Scientists are on the hunt for a biological control agent for New Zealand’s worst pastoral weed, the Californian Thistle.

In particular, they are after root pathogens that could potentially be used to curb the problematic plant. A pathogen is something that causes disease.

Previous research has shown that the Californian Thistle propagates (reproduces) via its roots, and not its seeds, weed ecologist Dr Graeme Bourdot says.

“The roots have buds that act as a survival system that gets it through the winter in established pastures.”

Cattle and sheep will not usually eat the thistle, which takes up large amounts of land that could be used more productively. An additional problem is that the thistle causes sheep to get scabby mouth when they try to graze around it.

It costs farmers hundreds of millions of dollars every year in herbicides to control it. Some of the herbicides also damage valuable crops such as clover.

Researchers hope the field survey team who will be collecting thistle samples from farms in the coming months may find a pathogen that proves useful.

“We are hopeful that one or more of the root pathogens may be the one we can study to understand how it works and develop it so it can be reapplied as a biological control agent,” Dr Bourdot says.