Multiple symbionts: Interactions between mycorrhizal fungi and parasitic plants colonizing the same host

Past studies have shown that arbuscular mycorrhizal fungi (AMF) and herbivores or parasitic plants can interact through their shared plant hosts. We examined interactions between a holoparasitic plant (Cuscuta indecora), host plant (Iva frutescens), and AMF (likely Glomus sp.) from Texas salt marshes in greenhouse experiments. We attempted to manipulate AMF with the fungicide Rovral, and documented the consequent performance of the host plants and Cuscuta. We also manipulated the presence of Cuscuta and determined how Cuscuta affected AMF over time. Root samples of Iva were stained with Trypan Blue and analyzed microscopically for percent colonization with the gridline intersect method of McGonigle et al. (1990). AMF were abundant (present in over 50% of slide views) and displayed multiple colonization growth forms including vesicles, coils and arbuscules. Applying the fungicide Rovral had little effect on AMF colonization; however, it increased Iva aboveground biomass, perhaps by negatively affecting pathogenic fungi in soils. Rovral, therefore, also indirectly increased Cuscuta biomass. The presence of Cuscuta significantly decreased Iva biomass, but overall had weak effects on AMF colonization.