Increasing Onion Yield and Internal Quality

Onion (Allium cepa L.) bulbs produced in the Pacific Northwest of the United States in 2014 and 2015 had unusually high incidence of internal decay. This decay was not detectable externally, leading to marketing problems when bulbs were packed and shipped to markets. In order to determine the cause of the decay, a trial was conducted to investigate the effect of onion bulb and soil temperatures on onion internal defects, internal decay, and bulb yield. Trials were conducted in 2016–18 at the Oregon State University, Malheur Experiment Station, located in the Treasure Valley of eastern Oregon and southwestern Idaho.

Various bacteria and fungi are known to cause internal decay of onion bulbs, and studies have found heat to be a factor enhancing disease expression by most of these pathogens. In this case, two techniques to reduce the heat load on the onions were studied: straw mulching and particle film, a liquid suspension of kaolinite clay. To increase onion bulb temperature, supplemental heating was applied using electric heat cables installed on the soil surface.

The treatments established significant midafternoon average bulb and soil surface temperature gradients in the following order of increasing temperature: straw mulch, kaolinite, untreated, and supplemental heat. Averaged over years and cultivars, straw-mulched onions had the highest yield of bulbs larger than 102 mm diameter and onions receiving supplemental heat had the lowest total and marketable yield. Straw -mulched onions had higher total and marketable yield than the bare soil check treatment in 2017, the hottest year. The higher yield of onion in the straw-mulched plots compared with plots with bare soil or plots that received supplemental heat is consistent with studies showing reductions in onion growth and yield due to excessive heat.

The kaolinite particle film reduced bulb and surface soil temperatures compared with the untreated onions ,but did not affect yield or internal decay. The delayed maturity of the straw -mulched onions relative to the bare ground and supplemental heat onions observed in the study agrees with research showing more rapid bulb development and earlier maturity with increasing temperature.

The causes of increased internal bulb decay in 2014 and 2015 were not determined in the study, but excessive heat is suggested as a contributing factor since 2014 and 2015 were as warm or warmer than 2016–18. Straw mulching may attenuate the negative effects of excessive heat on yield and bulb internal quality for long day onion production.

The incidence of internal bulb decay was low all 3 years. In 2017, onions receiving supplemental heat had the highest internal decay and the straw-mulched onions had among the lowest internal decay.

According to author Erik Feibert, “The high incidence of internal decay (in onions) in 2015 and 2016 prompted us to conduct the research”. Dr. Erik Feibert is Senior Faculty Research Assistant, Malheur Experiment Station at the Oregon State University, College of Agriculture.

The full study results can be found on the ASHS HortScience electronic journal website at: https://doi.org/10.21273/HORTSCI16829-22.