Evaluation of the effects of the insect pathogenic fungus Metarhizium anisopliae on microbial populations of disparate plant growing media

Amending plant growing media with biocontrol agents is becoming an increasingly popular strategy for protecting plants against insect pests and diseases. Although this practice offers potential for pest and disease control, very little is known about the influence of introduced microbes on the indigenous microbial communities and vice versa. This study examines fungal and bacterial community structures of diverse horticultural growing media and evaluates their interactions with a plant Euonymus fortunei and/or the insect pathogenic fungus Metarhizium anisopliae. Complementary physiological (Biolog™) and molecular (automated ribosomal intergenic space analysis – ARISA) techniques were applied to analyse community structure and diversity over 1 yr. These studies clearly showed distinct microbial communities in different plant growing media. Irrespective of media or any treatment, initial microbial diversity was very low but increased significantly over 1 yr. A succession of new fungal and bacterial species was observed, with species originally present being displaced. This succession led to the establishment of distinct microbial community structures for each medium over 6–12 months. ARISA studies showed that bacterial and fungal community profiles were influenced by the presence of plant and M. anisopliae, but in complex ways depending on the medium. Certain microbes were specifically associated or inhibited by the presence of M. anisopliae and/or E. fortunei. Physiological studies, however, showed that neither M. anisopliae nor plant had any significant effect on the communities' ability to decomposition a batch of carbon sources. Although diverse microbial communities were observed in different media, none of these communities influenced the efficacy of M. anisopliae.