Mycorrhizal fungus Rhizophagus irregularis mitigates symptoms of Verticillium wilt and enhances growth of olive trees (Olea europaea L.)

The interaction between arbuscular mycorrhizal fungus (Rhizophagus irregularis) and Verticillium dahliae was investigated in the susceptible olive cultivar "Picholine Marocaine" under controlled conditions. Olive plants were pre-inoculated with R. irregularis for three months, and subsequently post-inoculated with V. dahliae for nine months. The present study assessed the impact of combined inoculation on root colonization, plant growth parameters, nutrient uptake, and disease tolerance. Results showed that R. irregularis significantly increased mycorrhizal colonization, including vesicle and arbuscule formation, compared to non-mycorrhizal controls, even in the presence of V. dahliae. Enhanced colonization was associated with improved plant growth, including greater shoot and root lengths, increased numbers of leaves and branches, and higher total dry biomass. Additionally, R. irregularis significantly reduced disease severity as evidenced by lower dwarfing and leaf alteration indices. Mycorrhizal inoculation also led to a substantial increase in nutrient uptake, particularly potassium, calcium, and available phosphorus, in both shoots and roots of olive plants. Interestingly, sodium uptake was higher in plants solely colonized by R. irregularis, suggesting a potential influence on specific nutrient dynamics. Overall, this study demonstrates that R. irregularis effectively promotes root colonization, enhances plant growth and nutrient uptake, and significantly improves disease tolerance against Verticillium wilt in susceptible olive cultivars.