'Sprayers' are infected ogryns who served in PDF agri-corps (helped farmers, guarded crops, etc.) and went through series of mutations, which made them into walking tanks armed with poison-throwers.
Originally their weapons were agricultural pesticide sprayers, but power of the warp changed them into powerful acid/poison-throwers, turning a tool into a deadly weapon.
Sprayer's blight grenades were portable soil fertilizers. Ogryn placed the lower end of those cylinders in the ground, pushed the button on the top of the device, and distribution system quickly fertilized adjacent territory. But after the mutation Sprayers use their twisted versions of the device to poison soil, killing and contaminating everything around them.
Fearsome blades attached to every poison-thrower are made of combine harvester's blades and coated with deadly toxins.