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1 HORTSCIENCE 50(7): Irrigation Criteria and Sweetpotato Cultivar Performance in the Treasure Valley of Eastern Oregon Joel Felix 1,5, Clinton C. Shock 2, Joey Ishida 3, Erik B.G. Feibert 4, and Lamont D. Saunders 3 Malheur Experiment Station, Oregon State University, 595 Onion Avenue, Ontario, OR Additional index words. sweetpotato yield, drip irrigation, automated irrigation, soil water tension Abstract. In the United States, sweetpotato [Ipomoea batatas L. (Lam)] is predominately grown in the southeastern states and in California, but production farther north is limited. To determine if sweetpotato could be successfully produced in semiarid Pacific North West, four sweetpotato cultivars (Covington, Beauregard, Diane, and Evangeline) were subjected to four soil water tension (SWT) irrigation criteria treatments (40, 60, 80, 100 kpa in 2011 and 25, 40, 60, and 80 kpa in 2012) using drip irrigation at Ontario, OR. The four SWT criteria were maintained by an automated irrigation system. Sweetpotato cultivars were evaluated for the percentage of early groundcover, number of vines per hill, vine length, and yield. The total applied water decreased with the increase in the targeted SWT. The highest amount of water was applied at the 25 kpa criterion (1184 mm) and the least amount at the 100 kpa SWT criterion (146 mm). Cultivars varied in the average number of vines per hill, with Covington having the fewest at 6 vines per hill compared with Beauregard and Evangeline that averaged 10 vines and Diane averaging 11. The average vine length increased with the decrease in SWT criteria during both years. The total, marketable, and U.S. No. 1 sweetpotato yield was influenced by cultivars and varied among irrigation criteria and years. In general, the sweetpotato yield decreased with the increase in SWT, with the highest yield attained at the lowest SWT tested, 40 kpa in 2011 and 25 kpa in For Beauregard grown with irrigation onset criteria of 40 and 25 kpa, the marketable yields were 49 and 87 Mg ha L1 and U.S. No. 1 yields were 35 and 27 Mg ha L1 in 2011 and 2012, respectively. The results suggested that sweetpotato could be grown in eastern Oregon and would be capable of producing yields comparable to those obtained in California. However, yearly weather variations could delay transplanting and early harvest could be necessary to avoid frost damage. Sweetpotato is a warm-season root crop that is widely grown across the world under a wide range of environments and cultural practices. In the United States, commercial sweetpotato production is predominantly in the southeastern states and in California. In 2012, about 52,800 ha of sweetpotato were planted in the United States with a farm gate value of $462 million (U.S. Department of Agriculture, 2014). The average yield of sweetpotato grown under rain-fed conditions in North Carolina, Mississippi, and Louisiana in 2012 was 22 Mg ha 1,18Mg ha 1, and 23 Mg ha 1, respectively (USDA-NASS, 2014). Received for publication 25 Feb Accepted for publication 21 May This project was funded by the Agricultural Research Foundation, Oregon State University, Formula Grant no and Formula Grant no from the USDA National Institute of Food and Agriculture, and others. 1 Associate Professor. 2 Professor and Director. 3 Biological Sciences Research Assistant. 4 Senior Faculty Research Assistant. 5 To whom reprint requests should be addressed; joel.felix@oregonstate.edu. The root yield was comparatively higher in the irrigated fields of California, which averaged 38 Mg ha 1 in 2012 (USDA-NASS, 2014). The average yield for sweetpotato grown under furrow or drip irrigation conditions in California in 2012 (38 Mg ha 1 ) was 45% greater than the average yield of the rain-fed crop in North Carolina, Mississippi, and Louisiana. The relatively lower yield for sweetpotato grown under rain-fed conditions in the southeastern region of the United States could be attributed to fluctuating soil moisture conditions (Gajanayake et al., 2013; Villordon et al., 2012). Soil moisture stress is one of the major factors that limits sweetpotato growth and development and in turn affects storage root production and yield (Gajanayake et al., 2013). Critical factors for successful sweetpotato production include irrigation scheduling and the amount of water to be applied. Irrigation scheduling options rely on the measurement of soil water content or SWT. Precise irrigation scheduling that uses SWT criteria is a powerful method to optimize plant performance. Through the use of the ideal SWT and adjusting irrigation duration and amount, it is possible to simultaneously achieve high productivity and meet environmental stewardship goals for water use and reduce leaching of agricultural inputs (Shock and Wang, 2011). The primary sweetpotato production area in California is in Stanislaus and Merced Counties where transplanting is done between late April and early June (Stoddard et al., 2013). About 95% of the sweetpotato area in California is drip irrigated, with the remainder using furrow irrigation. Smith et al. (2009) indicated that the season water use total in California is 1200 to 2300 mm ha 1. The ideal daily maximum air temperatures for sweetpotato are between 30 and 35 C, but temperatures >38 C are not harmful as long as the crop is adequately irrigated and temperatures drop at night (Stoddard et al., 2013). These requirements closely match the weather conditions in the Treasure Valley of eastern Oregon and southwestern Idaho where the growing season is characterized by high evapotranspiration and low precipitation, making irrigation essential for production of most crops (Shock et al., 2000). The average daily high air temperature at the Oregon State University Malheur Experiment Station between 1 June and 30 Sept was 30 C, while the average daily low temperature was 13 C. The corresponding average daily high and low temperature from 1 June to 30 Sept was 31 and 13 C, respectively. The 20-year average daily high and low temperatures in both years were 30 and 13 C, respectively. These conditions suggest that there are >120 d of favorable weather for sweetpotato growth in the Treasure Valley of eastern Oregon. Newly developed commercial sweetpotato cultivars will produce mature roots in 80 to 90 d (Schultheis et al., 1999; Yencho et al., 2008), suggesting that sweetpotato transplanted in late May or early June could produce mature roots for harvest by the end of September or early October, around the time of the first killing frost in eastern Oregon. Information on the responses of different current commercial sweetpotato cultivars to a wide range of soil moisture regimes is limited. Therefore, the main goal of this study was to assess the possibility of producing sweetpotato in eastern Oregon. The specific objectives were to evaluate cultivars and develop the irrigation criterion suitable for sweetpotato production in the Treasure Valley of eastern Oregon and southwestern Idaho. Materials and Methods Field trials were conducted in 2011 and 2012 at the Oregon State University Malheur Experiment Station, Ontario, OR, on an Owyhee silt loam soil (course-silty, mixed, mesic, Xerollic Camborthid) each year following wheat (Triticum aestivum L.). The field was plowed and disked during the preceding fall to create a seedbed suitable for sweetpotato production. The soil was fumigated on 28 Mar using sodium methyldithiocarbamate (metam sodium) at 143 kg ai/ha through sprinklers. The 2012 study area HORTSCIENCE VOL. 50(7) JULY

2 was fumigated using 1,3 dichloropropene (Telone II) at 220 kg ai/ha in the fall of The 91-cm wide beds were formed 3 weeks after fumigation followed by fertilizer shanked into beds to supply 112 kg N/ha. Sweetpotato stems (also known as slips) of the cultivars Beauregard, Covington, Diane, and Evangeline were transplanted by hand on 3 June 2011 and 24 May 2012 at a depth of 0.15 and at 0.3 m spacing within the row. The irrigation drip tape (Toro Aqua-traxx; Toro Co., El Cajon, CA) was laid on top of each bed at transplanting and held in place by metallic pins. The tape used in these studies had emitters spaced 0.3 m apart and a flow rate of 0.5 L h 1 at 69 kpa pressure. To establish transplant/soil contact at planting, the field was immediately drip irrigated to provide 9 mm in 2011 and 15 mm in The field was drip irrigated three times until 5 July 2011 to provide 41.6 mm and twice until 25 June 2012 to provide 92.5 mm to allow plant establishment before imposing the irrigation regimes. Subsequent irrigations were automatically scheduled, depending on target SWT criteria, using a datalogger and controller. Irrigation treatments in 2011 consisted of four SWT criteria: 40, 60, 80, and 100 kpa. Because plants responded similarly to 80 and 100 kpa SWT in 2011 and to keep the study area manageable, the SWT criteria in 2012 were changed to 25, 40, 60, and 80 kpa. The study followed a split-plot design with treatments arranged in randomized complete block with three replications. Irrigation criteria formed the main plots onto which cultivars were randomly assigned as the split plots. Each split plot was 2.7 m wide (3 beds) by 9.1 m long. Soil water tension was measured in each main plot with four granular matrix sensors (GMS) (Watermark Soil Moisture Sensors Model 200SS; Irrometer Co. Inc., Riverside, CA) centered at 0.2 m depth in the center row of Beauregard cultivar in each main plot. Sensors had been calibrated to local soil water potential (Shock et al., 1998). The GMS watermark sensors were connected to a datalogger (CR10X datalogger; Campbell Scientific, Logan, UT) through a multiplexer (AM 410 multiplexer; Campbell Scientific). The datalogger was programmed to read and record the GMS data in each main plot every hour and irrigate the appropriate main plot every 12 h if needed (at 6 AM or 6 PM) to provide 12.7 mm of water if the SWT was above the targeted criterion. The irrigations were controlled by the datalogger using a controller (SDM CD16AC controller; Campbell Scientific) connected to solenoid valves in each main plot as described by Shock et al. (2000). The irrigation water was supplied by a well that maintained a continuous and constant water supply pressure of 241 kpa. The pressure in the drip lines was maintained at 69 kpa by pressure regulators in each main plot. The automated irrigation system was started on 8 July 2011 and 29 June 2012 and was turned off on 29 Sept and 1 Oct Posttransplant soil-active herbicides were not used because of the field proximity to sensitive crops. All plots were sprayed before transplanting with glyphosate at 860 g ae/ha on 26 May 2011 and 10 Apr to control all emerged weeds. The herbicide sethoxydim (215 g ai/ha) plus nonionic surfactant (0.25% v/v) was applied on 27 June 2011 and 19 June 2012 to control grassy weeds. Plots were hand weeded on 27 June and 28 July 2011 and 19 June 2012 to remove all broadleaf weeds. Late emerging weed cohorts were sparsely distributed and were periodically removed by hand as needed to keep the plots weed free. Sweetpotato vegetative groundcover was assessed subjectively based on 0% to 100% scale; where 0% = bare ground and 100% = complete groundcover. The number of sweetpotato vines was determined by counting the number of stems per hill just before harvesting. Average vine length was determined by measuring vines on five plants per split plot from the base to the tip of the vine. Sweetpotato vines were flailed on 4 Oct and 8 Oct and roots were subsequently lifted to the soil surface using a two-row harvester set at 0.46 m depth. Sweetpotato roots were picked by hand from the entire length of the center row to determine yield and quality and later graded following California standards (May and Scheuerman, 1998). In brief, California standards are as follows: U.S. No. 1 were of uniform size, 4.4 to 9 cm diameter and not less than 7.5 cm or greater than 23 cm long and weighed no more than 567 g each; mediums included misshapen roots with a minimum diameter of 4 cm, jumbo roots weighing more than 567 g each, and meeting the qualities for U.S. No. 1; and discards were those <4.4 cm in diameter. Marketable yield included U.S. No. 1, mediums, and jumbosized sweetpotato roots. Total yield for the purposes of this article included both marketable and discarded roots. Data were subjected to analysis of variance (ANOVA) using PROC MIXED procedure in Statistical Analysis Software (SAS Version 9.2.; SAS Institute, Inc., Cary, NC; SAS 2008) and means were compared using Fisher s protected least significant difference (LSD) (0.05). Regression equations were calculated using SigmaPlot software (Sigma- Plot, Systat Software, San Jose, CA). Results and Discussion Amount of water used. The average SWT increased with the increase in the targeted irrigation criterion (Table 1). The total amount of water applied from transplanting to harvest included the water used during the plant establishment phase (3 June to 8 July 2011 and 24 May to 25 June 2012) plus rainfall. Total applied water increased with the decrease in the targeted SWT. Plants irrigated at the 25 kpa SWT criterion in 2012 received a seasonal total of 1184 mm, which was greater than the seasonal total at the 40 kpa SWT criterion of 358 mm in 2011 and 862 mm in Substantially less water was used at 80 and 100 kpa SWT. The difference in water used between 2011 and 2012 could be attributed to the prevailing weather conditions during respective years (Fig. 1). The weather in 2012 was warmer and sweetpotato slips were transplanted 10 d earlier than in Significant differences were found in water use efficiency (WUE) among soil moisture treatments. The WUE increased with the increase in SWT, and ranged from 0.12 to 0.16 Mg mm 1 in 2011 compared with 0.05 to 0.14 Mg mm 1 in 2012 (Table 1). In 2011, increasing the SWT from 40 to 80 or 100 kpa SWT provided a 25% improvement in WUE, suggesting that the prevailing cooler Table 1. Mean hourly soil water tension, total water applied, average sweetpotato marketable yield, and average water use efficiency (Mg ha 1 marketable yield per millimeter of water applied) for four sweetpotato cultivars subjected to four irrigation onset criteria treatments, Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and Hourly soil water tension Total water applied z Sweetpotato marketable yield Water use efficiency y Soil water tension kpa kpa mm Mg ha 1 Mg mm LSD (0.05) z Total applied water for each criterion includes the amount applied uniformly to all treatments during plant establishment phase and rainfall from June to 29 Sept (27.9 mm) and May 24 to 8 Oct (37.8 mm). y Water use efficiency (WUE) was calculated using the formula WUE = Root yield/consumptive use of water (mm). LSD = least significant difference HORTSCIENCE VOL. 50(7) JULY 2015

3 conditions may have reduced sweetpotato evapotranspiration across the irrigation criteria. During 2012, which was warmer than 2011, irrigating at 25 and 40 kpa SWT resulted in similar WUE. However, increasing the SWT from 25 to 80 kpa improved the WUE by 64%. The high WUE at high SWT was always associated with reduced yield per unit area, suggesting that growers would have to balance WUE and the desired sweetpotato yield. The high consumptive water use at the 25 kpa SWT (1182 mm) was close to the range of the average water use in California reported by Smith et al. (2009) to be 1200 to 2300 mm. Vegetative groundcover. Visual evaluation at 49 and 60 d after transplanting in 2011 and 2012, respectively, indicated no significant difference in sweetpotato vegetative groundcover between years (Table 2). The pooled data indicated that the average vegetative groundcover differences were attributed to the irrigation criteria, cultivar, and irrigation criteria by cultivar. Differences in cultivar characteristics in terms of vine length and leaf size have been reported and the complete descriptions for Covington and Evangeline compared with Beauregard are provided by La Bonte et al. (2008) and Yencho et al. (2008). When irrigated at the 100 kpa SWT in 2011, Covington and Diane vegetative groundcover at 49 d after transplanting averaged 75% and 80%, respectively, compared with 94% and 93% for Beauregard and Evangeline. Similar differences were observed at the other irrigation criteria. These results are corroborated by the average length of vines for the different cultivars in 2011 and 2012 (Table 3). Plant vine length. Sweetpotato vine length was influenced by the irrigation criteria and cultivar, with no interaction between Fig. 1. Cumulative growing degree days (base 10 C) and mean air temperature at the Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and the two variables (Table 3). Vine length decreased with the increase in SWT for each of the four cultivars, regardless of the year. Vines were generally longer in 2012, likely as a response to warmer conditions compared with 2011 (Fig. 1). Cultivars Covington and Diane tended to have shorter vines compared with Beauregard and Evangeline. For example, when irrigated at the 40 kpa SWT in 2011, Covington and Diane vines averaged 88 and 78 cm compared with 178 and 141 cm for Beauregard and Evangeline, respectively. In 2012, the respective vine lengths for Covington, Diane, Beauregard, and Evangeline irrigated at 40 kpa were 151, 162, 261, and 198 cm. In 2011, increasing the SWT from 40 to 80 kpa reduced vine length by 22% for Covington, 30% for Beauregard, 37% for Evangeline, and 35% for Diane (Table 3). The corresponding decrease in 2012 for the respective cultivars was 31%, 23%, 24%, and 34%. These results corroborate the findings by Gajanayake et al. (2014) who reported a rapid decline in vine elongation with declining soil moisture. Increased vine length in response to increased irrigation was also reported in India by Nedunchezhiyan et al. (2012). The longer vines in 2012 were likely a result of warmer conditions in that year (Fig. 1), resulting in improved early season groundcover compared with Improved groundcover at lower SWT likely also contributed to the higher yields by providing needed resources to feed the developing roots, and would likely provide better weed suppression. The number of sweetpotato vines per hill at 117 d after transplanting was unaffected by the irrigation criteria or the interaction of irrigation by cultivar in 2011 and However, the differences in the number of vines per hill were attributed to cultivars in both years (Table 4). The greatest number of vines per hill in order was Diane > Evangeline = Beauregard > Covington (Table 4). Diane had a significantly greater number of vines per hill compared with the other cultivars. Yencho et al. (2008) stated that Covington has thicker stems with less branching compared with Beauregard, which is consistent with the results in this study. Table 2. Sweetpotato vegetative groundcover at 49 d after transplanting in 2011 and 60 d after transplanting in 2012 in response to differential irrigation criteria at Oregon State University Malheur Experiment Station, Ontario, OR. Groundcover Covington Beauregard Evangeline Diane Irrigation criteria Mean Mean Mean Mean (kpa) % Average LSD (0.05) Irrigation NS NS 5 NS NS 5 NS NS 5 NS NS 5 Cultivar NS NS 7 NS NS 7 NS NS 7 NS NS 7 Irrigation cultivar NS NS 4 NS NS 4 NS NS 4 NS NS 4 HORTSCIENCE VOL. 50(7) JULY

4 Total root yield. Total sweetpotato yield varied among irrigation criteria, cultivar, and years (Fig. 2). In both years, total sweetpotato root yield for Beauregard, Covington, Evangeline, and Diane decreased precipitously with the increase in SWT irrigation criteria. The highest yields were attained with the lowest SWT tested, 40 kpa in 2011 and 25 kpa in 2012 (Fig. 2). Beauregard and Evangeline had a relatively higher proportion of jumbo roots when irrigated at the 25 or 40 kpa SWT. Similarly, Covington produced a high proportion of jumbo roots when irrigated at 25 kpa SWT in Data for the cultivar Beauregard are presented in Table 5 to illustrate the high yield of jumbo-sized roots in Jumbo-sized roots composed 42% of the 94 Mg ha 1 yield produced by Beauregard irrigated at 25 kpa SWT in Comparatively, U.S. No. 1 roots formed 28% of the total yield at the 25 kpa SWT irrigation criterion. The total root yield for Beauregard irrigated at 40, 60, and 80 kpa SWT in 2012 was 80, 65.7, and 59.2 Mg ha 1, respectively. The corresponding percentages of jumbo-sized roots were 34, 24, and 19. These results indicated that increasing SWT disproportionately impacted jumbo root size compared to the other root sizes. Jumbo root yield decreased by 33% when the SWT increased from 40 to 80 kpa in 2011 Table 3. Mean sweetpotato vine length per hill and average length (at 117 d after transplanting) in response to differential irrigation criteria and cultivar at the Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and Average vine length Covington Beauregard Evangeline Diane Irrigation criteria (kpa) cm Average LSD (0.05) Irrigation Cultivar Irrigation cultivar NS NS NS NS NS NS NS NS (Table 5). The corresponding decrease in jumbo-sized root yield in 2012 was 72% when SWT increased from 25 to 80 kpa. In general, sweetpotato root size is affected by duration in the ground, prevailing weather, irrigation, plant spacing, and cultivar. The increase in jumbo-sized roots in 2012 may have been associated with relatively warmer weather conditions and late harvesting, which was 137 d after transplanting. La Bonte et al. (2008) reported an increase in jumbo-sized roots in Evangeline (which has a similar growth rate as Beauregard ) if harvested later than 120 d after transplanting. The increase in jumbosized root yield negatively affected the U.S. No. 1 yield in both years. The U.S. No. 1 yield was reduced by 35% when the SWT increased from 40 to 80 kpa in 2011 and decreased by 22% when SWT increased from 25 to 80 kpa in To increase the U.S. No. 1 yield and reduce the jumbosized root yield at lower SWT in this study, the crop would have been harvested earlier. Most fields in California are harvested when 50% to 75% of the total root production is at U.S. No. 1 or when there is a risk of chilling injury from a later fall harvest (Stoddard et al., 2013). The results of this study suggested that the irrigation regimen chosen will depend on the targeted sweetpotato market. However, it should be noted that irrigation at lower SWT encourages the heavy use of water and could be an economically poor choice given that the U.S. No.1 yield was similar at 25 and 40 kpa. It is also noteworthy that the proportion of discarded roots increased by 26% in 2012 when the SWT increased from 25 to 80 kpa. It is likely that roots from plants irrigated at 80 kpa lacked enough moisture to bulk up and attain marketable size. Marketable root yield. Marketable sweetpotato yield followed the same pattern as the total yield (Fig. 3). The highest yield was obtained at the lowest SWT tested in both years, except for Diane, which had a curvilinear response in The cultivar Diane tended to tolerate drier conditions better than Covington and Evangeline. All cultivars produced much lower marketable yield at 80 and 100 kpa. For example, the marketable yield for Beauregard in 2011 ranged from 49.3 to 27.8 Mg ha 1 when plants were irrigated at 40 to 100 kpa SWT (Table 5). In 2012, the marketable yield for Beauregard ranged from 86.9 to 49.4 Mg ha 1 for plants irrigated at 25 to 80 kpa SWT. The higher marketable yield in 2012 could be attributed to warmer conditions compared with 2011 (Fig. 1) and delayed harvesting that resulted in a higher proportion of jumbo-sized roots. Even though jumbo-sized roots are processed into fries, they do not command a premium price. The U.S. No. 1 sweetpotato command the highest price, followed by jumbos, and then mediums (May and Scheuerman, 1988). The optimal time for harvesting Beauregard has been reported to be 100 to 110 d after transplanting, whereas acceptable yields could be obtained as early as 90 d after transplanting depending on market demands (Schultheis et al., 1999). Our delay in harvest was intended to allow Covington extra days to size up because it typically requires an extra 5 to 10 d compared with Beauregard (Yencho et al., 2008). Previous studies by May and Scheuerman (1998) in California indicated improved yield when sweetpotato were irrigated at 25 kpa throughout the season or 25 kpa during plant development followed by irrigation at the 100 kpa during root bulking stage. Our results in part corroborate the findings by May and Scheuerman (1998). The U.S. No. 1 root yield. The U.S. No. 1 sweetpotato yield was influenced by cultivar Table 4. Mean number of sweetpotato vines per hill (at 117 d after transplanting) in response to differential irrigation criteria and cultivar at the Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and Number of vines per hill Covington Beauregard Evangeline Diane Irrigation criteria Mean Mean Mean Mean (kpa) Number per hill LSD (0.05) Irrigation NS NS NS NS NS NS NS NS NS NS NS NS Cultivar NS 3 NS 3 NS 7 NS 3 NS 3 NS 3 Irrigation cultivar NS NS NS NS NS NS NS NS NS NS NS NS 1014 HORTSCIENCE VOL. 50(7) JULY 2015

5 Fig. 2. Total sweetpotato root yield in response to irrigation criteria at the Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and Table 5. Sweetpotato yield by grade for cultivar Beauregard in response to differential irrigation criteria at the Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and Yield by grade Total Marketable U.S. No. 1 Mediums Jumbo Discard Irrigation criteria (kpa) Mg ha LSD (0.05) NS 1.8 and varied among irrigation criteria and years (Fig. 4). Just like the total and marketable yield, the U.S. No. 1 yield decreased with the increase in SWT, with the highest yield attained at the lowest SWT, 40 kpa in 2011 and 25 kpa in Again we used Beauregard as an example to illustrate the U.S. No. 1 root yield in response to SWT in 2011 and 2012 (Table 5). The U.S. No. 1 root yield in 2011 ranged from 34.6 to 20.8 Mg ha 1 for plants irrigated at 40 to 100 kpa SWT. The U.S. No. 1 root yield in 2012 was 26.4 to 20.7 Mg ha 1 when plants were irrigated at 40 to 100 kpa. The U.S. No. 1 root yield in 2012 for plants irrigated at 25 kpa SWT was 26.7 Mg ha 1. The high U.S. No. 1 yields attained in 2011 (Fig. 4) could be attributed to relatively cooler conditions during the growing season compared with 2012 (Fig. 1). The warmer weather conditions in 2012 may have increased the bulking of sweetpotato roots from U.S. No. 1 to jumbo size compared with the growth in Smittle et al. (1990) using an older sweetpotato cultivar Georgia Jet reported an increase in U.S. No. 1 root yield when plots were irrigated at 25 kpa SWT throughout their growth. But they also found that yields of U.S. No. 1 roots were not significantly reduced if the plants were irrigated at 25 kpa during early plant development and subsequently switched to 100 kpa throughout the root enlargement period. It is important to note that the studies by Smittle et al. (1990) were conducted on loamy sand and sandy soils. Our studies were conducted on a silt loam soil, which may require maintaining a 25 or 40 kpa SWT irrigation criterion throughout the season to avoid root deformities that could result when roots are exposed to extended drier soil conditions. Timely irrigation tends to loosen the soil and can improve the root bulking (Miller and Donahue, 1992) resulting in better yields. The use of a low SWT irrigation criterion in our study increased the irrigation frequency and also maintained moist soil conditions, which increased sweetpotato root yield (Fig. 4). In India, Nedunchezhiyan et al. (2012) reported 24.9% and 28.1% increases in sweetpotato root yield when the frequency of 4-cm irrigations supplemental to rainfall were increased to 3 and 5 irrigations per season, respectively, compared with nonirrigated control. Conclusions This study indicated that sweetpotato can be grown successfully in the Treasure Valley of eastern Oregon and southwestern Idaho. However, further studies are needed to understand the effects of yearly weather variations on the time of transplanting and harvest to avoid frost damage. Cultivar differences in terms of growth habits and yield in response to applied water were noted. Both Beauregard and Covington responded well to HORTSCIENCE VOL. 50(7) JULY

6 Fig. 3. Marketable sweetpotato yield in response to irrigation criteria at the Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and Fig. 4. Sweetpotato U.S. No. 1 root yield in response to irrigation criteria at the Oregon State University Malheur Experiment Station, Ontario, OR, 2011 and lower SWT irrigation criteria and produced greater yield in 2012 compared with Cultivars Covington, Beauregard, and Evangeline responded well to the lower SWT of 25 and 40 kpa compared with Diane, which performed well under drier conditions in 2012, but did not have the yield potential of the other cultivars at our location when moisture was less limited. The results also suggested that Beauregard could be harvested earlier compared with Covington to reduce the proportion of jumbo-sized roots. Stoddard et al. (2013) suggested halting irrigation when jumbo-sized roots exceed 33% of the total yield, unless the crop is slated for processing or when maximum total yield is desired. The use of drip irrigation coupled with moisture monitoring equipment for timely irrigation provided uniform moisture throughout the season and resulted in a low proportion of misshapen roots even 1016 HORTSCIENCE VOL. 50(7) JULY 2015

7 under silt loam soil conditions of these studies. Literature Cited Gajanayake, B., K.R. Reddy, R.W. Shankle, and R.A. Arancibia Early-season soil moisture deficit reduces sweetpotato storage root initiation and development. HortScience 48: Gajanayake, B., K.R. Reddy, M.W. Shankle, and R.A. Arancibia Growth, development, and physiological responses of two sweetpotato cultivars to early season soil moisture deficit. Sci. Hort. 168: La Bonte, D.R., P.W. Wilson, A.Q. Villordon, and C.A. Clark Evangeline sweetpotato. HortScience 43: May, D. and B. Scheuerman Sweet potato production in California. Vegetable Research Information Center. Vegetable Production Series, Univ. Calif., Div. Agr. Natural Res. Publ Miller, R.W. and R.L. Donahue Soils: An introduction to soils and plant growth, Prentice Hall of India, New Delhi, India. Nedunchezhiyan, M., G. Byju, and R.C. Ray Effect of tillage, irrigation, and nutrient levels on growth and yield of sweet potato in rice fallow. ISRN Agronomy 2012:291285, doi: /2012/ SAS Institute Inc SAS/STATH 9.2 User s guide. SAS Institute Inc., Cary, NC. Shock, C. and F. Wang Soil water tension, a powerful measurement for productivity and stewardship. HortScience 46: Shock, C., E.B.G. Feibert, and L.D. Saunders Irrigation criteria for drip-irrigated onions. HortScience 35: Shock, C.C., J.M. Barnum, and M. Seddigh Calibration of watermark soil moisture sensors for irrigation management. Proceedings of the International Irrigation Show, Irrigation Association, San Diego, CA. p Schultheis, J.R., S.A. Walters, and D.E. Adams In-row plant spacing and date of harvest of Beauregard sweetpotato affect yield and return on investment. HortScience 34: SigmaPlot SigmaPlot (Systat software). San Jose, CA. Smith, T.P., S. Stoddard, M. Shankle, and J. Schultheis Sweetpotato production in the United States, p In: G. Loebenstein and G. Thottappilly (eds.). The sweetpotato. Springer- Verlag, Netherlands. Smittle, D.A., M.R. Hall, and J.R. Stansell Irrigation regimes on yield and water use by sweetpotato. J. Amer. Soc. Hort. Sci. 115: Stoddard, C.S., R.M. Davis, and M. Cantwell Sweetpotato production in California. University of California Vegetable Research and Information Center, Division of Agriculture and Natural Resources, University of California, Vegetable production series, Publication 7237, Richmond, CA. U.S. Department of Agriculture (NASS) Potatoes and sweetpotatoes final estimates SB Nov cornell.edu/usda/nass/sb998/sb1047.pdf. Villordon, A., D.R. LaBonte, J. Solis, and N. Firon Characterization of lateral root development at the onset of storage root initiation in Beauregard sweetpotato adventitious roots. HortScience 47: Yencho, G.C., K.V. Pecota, J.R. Schultheis, Z. VanEsbroeck, G.J. Holmes, B.E. Little, A.C. Thornton, and V. Truong Covington sweetpotato. HortScience 43: HORTSCIENCE VOL. 50(7) JULY