Arbuscular mycorrhizal fungi (AMF) have multiple functions in agroecosystems and affect
many processes below- and aboveground, including plant productivity. Mycorrhizal symbiosis is
not necessarily beneficial for the host plant and the growth response can be not only positive but
also neutral or negative. Among other factors, the responsiveness of plants to AMF depends on the
plant-fungus combination. To find out whether the AMF species or isolate is a decisive factor for
growth responses of weeds, 44 AMF isolates were tested in a pot experiment for their effects on three
agricultural weeds: Echinochloa crus-galli, Solanum nigrum and Papaver rhoeas. The 44 isolates cover
18 AMF species from 13 genera and all 5 orders of the Glomeromycota. The aboveground biomass of
the weeds was determined after different times of growth of each weed. In most cases, the effects of
AMF isolates on weed growth were negative or neutral. We conclude that some weed species do not
benefit from AMF in terms of growth. AMF species can even cause negative growth responses, an
effect that may be of practical interest for organic farming where the aim is to obtain a high diversity
and concentration of native AMF for the benefit of the cultivated crops without increasing the labor
for mechanical weeding.