From Ryzom Wiki
Rot is one of the 7 elemental damage types.
Rot damage is usually the third elemental spell a mage learns, often while still on Silan. It is generally the preferred type of damage against intelligent plants until the mage reaches level 125 and can learn the racial damage types (fire, poison, shockwave and electric). And even after that some player prefer it against plants.
Most intelligent plants have no protection against Rot magic; the only exceptions are those in the Prime Roots and those infected by the Goo. By contrast, all animals and all kitin have significant protection against Rot magic. All creatures in the Prime Roots have particularly high protection against Rot, and being infected by Goo also gives creatures high protection against Rot.
Protection from Rot
All player characters have a default 10% protection from Rot damage. You can protect yourself against Rot damage further by wearing jewelry that gives additional Rot protection. You can have at maximum 70% protection.
You can also increase your chances of resisting a Rot elemental spell by wearing jewelry that raises your Prime Roots resistance.
Non-magical Rot damage
Najabs have a spit attack which does Rot damage, but this attack itself is physical and not magical in nature. That means Rot protection will protect you against it, but Prime Roots resistance will not help you resist it.
Many bandits and tribes guards also have random elemental damage added to their melee and ranged attacks, including Rot. If you're hit with Rot damage that way, Rot protection will decrease that damage, but Prime Roots resistance will not help you resist it.