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 communiti es. There have been reports of negative effects of phosphite on arbuscular mycorrhiza (AM), so there are concerns that it may have a deleterious effec t on other mycorrhizal fungi. Two glasshouse experiments were undertaken to determine the impact of phosphite on eucalypt-associated ectomycorrhizal f ungi. In the first experiment, non-mycorrhizal seedlings of Eucalyptus marg inata, Eucalyptus globulus and Agonis flexuosa were sprayed to runoff with several concentrations of phosphite, and then planted into soil naturally i nfested with early colonising mycorrhizal species. Assessments were made of percentage of roots infected with mycorrhizal fungi. There was no signific ant 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, Scler oderma and Descolea were treated with different levels of phosphite and inf ection of new roots by ectomycorrhizal fungi was assessed. There was no sig nificant effect on ectomycorrhizal formation when phosphite was applied at the recommended rate (5 g L (1)), while at 10 g L (1) phosphite significant ly decreased infection by Descolea.