Abstract
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. The onion growing seasons in 2014 and 2015 were unusually hot, suggesting a connection of heat stress to bulb internal decay. Field studies to investigate the effect of temperature on onion bulb internal decay and yield were conducted in 2016–18 with drip-irrigated onions at the Oregon State University, Malheur Experiment Station located in eastern Oregon. Two long-day onion cultivars were submitted to four cultural practice variations to affect soil and bulb temperatures: bare soil check, supplemental heat using electric heat cables, white kaolinite clay application to the bulb sides and soil surface, and wheat (Triticum aestivum L.) straw mulch. The treatments established significant midafternoon average bulb and soil surface temperature gradients in the following order of increasing temperature: straw mulch, kaolinite, check, and supplemental heat. Averaged over years and cultivars, straw-mulched onions had the highest yield of bulbs larger than 102 mm diameter. Averaged over years and cultivars, onions receiving supplemental heat had the lowest total and marketable yield with no difference among the other treatments. Straw mulched onions had higher total and marketable yield than the bare soil check treatment in 2017, the hottest year. Averaged over the 3 years and two cultivars, marketable yield and yield of bulbs larger than 102 mm diameter decreased with increasing midafternoon bulb temperatures. Kaolinite application did not increase bulb yield nor bulb size compared with the bare soil check. 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. There was little difference in the measured soil moisture among treatments. Straw mulching may attenuate the negative effects of excessive heat on yield and bulb internal quality for long day onion production.
In 2014 and 2015, there was an increase in onion bulb internal dry scale and internal decay in the onion production areas of Washington, Oregon, and Idaho (du Toit et al., 2016). In bulbs with dry scale, the top 6 mm or more of one or more internal, fleshy scales collapses partially or fully into a paper-thin layer. These internal dry scales may be colonized by bacteria or fungi that may progress to internal decay that, unlike Botrytis neck rot (caused primarily by Botrytis aclada and B. allii in the Pacific Northwest, United States) or Fusarium basal rot (caused by Fusarium oxysporum f. sp. cepae), is difficult to detect externally and can result in quality issues in marketing.
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. A report from Australia found numerous endophytic bacteria associated with internal decomposition of onion bulbs, and the authors suggest that development of these bacteria is enhanced when bulbs are subjected to excessive heat (Cother and Dowling, 1986). Burkholderia gladioli pv. alliicola, Enterobacter cloacae, and Pantoea agglomerans are the predominant bacterial pathogens causing internal decay of onion bulbs in Washington State (du Toit et al., 2016; Schroeder et al., 2009). Internal decay of onion caused by these thermophilic bacteria has been found to increase with increasing bulb curing temperature (Schroeder and du Toit, 2010; Schroeder et al., 2012; Vahling-Armstrong et al., 2015) and has been associated with excessive heat during bulb development (Bishop, 1990; Pfeufer and Gugino, 2018; Schwartz and Mohan, 2008). The fungus Fusarium proliferatum has also been found to cause bulb decay in Washington State (du Toit et al., 2003), and Fusarium basal rot can progress to bulb decay in storage (Schwartz and Mohan, 2008).
The 2014 and 2015 onion growing seasons in the Pacific Northwest of the United States were unusually warm (du Toit et al., 2016), suggesting, as previous studies have shown, that excessive heat could be associated with the various causes of internal decomposition. Onions are relatively slow growing and leaf production is reduced after bulb initiation. In a study comparing growth patterns of lettuce (Lactuca sativa L.), red beets (Beta vulgaris L.), and onions, onions had the slowest leaf growth rates, lowest leaf area index, and least groundcover (Tei et al., 1996). These onion plant characteristics result in a prolonged period of bare soil, before row closure, in June to mid-July and in a smaller final percentage of groundcover than in other crops. The reduced groundcover with onions could potentially allow more solar radiation to be absorbed by the soil, enhancing ambient temperatures. The objective of this trial was 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.
Two techniques to reduce the heat load on the onions were chosen: straw mulching and particle film. Studies have shown that mulching during the summer can reduce soil temperature (Kader et al., 2017). Liquid suspensions of kaolinite clay have been used to create a particle film on apples (Glenn et al., 2002) and onion bulbs during field curing (Shock et al., 2005) to reduce temperature and sun scald. Particle films applied to onion foliage have also been used for control of onion thrips (Thrips tabaci) (Larentzaki et al., 2008a). However, in this study, particle film was applied to the base of the bulbs and the soil surface to reflect sunlight and reduce bulb temperature.
To increase onion bulb temperature, supplemental heating was applied using electric heat cables installed on the soil surface.
Materials and Methods
Onions were grown in 2016, 2017, and 2018 on Owyhee silt loam (coarse-silty, mixed, mesic Xerollic Camborthid) at the Malheur Experiment Station. The fields had previously been planted to wheat. The wheat stubble was shredded and the field deep-chiseled, disked, irrigated, moldboard-plowed, roller-harrowed, and bedded in the fall before spring planting. Each year, before fall plowing, fertilizer was broadcast based on soil analyses. The soil had a pH of 7.5 and 3.4% organic matter in 2016, a pH of 8.1 and 3% organic matter in 2017, and a pH of 7.8 and 2.5% organic matter in 2018. In 2016, P at 30 kg·ha−1, S at 83 kg·ha−1, and B at 0.23 kg·ha−1 were broadcast before plowing. In 2017, P at 10 kg·ha−1, K at 19 kg·ha−1, S at 91 kg·ha−1, Zn at 0.9 kg·ha−1, Mn at 0.9 kg·ha−1, and B at 0.5 kg·ha−1 were broadcast before plowing. In 2018, P at 33 kg·ha−1, K at 74 kg·ha−1, S at 26 kg·ha−1, Zn at 0.5 kg·ha−1, Mn at 2.3 kg·ha−1, Cu at 0.5 kg·ha−1, and B at 0.9 kg·ha−1 were broadcast before plowing.
Each fall, at bedding, the field was fumigated with 97 kg·ha−1 a.i. of potassium methyldithiocarbamate (K-Pam; Amvac, Los Angeles, CA, USA). After fumigation, the fields were left without further tillage until spring.
Onion planting.
Onion seed was planted in excess in double rows on 0.56-m beds. The single rows were spaced 76 mm apart. Planting was done with customized planter units equipped with disc openers (Flex Planter; John Deere and Company, Moline, IL, USA). Immediately after planting, plots received 0.92 g·ha−1 a.i. of chlorpyrifos (Lorsban 15G; Corteva Agriscience, Calgary, Canada) to control onion maggot (Delia antiqua). Onion emergence started on 7 Apr 2016, 20 Apr 2017, and 8 Apr 2018. In mid-May each year, alleys 0.92 m wide were cut between the cultivar split plots, leaving plots 7 m long. In mid-May each year, the seedlings were hand thinned to a spacing of 12 cm between individual onion plants in each single row, or 297,000 plants/ha.
Experimental designs.
The experiments each had a randomized complete block design with split plots and six replicates. There were four treatments to affect temperature applied to the main plots and two cultivars planted in split plots within each main plot. Each split plot was four double onion rows wide (four beds wide) and 7 m long. The two cultivars were Joaquin and Granero (Nunhems, Parma, ID, USA). Anecdotal observations by growers in the Treasure Valley suggested that Granero might be more susceptible to bulb rots than Joaquin. The four treatments were 1) an untreated check, 2) supplemental heat, 3) kaolinite clay, and 4) wheat straw mulch.
Kaolinite and straw-mulch treatments were intended to reduce the heat load on the onions. Kaolinite clay (Surround WP; Novasource, Phoenix, AZ, USA) was applied at 50 kg·ha−1 in a suspension of 0.05 kg kaolinite/L of water. The kaolinite was applied with a backpack sprayer by aiming the nozzle at the base of the onion plants on the south side of each double row. Kaolinite created a white film on the soil surface, which reflected sunlight. Because the white film wears off, kaolinite needed to be applied two to five times each year, starting in early June and ending in late July. For the straw-mulch treatment, wheat straw bales, each weighing 22 kg and measuring 0.21 m3, were used. Each year the wheat straw was applied on 1 Jun between the onion double rows to cover the entire plot area. The straw was applied at a rate of 17 m3·ha−1 (1747 kg·ha−1).
The supplemental heat was applied using commercial self-regulating heat cables with a maximum temperature of 65 °C (CPR Heat Trace; Chromalox, Pittsburgh, PA). The heat cables were laid next to each of the middle two double rows in the center of each plot to be heated. The heat cables were turned on and run continuously starting in late June and ending on 31 Aug.
The field had drip tape laid at 10-cm depth between pairs of double rows during planting. The drip tape had emitters spaced 30 cm apart and an emitter flow rate of 0.5 L·h−1 (Aqua-Traxx; Toro Co., El Cajon, CA, USA). The distance between the tape and the center of each double row of onions was 28 cm.
Cultural practices.
The onions were managed to minimize yield reductions from weeds, pests, foliar diseases, water stress, and nutrient deficiencies. Weeds were controlled with conventional low-rate herbicide applications as needed until late June, when onion foliar growth precluded further tractor traffic. Herbicides included oxyfluorfen (GoalTender; Dow AgroSciences, Indianapolis, IN, USA), bromoxynil (Brox 2EC; Albaugh LLC, Ankeny, IA, USA), pendimethalin (Prowl H2O; Bayer Corp., Res. Triangle Park, NC, USA), and clethodim (Shadow 3EC; Arysta LifeScience North America, Cary, NC, USA), which were applied according to product labels. Onion thrips were controlled with weekly applications of insecticides including spirotetramat (Movento; Bayer CropScience, St. Louis, MO, USA), spinetoram (Radiant; Dow AgroSciences), azadirachtin (Aza-Direct; Gowan, Yuma, AZ, USA), abamectin (Agrimek; Syngenta Crop Protection, Greensboro, NC, USA), and methomyl (Lannate; Dupont de Nemours and Company, Wilmington, DE, USA) from late May through early August. Ten insecticide applications for onion thrips control were made in 2016 and 2018, and nine applications were made in 2017.
Starting in early June, root tissue and soil solution samples were taken every week from borders of check plots and analyzed for nutrient concentrations by Western Laboratories, Inc. (Parma, ID, USA). Nutrients judged to be deficient according to extension guidelines (Sullivan et al., 2001) were applied each week through the drip tape. In 2016, N at 67 kg·ha−1 and K at 90 kg·ha−1 were applied. In 2017, N at 118 kg·ha−1 and K at 221 kg·ha−1 were applied. In 2018, N at 157 kg·ha−1, P at 11 kg·ha−1, K at 112 kg·ha−1, and Mg at 24 kg·ha−1 were applied.
Onions were irrigated automatically to maintain the soil water potential (SWP) in the onion root zone below 20 kPa (Shock et al., 2000). Soil water potential in each main plot was measured with two soil moisture sensors (Watermark Soil Moisture Sensors Model 200SS; Irrometer Co., Inc., Riverside, CA) installed in ‘Vaquero’ split plots at 20-cm depth in the center of the double row. In 2016, SWP was measured only in the check plots. Sensors had been calibrated to SWP (Shock et al., 1998). The datalogger (CR10X; Campbell Scientific, Logan, UT, USA) read the sensors and recorded the SWP every hour and automatically made irrigation decisions every 12 h. Each year the field was irrigated when the average of the 24 sensors in the check and kaolinite plots (only check plots in 2016) reached a SWP of 20 kPa or higher. Irrigation durations were 8 h, 19 min to apply 1.2 cm of water. The automated irrigation system was started in early June and irrigations ended on 31 Aug each year.
Onion bulb temperatures and soil surface temperatures were measured once a week in the midafternoon using an infrared (IR) thermometer (Close Focus IR; ThermoWorks, Salt Lake City, UT, USA) starting in mid-June and ending mid-August each year. The IR thermometer measured temperature on a 2.5 mm spot on the bulb and soil surface at a distance of 18 mm. Bulb and soil temperature measurements were made as close as practical to 2:00 PM (12:30 PM to 3:30 PM) on clear days. The bulb temperature was measured on the south side of the bulbs furthest from the drip tape and ∼1.3 cm above the soil surface. The soil surface temperature was measured ∼1.3 cm to the south of the same bulbs. In the straw-mulched plots, the straw was temporarily pushed aside to measure the soil surface temperature. Each weekly bulb or soil temperature measurement was the average of four measurements in each plot. Additionally, the soil temperature at 10-cm depth was measured in the midafternoon once a week in 2017 and 2018 at 1.3 cm to the south of onion bulbs in each plot using digital thermometers (Hanna Instruments, Limena, Italy). Air temperatures concurrent with the bulb and soil measurements, were obtained from a weather station (Agrimet; Bureau of Reclamation, Boise, ID, USA) located at the Malheur Experiment Station.
Onions were evaluated for maturity and symptoms of infection by Iris yellow spot virus (IYSV) on 8 Aug in 2016 and 2017, and on 14 Aug in 2018. Onions in each plot were evaluated for maturity by visually rating the percentage of plants with the tops down and the percentage of dry leaves. The number of bolted onion plants was counted in each plot. For the IYSV evaluations, onions in each plot were given a subjective rating on a scale of 0 to 5 for severity of IYSV symptoms. The rating was 0 if there were no symptoms, 1 if 1% to 25% of the foliage was symptomatic, 2 if 26% to 50% of the foliage was symptomatic, 3 if 51% to 75% of the foliage was symptomatic, 4 if 76% to 99% of the foliage was symptomatic, and 5 if 100% of the foliage was symptomatic.
The onions were undercut in mid-September to dry in the field. Approximately 1 week after undercutting, onion bulbs from the middle two double rows in each split plot were topped by hand and bagged. The bags were put into storage at the end of September. The storage shed was ventilated and the temperature was slowly decreased to maintain air temperature as close to 1 °C as possible.
Onions were graded out of storage in early November each year. During grading, bulbs were separated according to quality: bulbs without blemishes (No. 1s), split bulbs (No. 2s), bulbs infected with Botrytis neck rot (Botrytis allii), bulbs with symptoms of Fusarium basal rot (Fusarium oxysporum), bulbs infected with black mold (Aspergillus niger), and bulbs infected with bacteria in the external fleshy scales. Disease identification was based on typical visual symptoms for each disease. The No. 1 bulbs were graded mechanically (Kerian Speed Sizer; Kerian Machines, Inc., Grafton, ND, USA) according to diameter: small (<57 mm), medium (57–76 mm), jumbo (76–102 mm), colossal (102–108 mm), and super colossal (>108 mm). Marketable yield consisted of No.1 bulbs larger than 57 mm.
During grading, two bags of No. 1 bulbs (with no symptoms of external decay) from each split-plot were put back into storage for evaluation of internal bulb quality. After 2 months of storage, 25 bulbs from each split-plot were cut longitudinally and evaluated for the presence of incomplete scales, dry scales, internal bacterial rot, and internal rot caused by F. proliferatum or other fungi. Disease identification was based on typical visual symptoms caused by each pathogen. Bulbs with exterior symptoms of decay were excluded from the evaluations for internal decay. Incomplete scales were defined as scales that were incomplete 6 mm from the center of the neck or any part of a scale incomplete lower down in the bulb. Bulbs were defined as having dry scales if one or more fleshy scales had dried at the top of the bulb or lower down into the bulb.
Treatment differences were determined using analysis of variance and means separation was determined using Fisher’s protected least significant difference test at the 5% probability level (NCSS Statistical Software, Kaysville, UT, USA). The effects of midday bulb temperature on bulb yield and grade were determined by regression analysis.
Results
The number of hours that air temperatures were above 30 °C in June and July, calculated from hourly air temperature data measured by the Agrimet weather station at the Malheur Experiment Station, can provide an estimate of heat stress among years. The hourly weather data showed that 2017 was hotter than average and 2016 and 2018 were closer to average with 277, 308, and 265 h >30 °C in June and July in 2016, 2017, and 2018, respectively (29-year average is 249 h >30 °C). In comparison, 2014 had 282 and 2015 had 330 h >30 °C in June and July. Soil surface and bulb temperatures measured midafternoon were highest in 2017, lower in 2016, and lowest in 2018 (Table 1).
Onion bulb temperature, soil temperature at the surface and at 10-cm depth (mid-June to mid-August) and soil water potential (SWP) measurements for four treatments to alter onion bulb and soil temperatures in 2016, 2017, and 2018 at the Oregon State University, Malheur Experiment Station. Average ambient air temperature was 31, 33, and 32 °C in 2016, 2017, and 2018, respectively.
Soil surface and onion bulb temperatures for all treatments were, on average, higher than the average midafternoon air temperature each year (Table 1). Each year, the treatments established a statistically significant midafternoon average bulb and soil surface temperature gradient in the following order of increasing temperature: straw mulch, kaolinite, check, and supplemental heat (Table 1). On average, plots that had straw mulch applied, had bulb temperatures that were 2.3 and 5.2 °C lower than bulbs in the bare soil check and the supplemental heat plots, respectively.
Treatment, cultivar, year, and the interaction of cultivar and year all had statistically significant effects on onion maturity (Table 2). Compared with the bare soil check treatment, straw mulch delayed maturation of onion plants, but applications of kaolinite did not delay maturation of onion plants. Averaged over cultivars and years, straw-mulched onions had fewer tops down and dry leaves in early August than onions in the check plots and plots with supplemental heat. Onions receiving supplemental heat had among the earliest maturation.
Yield and grade after one mo. of storage, and maturity of onions subjected to four treatments to alter temperature, in field trials in 2016, 2017, and 2018 at the Oregon State University, Malheur Experiment Station. Data are the average of two onion cultivars, Joaquin and Granero.
Treatment, cultivar, year, the interaction of cultivar and year, and the interaction of treatment and year all had statistically significant effects on onion yield and grade (Table 2). Averaged over cultivars, supplemental heat reduced total yield and marketable bulb yield compared with the bare soil check treatment in both 2017 and 2018. Straw-mulched onions had higher total and marketable yield than the bare soil check treatment in 2017, the hottest year. Averaged over the 3 years, straw mulching resulted in the highest yield of colossal plus super colossal bulbs (>102 mm diameter). Kaolinite applications did not increase bulb yield or bulb size compared with the bare soil check treatment in any of the 3 years. Averaged over cultivars and years, marketable yield and yield of bulbs >102 mm diameter (colossal plus super colossal bulbs) decreased with increasing bulb temperatures (Figs. 1 and 2). Cultivar Joaquin had higher yield and bulb size than Granero.
Marketable onion yield response to average midday bulb temperature (mid-June to mid-August) averaged over two cultivars and 3 years at the Oregon State University, Malheur Experiment Station.
Citation: HortScience 57, 11; 10.21273/HORTSCI16829-22
Colossal plus super colossal (>102 mm) bulb yield response to average midday onion bulb temperature (mid-June to mid-August) averaged over two cultivars and 3 years at the Oregon State University, Malheur Experiment Station.
Citation: HortScience 57, 11; 10.21273/HORTSCI16829-22
The incidence of bulbs with external symptoms of decay (after 1 month of storage) was unaffected by the treatments (Table 2). Total internal bulb decay after 3 months of storage was minimal all 3 years, with averages of 2.3%, 5.1%, and 3.6% of the bulbs developing symptoms of internal decay in 2016, 2017, and 2018, respectively (Table 3). Total internal decay was only affected significantly by the treatments in 2017 (Table 3). In 2017, averaged over cultivars, onions receiving supplemental heat had the highest internal decay and the straw-mulched onions had among the lowest internal decay.
Internal bulb defects (incomplete scales and dry scales) and internal bulb decay in field trials in 2016, 2017, and 2018 for onions subjected to four treatments to alter soil and bulb temperature at the Oregon State University, Malheur Experiment Station. Data are the average of two onion cultivars, Joaquin and Granero. Bulbs were evaluated after three mo. of storage.
There was no significant difference among the four treatments in the percentage of bulbs with internal defects (incomplete scales and dry scales; Table 3). Only cultivar and year had an effect on internal defects after 3 months of storage. Averaged over treatments and cultivars, internal defects were lower in 2018 (47%) than in 2016 (82%) and 2017 (79%). Averaged over years and treatments, cultivar Granero had a higher percentage of internal defects (78%) than Joaquin (63%). Averaged over years, cultivars, and treatments, 70% of the bulbs had incomplete scales, of which 36% also had dry scales. Although the incidence of internal defects was high, the severity was low with most bulbs having only a short length of incomplete scales. There was no association between internal defects and internal decay. In 2016 and 2017, internal decay was observed only in bulbs with incomplete scales (Table 3). In 2018, internal decay occurred in bulbs with and without incomplete scales.
Each year (and averaged over the 3 years and two cultivars), most of the internal decay was caused by Botrytis neck rot (Table 4). Averaged over years, treatments, and varieties, internal decay caused by bacteria, Fusarium proliferatum, Botrytis neck rot, and black mold was 0.3%, 0.3%, 4.0%, and 0.5% of the bulbs, respectively. Although Botrytis neck rot and black mold can include symptoms on the exterior of the bulbs, these diseases were not detected externally because bulbs with external symptoms were excluded from the evaluations of internal decay.
Type of internal onion bulb decay after 3 months of storage for onions subjected to four treatments in 2016, 2017, and 2018 to alter bulb and soil temperature at the Oregon State University, Malheur Experiment Station. Data are the average of two onion cultivars, Joaquin and Granero. Bulbs with symptoms of external decay were not included in the evaluations for internal decay.
Straw mulching likely reduced evaporation from the soil, which could have resulted in higher soil moisture, but there was no statistically significant difference in average SWP among treatments in 2017 and 2018, the years when SWP was measured in plots of all treatments (Table 1). Regressions of marketable bulb yield and of yield of bulbs larger than 102 mm against average SWP did not reveal any significant correlation. The severity of IYSV symptoms was low with no significant difference between treatments. All treatments had an average IYSV symptom severity rating of 1 (1%–25% of plants with symptoms) with most plants showing only one or two lesions.
Discussion
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. Brewster (1979) grew onion plants in a growth chamber at a range of temperatures and found that growth peaked at 23 °C. In Ontario, Canada, onion yield was negatively correlated with number of days with maximum air temperatures ≥30 °C (Tesfaendrias et al., 2010). Onion yield was also negatively correlated with increasing number of hours >30 °C from June to mid-July at the Malheur Experiment Station (Feibert, 2022). In our study, average maximum air temperatures from mid-June to mid-August exceeded 30 °C each of the 3 years, and soil surface and bulb temperatures were considerably higher. Midafternoon soil surface and bulb temperatures exceeded 30 °C for all treatments, whereas temperatures were lower for the kaolinite and straw-mulched onions. Other research has also shown increased onion yields in mulched soils. In a study with drip-irrigated onion in Ghana during the summer, onions mulched with grass straw had higher yield and later maturation than onions on bare ground (Inusah et al., 2013). Our results also are consistent with those of Igbadun et al. (2012), who found onion yields were highest with either rice straw or black plastic mulch compared with bare ground during the dry summer season in Nigeria. In that study, seasonal water use was unaffected by mulch treatment, similar to the results of this study.
Application of straw mulch to the soil surface of onion crops has also been shown to increase onion yields as a result of a reduction in onion thrips populations and the associated incidence of IYSV (Larentzaki et al., 2008b; Schwartz et al., 2009). Although we did not measure onion thrips damage, the low severity of IYSV did not appear to influence bulb yield. Larentzaki et al. (2008b) showed reduced onion thrips populations with straw-mulching compared with bare soil in a study in which onions were not treated with insecticides. Our study used a relatively intensive onion thrips control strategy, in an attempt to eliminate differences in onion thrips damage among treatments.
In our study, kaolinite reduced bulb and surface soil temperatures, compared with the bare soil check but did not affect yield or internal decay. In a 2004 bulb curing study at the same location, kaolinite applied to onions bulbs at harvest reduced bulb temperatures and decreased bulb decay (Shock et al., 2005). This was consistent with higher internal bacterial decay severity resulting from higher postharvest bulb curing temperatures observed in Washington State (Schroeder and du Toit, 2010; Schroeder et al., 2012).
The delayed maturity of the straw mulched onions relative to the bare ground and supplemental heat onions observed in the current study agrees with research showing more rapid bulb development and earlier maturity with increasing temperature (Brewster, 1990; Inusah et al., 2013).
The level of internal bulb decay observed in this study was low (average of 5% over all 3 years) compared with levels of 30% observed in some commercial fields in the Pacific Northwest in years with very warm summers or other adverse conditions or practices that are highly conducive to bulb rots (du Toit et al., 2003, 2016), but similar to the typical levels of ≤5% in most years in the Pacific Northwest and California (Bishop, 1990; Schroeder and du Toit, 2010). The low incidence of internal bulb decay observed in the 3 years of this study concur with anecdotal evidence of limited onion internal decay in the Pacific Northwest during these years (S. Reitz, personal communication). The low levels of decay observed in this study could also be related to the precise irrigation provided by the automated irrigation system used in the study. The onions were kept at a relatively constant soil moisture status all season, without under- or overirrigation. Excessive irrigation at the end of the season has been associated with greater disease pressure in onion bulb crops in the Pacific Northwest, particularly during the very warm seasons of 2014 and 2015 and especially with overhead irrigation, which is more commonly used in the Columbia Basin of Oregon and Washington (du Toit et al., 2016). In addition, IYSV pressure in this Treasure Valley study was minimal. Iris yellow spot can lead to premature leaf dieback, which hinders the foliage falling over and maturing, increasing the susceptibility of bulbs to infection by bulb rot pathogens (du Toit et al., 2016). Iris yellow spot can also expose soil and bulbs to higher solar radiation from premature leaf dieback. Even though the levels of internal bulb decay in this study were low, there was a significant association of the incidence of internal neck rot with temperature in 2017. More important, even though straw mulch did not reduce the incidence of decay below that detected in the control plots with bare soil, application of straw mulch increased bulb yield compared with the yield of bulbs in the bare soil plots.
The causes of increased internal bulb decay in 2014 and 2015 were not determined, but excessive heat is suggested as a contributing factor since 2014 and 2015 were as warm or warmer than 2016–18.
Soil and bulb temperature were associated negatively with bulb yield and bulb grade in this study in the Treasure Valley. As a result, application of straw mulch attenuated the negative effects of heat on bulb yield and bulb rot under the semiarid conditions in this long-day region of onion production in eastern Oregon.
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