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Impact of Arbuscular Mycorrhizal Fungi on Phosphorus Use Efficiency and Root Diseases of Onion Crops

Onion growers in the semi-arid, irrigated Columbia Basin produce 27% of the storage onions in the USA, with a crop farmgate value ranging from $4,000-$7,000/acre. Symbiotic arbuscular mycorrhizal fungi (AMF) colonize roots of many plant species and help the plants mine soil more effectively for immobile nutrients, particularly phosphorus (P). AMF can also help defend plants against some root pathogens. Onions depend significantly on AMF as the symbiotic association compensates for the relatively sparse, unbranched roots with few root hairs. However, the widespread use of soil fumigation and high rates of P fertilizer used for onion crops in the Columbia Basin may negate or reduce significantly this symbiotic association of onion plants with AMF. The objectives of this proposal are to determine the AMF species and prevalence in organic and conventional onion crops in the Columbia Basin, and to use greenhouse trials as well as grower-cooperator field trials to determine whether inoculation of onion plants with AMF improves onion growth, increases P use efficiency, and/or reduces soilborne disease pressure (particularly Rhizoctonia stunting and other prevalent soilborne diseases of onion such as pink root). A long-term goal is to examine how AMF can contribute to improved soil quality in the Columbia Basin by facilitating reduced use of soil fumigation and soil application of fungicides, as well potential reduced rates of P fertilizers.

Grant Information

  • Project ID: 137
  • Project Status: Complete


  • Principal Investigator(s): du Toit, L.
  • Investigator(s): Paulitz, T., Poudyal, D.
  • Grant Amount: $40,000
  • 2014 Progress Report


Sharma-Poudyal, D., Paulitz, T.C., Linderman, R.G., and du Toit, L.J. 2014. Effect of arbuscular mycorrhizal fungi on onion growth and onion stunting caused by Rhizoctonia solani, 2013. Plant Disease Management Reports 8:V305.

Schroeder, B.K., du Toit, L.J., Waters, T., and Reitz, S. 2014. Tackling challenges facing Pacific Northwest onion growers. Onion World Dec. 2014:20-23.

Knerr, A.J., Wheeler, D., Schlatter, D., Sharma-Poudyal, D., du Toit, L.J., and Paulitz, T.C. 2018. Arbuscular mycorrhizal fungal communities in organic and conventional onion crops in the Columbia Basin of the Pacific Northwest USA. Phytobiomes Vol 2:No 4 .

du Toit, L.J., Derie, M.L., Holmes, B.J., Miller, C.E., Brouwer, L.R., Waters, T.D., and Darner, J. 2019. Effects of the arbuscular mycorrhizal fungi inoculant Mykos Gold Granular on pink root and yield in commercial onion bulb crops near Irrigon, OR, 2017. Plant Disease Management Reports 13:V008.

du Toit, L.J., Derie, M.L., Holmes, B.J., Morgan, P., Brouwer, L.R., and Waters, T.D. 2019. The influence of soil phosphorus levels on onion root colonization by mycorrhizal fungi from commercial inoculants, 2017. Plant Disease Management Reports 13:V009.

Winkler, L., du Toit, L., and Waters, T. 2019. Mycorrhizal inoculants: Yea or nay? The effects of mycorrhizal inoculants on onion crops in the Columbia Basin. Onion World Feb. 2019:10-14.

Additional Funds Leveraged

Additional funding was not applied for or secured in 2014 since some of the trials are still in progress. However, we plan to submit a proposal for a Washington State Department of Agriculture Specialty Crop Block Grant or a Western Region IPM grant in 2015.


 Short-Term:
The study is revealing the prevalence and diversity of AMF in conventional and certified organic onion bulb crops in the Columbia Basin. Since onion production is an intensive, high input, high value system, the study should elucidate the potential value of AMF for enhancing P nutrition and suppressing onion root diseases. Information generated on the effects of fungicides applied as seed treatments or incorporated into soil will help growers optimize fungicide use to minimize negative impacts on AMF colonization in onion crops without compromising root disease management.
 Intermediate-Term:
Dissemination of results to consultants, extension personnel, and growers through field days, conferences, and publications will increase awareness of the potential role of AMF in soil management and crop productivity. Depending on results, the study may lead to increased adoption of AMF inoculation in onion crops and avoidance of production practices that adversely affect AMF. Since this project was initiated, more onion growers in the Columbia Basin are evaluating AMF products on their farms. This highlights the need for independent, robust trials to determine the cost-benefit of such treatments.
 Long-Term:
This research is expected to provide scientific justification for the significance of robust AMF communities in the intensely cultivated Columbia Basin, to potentially increase availability of P that should enhance onion growth and increase suppression of onion root diseases. This could reduce rates of P fertilizer application and dependency on fungicides, and possibly even soil fumigation and soil quality. This will, in turn, reduce onion production expenses. Results of this project could also be applied to other high value crops in the Columbia Basin that form mycorrhizae, e.g., potato and carrot.