Impact of Arbuscular Mycorrhizal Fungi on P Use Efficiency and Root Diseases of Onion Crops in the Columbia Basin

Onion growers in the semi-arid, irrigated Columbia Basin produce ~27% of the US storage onions annually at a farmgate value of $4,000-$7,000/acre. Symbiotic arbuscular mycorrhizal fungi (AMF) colonize roots of many plant species and help the plants mine soils for immobile nutrients, particularly phosphorus (P). Onions depend significantly on AMF since the symbiotic association compensates for the relatively sparse, unbranched roots with few root hairs. However, widespread use of soil fumigation and high rates of P fertilizer applications in onion crops in the Columbia Basin may negate or reduce significantly this symbiotic association with AMF.

The objectives of this proposal were to determine the prevalence and species of AMF in certified organic and conventional onion bulb crops in the Columbia Basin, and to use greenhouse trials and grower field trials to determine whether inoculation of onion plants with AMF can improve onion growth, increase P use efficiency of onion crops, and/or reduce soilborne disease pressure (particularly stunting caused by Rhizoctonia solani, and other prevalent soilborne onion diseases such as pink root caused by Phoma terrestris).

A longer-term goal is to examine whether AMF can contribute to improved soil quality in the Columbia Basin by facilitating reduced reliance on soil fumigation, soil applications of fungicides, and rates of P applications in onion crops.

AMF were detected in all 9 fields surveyed in 2014 (5 certified organic and 4 conventional fields) in the Columbia Basin, at incidences ranging from 42 to 90% colonization of onion roots. This suggests AMF may be more prevalent than expected in onion crops in the Basin despite the widespread use of soil fumigants (synthetic fumigants or biofumigant cover crops), although the use of AMF inoculants might help increase AMF prevalence to enhance the beneficial effects of AMF on onion production. A survey to evaluate the effects of soil fumigation on AMF in growers’ fields is in progress.

The commercial AMF product BioTerra Plus significantly increased onion growth, but no benefit was observed with the product MycoApply Ultrafine Endo in growth chamber trials. BioTerra Plus also suppressed stunting caused by R. solani in growth chamber trials. Pink root AMF trials were inconclusive because the inoculation protocol resulted in such severe pink root that no AMF effects were detected. The inoculation protocol will need to be modified to facilitate detection of any potential suppressive effect of AMF on pink root.

Onion fungicide seed treatments commonly used in conventional onion crops in the Basin (Apron + Thiram, Farmore D300, and Farmore I500) all negated the beneficial effect of AMF on onion growth. However, soil application of the fungicide Quadris was either partially inhibitory or not inhibitory to AMF, and Fontelis did not inhibit AMF in either trial. These results have important ramifications for growers who routinely use fungicide seed and/or soil treatments for control of soilborne onion diseases.

Attempts to evaluate AMF products in grower-cooperator trials in 2014 were complicated by incompatibility issues of the product formulations with growers’ planting equipment. Although one or two AMF products were evaluated in three growers’ fields, difficulties with application did not result in a reasonable evaluation of the potential value of these AMF inoculants for onion production. Additional research is needed to address these issues in order to obtain effective, robust evaluation of the potential value and cost-benefit of onion growers using AMF products.