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A rust fungus, Puccinia komarovii var. glanduliferae, is being used as a biological control for invasive Himalayan balsam by infecting its leaves and stems, reducing its ability to photosynthesize and compete with native plants, and even killing seedlings in spring, offering a self-sustaining method to control the weed's spread by making it less dominant in ecosystems. 

How the Fungus Works

1. Seedling Infection (Spring): The rust fungus attacks young seedlings, often killing them before they can establish, notes the North Wales Wildlife Trust and Natural Resources Wales.

2. Mature Plant Infection (Summer): It forms rust-colored pustules on the leaves of mature plants, which hinders photosynthesis and weakens the plant, making it less competitive, explains CABI.org and CABI.org.

3. Overwintering & Spread: The fungus survives winter in decaying balsam leaf litter and releases spores in spring to start the cycle again, spreading naturally by wind, water, and animals, say CABI.org and CABI.org. 

Key Benefits

• Host-Specific: Extensive testing confirms the fungus only affects Himalayan balsam, posing no threat to native plants, according to CABI.org and CABI.org.

• Self-Sustaining: Once established, it spreads naturally, reducing reliance on manual removal, notes CABI.org.

• Eco-Friendly: It aims to reduce balsam dominance, allowing native flora to recolonize riverbanks and restoring biodiversity, says CABI.org and The Ecologist. 

Implementation

• Scientists from organizations like CABI.org and CABI.org www.cabi.org

Scientists are using a specialized 

rust fungus (Puccinia komarovii var. glanduliferae) as a biological control agent to combat Himalayan balsam across the UK. This fungus, native to the plant's original range in the Himalayas, was first approved for release in England and Wales in 2014 after years of rigorous safety testing. 

As of 2026, here is how the fungus is being utilized:

1. Mechanisms of Attack

The rust fungus employs a "two-pronged attack" to suppress the plant's growth and spread: 

• Seedling Infection: In the spring, the fungus infects and often kills young Himalayan balsam seedlings as they emerge.

• Leaf Infection: Throughout the growing season, it infects the leaves of mature plants, forming brown pustules that disrupt photosynthesis. This stunts the plant's growth, making it less competitive against native species and reducing its overall height and seed production. 

2. Application and Spread

• Inoculation: Scientists mix rust spores with water and spray them onto the underside of leaves, which are more susceptible to infection than the tops.

• Natural Dispersal: Once established at a site, the fungus reproduces via spores that are naturally spread to adjacent plants and distant populations by wind, water, and animals.

• Overwintering: The fungus persists in decaying leaf litter over the winter, reactivating in the spring to infect the next generation of seedlings. 

3. Current Status and Trials (2024–2026)

• Recent Successes: Trials in Yorkshire and Hertfordshire during 2024 and 2025 have shown "early success," with the fungus successfully overwintering and naturally re-infecting new seedlings in the 2025 growing season.

• Strain Matching: Because different Himalayan balsam populations in the UK have different genetic origins, researchers have released multiple strains of the fungus—including ones from India and Pakistan—to ensure a better match for local plant biotypes.

• Ongoing Monitoring: Organizations like CABI and various Wildlife Trusts (e.g., North Wales, Yorkshire) continue to monitor 2026 field sites to determine how environmental factors like shade and humidity affect the fungus's ability to suppress the weed long-term.