In September 2023, the North Wales Wildlife Trust’s Invasive Non-Native Species (INNS) team set out on an exciting mission along the River Dee. As part of the Preventing Aliens Taking Hold (PATH) Project, we were scouting for suitable sites to release a natural enemy of Himalayan balsam—a plant that’s been wreaking havoc on our countryside.

The star of this story? Himalayan balsam rust fungus—a biocontrol agent that has co-evolved with balsam in its native range. After rigorous safety testing by CABI (Centre for Agriculture and Bioscience International), this rust fungus was approved for release in the UK. It offers a promising, nature-based solution to reduce the ecological damage caused by balsam invasions. 

How it works

In spring, rust fungus infects vulnerable seedlings, often killing them. It then spreads to mature plants, forming rust-coloured pustules on leaves that disrupt photosynthesis and reduce competitiveness.

The fungus reproduces via spores, easily spread by wind, water, or animals. Once established, it can quickly colonize adjacent balsam plants, creating a ripple effect across the landscape.

As temperatures drop, the rust fungus overwinters in decaying leaf litter, which shelters it through winter. In spring, it reactivates, releasing spores to infect the next generation, continuing its cycle year after year.

Winter testing & summer inoculations

After a winter of lab testing, two sites emerged as strong candidates. CABI experts visited both locations to train our team on the inoculation protocol. Throughout summer 2024, both sites were inoculated multiple times, and we conducted regular surveys to monitor progress.

The results

Unfortunately, the rust fungus struggled to establish itself at both sites, likely due to the unseasonably hot weather early in 2025, conditions that may have disrupted both the balsam’s growth and the fungus’s development. With this in mind, we concentrated all remaining vials at the site that had shown some promise.