A semi-natural evaluation of the potential of the rust fungus Puccinia komarovii var. glanduliferae as a biocontrol agent of Impatiens glandulifera.
Himalayan balsam (Impatiens glandulifera) is one of the most prolific non-native species in Europe. Since 2014, the highly-specific rust fungus, Puccinia komarovii var. glanduliferae, has been released into Great Britain as a classical biological control agent for this invasive weed. Prior to its release, research focused on ensuring the safety of the pathogen, elucidating its life cycle and verifying its host-specificity. However, limited studies were conducted to determine the likely impact of the rust on its host. Due to difficulties in assessing field populations, unforeseen complexities in the plant-pathogen relationship and the requirement for long-term monitoring, inoculation experiments were conducted to evaluate the effect of the rust on both seedlings and mature plants of Himalayan balsam under semi-natural conditions. The impact of the rust was determined through measuring plant-growth parameters in addition to assessing reproductive output. The rust significantly increased seedling mortality, with up to 80% of seedlings dying as a result of rust infection, or indirectly through the colonisation of secondary pathogens. Mature plants infected with the rust produced fewer leaves with a decrease in total plant biomass. Reproductive output was negatively affected through a reduction in both flower and seed production, but with no observed effects on seed viability. This study demonstrates the high potential of the rust for controlling fully susceptible field populations of Himalayan balsam.