Evaluation of Arbuscular Mycorrhizal Fungi for Enhanced Growth, Phosphorus Use Efficiency, and Management of Soilborne Pathogens of Onion and Carrot in the Columbia Basin

This WSDA Specialty Crop Block Grant final report evaluated commercial arbuscular mycorrhizal fungi (AMF) inoculants for improving onion and carrot production in the Columbia Basin. Across 18 field trials (2016–2018) and multiple growth chamber studies, researchers assessed AMF effects on crop growth, nutrient uptake, disease, and root colonization under typical high-input production systems. Results showed little to no consistent benefit of AMF inoculants on crop performance, despite measurable colonization in some cases. Native AMF populations were present even in fumigated soils, and crop response was driven more strongly by fertilizer practices than inoculation. High soil phosphorus levels and common fungicide seed treatments reduced AMF effectiveness, while liquid formulations failed to establish colonization. Overall, the study concluded that AMF inoculants offer limited economic or agronomic value under typical Columbia Basin conditions, though they may have potential under lower-input or stress conditions. Findings provide independent data to inform grower decision-making and reduce unnecessary input costs.