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Potato (Solanum tuberosum) is one of the largest food crop worldwide, the production of which is also threatened by Phytophthora infestans, the causal agent of late blight. In organic farming, copper is used to control the disease. However, it has a long remanence and may accumulate in soil, leading to harmful effects on the environment. A potential alternative or combined approach to copper is the use of beneficial microorganisms such as arbuscular mycorrhizal fungi (AMF). In the present study three objectives were pursued: (1) the development of a molecular marker based on the mitochondrial large subunit (MtLSU) allowing inoculum traceability, and it's validation under field conditions with Rhizophagus irregularis MUCL 41833, (2) the evaluation of this AMF on late blight development in the cultivars Bintje and Nicola over two cropping seasons, (3) the assessment of the role of common mycorrhiza networks (CMN) on transfer of defense signals from diseased to healthy plants. We demonstrated that the MtLSU marker was efficient to trace R. irregularis inoculant into the field. This AMF was able to decrease the pressure caused by P. infestans under dry and hot conditions, while under high humidity and low temperatures conditions no effect was noticed. Finally, we demonstrated under in vitro culture conditions the activation of jasmonic acid (JA) or JA/ethylene (JA/ET) pathways defense related genes in healthy plants connected via the CMN of R. irregularis to plants infected by P. infestans within 24 h following infection. In conclusion, our results provided a glimpse on the link between root colonization by a specific AMF strain and its effects on potato resistance to P. infestans under field conditions and raised the question on the ecology of AMF and roles of CMN during late blight outbreak.