the extra poison damage from Symbiotic Entity? Or does being "hit" by an effect necessarily involve an attack roll?

Answer 1

There are few reasons the poison damage probably does not apply to the splash damage target

1. Symbiotic Entity requires you to actually hit your target.

   The Symbiotic Entity feature states:

   Your melee weapon attacks deal an extra 1d6 poison damage to any target they hit...

   The books use the term "hit" not as its standard English meaning but to describe when an attack roll exceeds or equals the AC of a target creature. The secondary effect of green-flame blade does not involve making an attack, and so you have not actually hit this second target.

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2. The secondary damage of green-flame blade likely is not part of the actual melee weapon attack:

   The Symbiotic Entity feature states:

   Your melee weapon attacks deal an extra 1d6 poison damage to any target they hit...

   One way we could hope to know an answer here is by looking at the section on "Critical Hits" which states:

   When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together.

   We know that critical hits only modify the attack's damage so if a critical hit doubled green-flame blade's secondary damage, that damage would have to be considered part of attack's damage and thus part of the melee weapon attack required by Symbiotic Entity. Unfortunately this is not very clear...

   One thing we do know is that damage gated by a saving throw does not double when you score a critical hit. This is supported by this Q/A ("Is Ice Knife's explosion damage affected by rolling a critical?") and also this Q/A ("How does extra damage work for critical hits?"). This is also supported by lead game designer, Jeremy Crawford, though his idea are not official rulings:

   Q. DMG Poisons: If you crit with a poison coated weapon, do you double poison dice because crit, or not because saving throw?
   A. The intent is no. The saving throw, not the attack, determines whether the poison takes effect after a hit.

   However, with green-flame blade there is no save that is gating the additional damage, so perhaps critical hits would double its damage like they do with sneak attack. Crawford has also had input on this specific question stating:

   Q. If you critically hit with the attack part of Green Flame Blade, and are greater than level 5, do you roll extra dmg against the 2nd target?
   A. The splash damage of green-flame blade isn't affected by the attack critting. Think of the attack as process X & the splash damage as Y.

   Thus we at least know the intent is that green-flame blade will not double it's additional die when critting meaning it does not count as part of the attack's damage. Because of this we can conclude that it is not part of the melee weapon attack and thus cannot cause the additional 1d6 damage from Symbiotic Entity.

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3. Symbiotic Entity only effect a target of a melee weapon attack not all of its targets, plural.

   The Symbiotic Entity feature states:

   Your melee weapon attacks deal an extra 1d6 poison damage to any target they hit...

   Because this feature only applies to a singular target which we hit, it cannot apply to more than one person affected by our attack. This idea also would allow us to rule that critically hitting with green-flame blade does not double the additional damage against the second target because critical hits have the same wording, using the singular form of "target".

A note: This does of course require that we assume the phrase "any target" is, in fact, singular. As user @thedarkwanderer has pointed out this phrase could be read as already being plural in which case my argument for point 3 would not apply. There's ambiguity in the meaning as the word "any" in English - It can be used as the plural over the alternative options of "each/every".

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A small note on the exceptional state of green-flame blade:
This spell is rather exceptional; you make only one attack but end up affecting multiple creatures. The Sage Advice Compendium has said the following regarding the Ranger's Whirlwind Attack and Volley features:

Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls...

These two features and green-flame blade are the only cases I know of where a single attack affects multiple creatures and so they probably have some odd interactions such as the one asked about in this Q/A ("How does the Sharpshooter feat interact with the Hunter ranger's Volley feature?")

Answer 2

Any additional attack damage is included in "the attack's normal effects"

By my reading, when Green Flame Blade refers to "the attack's normal effects", it means whatever effect the attack would have had without the spell. So, while the druid's Halo of Spores is active, the attack's normal effects include the additional 1d6 poison damage. Since none of the attack's normal effects apply to the secondary target, and the secondary target is not targeted by an attack, the poison damage from Halo of Spores does not apply to that target. To put it another way, when you use Green-Flame Blade, the spell hits both targets, but the weapon used in the spell only hits the first target.

Similarly, if some ability gave your weapon attacks an additional effect such as inflicting a condition on the target or pushing it, this would only apply to the target of the attack, not the secondary target of the spell.