Arbuscular Mycorrhizal Fungi: A Blessing or A Curse for Weed Management in Organic Olive Crops?

Arbuscular Mycorrhizal (AM) fungi form symbiotic associations with the roots of many plants including important weeds. Cultural practices affect the populations and the infectivity of AM fungi. Density, biomass, root density, AM root colonization and N% of weeds were compared in the two olive production systems (organic and conventional). Weed species differed in the response to AM root colonization. The highest AM root colonization was found for Lactuca serriola, Picris echioides, Plantago lanceolata and Gallium aparine. In addition, Avena sterilis, Fumaria officinalis and Stellaria media had the lowest AM root colonization. The highest AM root colonization of weeds was found in organic olive fields. AM root colonisation of weeds influences the density and biomass of competitive weeds. A positive correlation was found between AM root colonization and weed growth. Moreover, the different cultural practices in two production systems influence the weed AM root colonization. There were significant differences in the density and biomass of competitive weeds (Avena sterilis L.; Galium aparine L.; Lactuca serriola L.; Picris echioides L.; Plantago lanceolata L.; Sonchus oleraceus L.), with the highest values being found in organic olive fields. On contrast, there were no significant differences between the organic and conventional systems in the density and biomass of non-competitive weeds (Anthemis arvensis L.; Fumaria officinalis L.; Lamium aplexicaule L.; Lolium rigidum Gaudin.; Stellaria media (L.) Vill.; Veronica hederifolia L). Our results indicate that organic cultural practices significantly increased weed biomass and AM root colonization. The mycorrhizal symbiosis is an important factor influencing weed growth