Phosphite and mycorrhizal formation in seedlings of three Australian Myrtaceae

Currently in Western Australia, phosphite is being used to contain the root and collar rot pathogen, Phytophthora cinnamomi, in native plant communities. There have been reports of negative effects of phosphite on arbuscular mycorrhiza (AM), so there are concerns that it may have a deleterious effect on other mycorrhizal fungi. Two glasshouse experiments were undertaken to determine the impact of phosphite on eucalypt-associated ectomycorrhizal fungi. In the first experiment, non-mycorrhizal seedlings of Eucalyptus marginata, Eucalyptus globulus and Agonis flexuosa were sprayed to runoff with several concentrations of phosphite, and then planted into soil naturally infested with early colonising mycorrhizal species. Assessments were made of percentage of roots infected with mycorrhizal fungi. There was no significant effect on ectomycorrhizal formation but there was a four-fold increase in AM colonisation of A. flexuosa roots with phosphite application. In the second experiment, E. globulus seedlings mycorrhizal with Pisolithus, Scleroderma and Descolea were treated with different levels of phosphite and infection of new roots by ectomycorrhizal fungi was assessed. There was no significant effect on ectomycorrhizal formation when phosphite was applied at the recommended rate (5 g L–1), while at 10 g L–1 phosphite significantly decreased infection by Descolea.