Caustic Arrow

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Damage
The base damage per second dealt by a given application of poison equals 10% of the combined physical and chaos damage dealt by the hit which applied that particular instance of poison.

As a damage over time effect, Poison's damage can be affected positively or negatively by some damage stats. Notably, stats modifying spell damage and attack damage (also often referred to as damage with x weapon) do not apply to Poison as Poison is not a spell or attack. The following stats will directly affect the damage of Poison; note that some of these stats apply only to the Poison whereas some may also apply to the skill that triggered it:

Damage over Time
Poison Damage
Damage
Chaos Damage
Area Damage if applied by an Area skill
Projectile Damage if applied by a Projectile skill
Trap Damage if applied by a Trap
Mine Damage if applied by a Mine
Totem Damage if applied by a Totem
Minion Damage if applied by a Minion

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dudiobugtron wrote:

The main arrow is a bow attack, and so will be increased by projectile damage nodes, and also bow nodes.

The cloud is not an attack (it doesn't use your weapon's stats), so it doesn't count as 'bow' damage. But it does count as 'projectile' damage for some reason. It will also be affected by 'AoE' and 'Chaos' damage modifiers.

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dudiobugtron wrote:

Concentrated effect should increase the damage per second of the cloud. If it's not showing the the tooltip, then the tooltip is bugged yeah. Most tooltips are not that reliable though anyway.