Arbuscular mycorrhizal fungal communities in organic and conventional onion crops in the Columbia Basin of the Pacific Northwest United States

Authors: KNERR, ANNE JEAN'NE - University Of Idaho; WHEELER, DAVID - Washington State University; Schlatter, Daniel; SHARMA-POUDYAL, DIPAK - Oregon Department Of Agriculture; DU TOIT, LINDSEY - Washington State University; Paulitz, Timothy

Submitted to: Phytobiomes Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2018
Publication Date: 1/18/2019
Citation: Knerr, A., Wheeler, D., Schlatter, D.C., Sharma-Poudyal, D., Du Toit, L.J., Paulitz, T.C. 2019. Arbuscular mycorrhizal fungal communities in organic and conventional onion crops in the Columbia Basin of the Pacific Northwest United States. Phytobiomes Journal. 2(4):194-207. https://doi.org/10.1094/PBIOMES-05-18-0022-R.
DOI: https://doi.org/10.1094/PBIOMES-05-18-0022-R

Interpretive Summary: Much of the US onion crop is grown in irrigated circles in the Columbia Basin of the Pacific Northwest. Onions are highly depending on arbuscular mycorrhizal fungi (AMF), symbiotic fungi that colonize the roots and help the plants take up phosphorus. There is an increasing production of organic onions, using non-synthetic forms of fertilizer. This study used molecular methods to describe the AMF communities in organic and conventional fields, and the effect of metam sodium, a commonly used soil fumigant. Differences in communities between organic and conventional were detected from fields and roots sampled in the mid summer. There was slightly more colonization of roots in the organic treatment, and in the non-fumigated treatments. Three to four groups of AMF were dominant in all treatments, but there were some less abundant groups that were present in the organic treatment but absent in the conventional treatments. But overall, soil fumigation had little effect on communities compared to the organic vs non-organic. The AMF community has probably adapted to the higher levels of P in the agriculture soils.

Technical Abstract: Irrigated onion crops are grown in the Columbia Basin of Washington and Oregon. Little is known about arbuscular mycorrhizal fungi (AMF) communities in onion crops in this region, including impacts of synthetic fertilizers, fungicides, and soil fumigation with metam sodium. AMF communities and root colonization were compared in organic and conventional onion fields using soil samples collected in spring and baited for AMF by planting onion seed, and in onion plants sampled mid-summer from the crop in each field. AMF communities also were compared between paired conventional fields that were non-fumigated or fumigated with metam sodium. AMF were detected in all fields at all sampling times with no differences in organic vs. conventional and fumigated vs. non-fumigated fields, except in the roots of plants sampled mid-summer in the fields (67% vs 51% and 45% vs 67%, respectively). Pyrosequencing identified four AMF genera (Glomus, Claroideoglomus, Paraglomus, and Diversispora) and four dominant OTUs (Glomus mosseae, Glomus Whitfield type 17, Claroideoglomus lamellosum, and Glomus MO_G17). AMF community structure in plants collected mid-summer was significantly different in organic vs. conventional crops, and AMF diversity and richness were greater in organic than conventional crops. There was no effect of organic vs. conventional crops on dominant OTUs, but several OTUs in limited abundance in organic fields were not detected in conventional fields. There was no consistent effect of metam sodium on AMF communities.