Antagonistic interactions between native fungi of Minnesota and the root rot pathogen Heterobasidion irregulare

Abstract

The forest pathogen, Heterobasidion irregulare, is a serious threat to conifers in North America including Minnesota. Fungi native to Minnesota were isolated and tested in laboratory and field assays to evaluate their antagonism towards H. irregulare. One management strategy for plant pathogens, and especially H. irregulare, is to use fungi as biological control agents. A successful biological control agent used to manage root rot disease caused by H. irregulare is the fungus Phlebiopsis gigantea. The goal of this research was to screen different native fungi, including P. gigantea, against H. irregulare and examine and quantify their interactions in vitro and ex vitro. A set of four different antagonism assays were conducted. These assays served as a screening process involving both the laboratory and the field. Interactions were first examined with dual inoculation studies on media and wood discs of red pine (Pinus resinosa). These assays demonstrated strong inhibition and limited growth of H. irregulare by select fungi, including Phanerochaete livescens and P. gigantea. Another assay involved using soil microcosms and wood wedges of red pine. This allowed for a different examination, as wood wedges were inoculated with a candidate antagonistic fungus and placed in soil microcosms with H. irregulare. The opposite interaction was also examined with wedges inoculated with H. irregulare and then placed in soil microcosms containing different candidate fungi. In the field, large wood discs were placed around stumps and inoculated with candidate fungi in a red pine plantation infected with H. irregulare. Certain fungi performed well in different assays, but across all assays, P. gigantea performed the best. The antagonism of P. gigantea was most noticeable on wood discs and wood wedges used in vitro, as H. irregulare was not able to be reisolated from these substrates. Overall, these results provide more information on the fungi that appear to be acting as antagonists in forests to prevent H. irregulare from colonizing and provide new information on potential candidate fungi that could be used as a new biological control agent.