Impact of Rhizophagus irregularis MUCL 41833 on disease symptoms caused by Phytophthora infestans in potato grown under field conditions

In organic potato production in Europe, only copper-based fungicides allow to directly control Phytophthora infestans (the causal agent of late blight). Due to environmental concerns caused by the repeated and excessive use of Cu before the nineties, the EU legislation has promoted alternatives approaches such as the use of biocontrol agents (e.g. arbuscular mycorrhizal fungi - AMF). Here, two field trials were conducted over two climatic-contrasting growing seasons. Trial 1 was characterized by a dry and hot cultural season with low pressure of P. infestans, while trial 2 was conducted under high humidity and relatively low temperatures with high pressure of the pathogen. In both trials, sprouted potato tubers were inoculated with AMF in the greenhouse before transplanting to the field. A Real-Time quantitative PCR assay was designed to target the inoculant strain Rhizophagus irregularis MUCL 41833 as well as the native Rhizophagus irregularis strains. In both trials, the inoculated AMF was detected in the roots at harvest, demonstrating the capacity of the inoculated strain to incorporate the microbiome of the potato plants. In the first trial, disease severity in AMF pre-colonized potato plants was markedly decreased and the onset of late blight symptoms was delayed by 10 days. In contrast, in the second trial no differences were noticed between AMF pre-colonized and control plants. In both trials, no mycorrhizal effect was noticed on tuber yield. As a conclusion, disease severity of P. infestans, measured by symptoms development on leaves, was decreased in AMF pre-colonized plants under conditions of low pressure of late blight and over a short period of time, while under conditions more adequate to the pathogen, no reduction in symptoms was noticed.