From Lexicanum

Antiplant grenades are specially designed for removing foliage and natural material. They release viral gasses which cause plants to rot, although many only work on plants with a certain genetic structure and so the correct grenade type is required before every battle. They are especially useful for clearing large open areas to either deprive the enemy of cover or provide camp sites or landing strips.¹

Sources

• ¹:Warhammer 40,000: Wargear (2nd Edition)