Potential of a coevolved rust fungus for the management of Himalayan balsam in the British Isles: first field releases

Abstract

In 2014, the rust fungus Puccinia komarovii var. glanduliferae, native to the foothills of the Himalayas from Kashmir to Western Nepal, became the first fungal agent to be released into Europe for the classical biological control of a non-native weed. The target, Impatiens glandulifera or Himalayan balsam, is a prolific invader of riparian habitats in Europe and North America. During the period 2015–2018, a strain of the rust from India was released at a total of 36 sites, in 17 counties in England and Wales. There was limited field infection in 2015 and inoculation experiments conducted under controlled conditions revealed significant variation in the susceptibility of plant populations to the rust, with some showing immunity. Subsequently, a second strain of the rust from Pakistan was released in 2017 and was found to infect a different cohort of Himalayan balsam populations. The rust mass production methodology, and field inoculation and monitoring protocol, are detailed, and plants were tested for susceptibility to both rust strains prior to field release. Levels of foliar infection at selected sites in 2017 and 2018 are presented, as well as seedling infection rates in the spring of 2019. The results show that the rust is able to overwinter and establish populations in stands of Himalayan balsam in England. The issues involved with measuring the impact of the biological control agent are discussed.