Field studies were conducted in North Carolina in 2010/2011 and 2012/2013 to determine the effect of flumetralin applied for sucker control in tobacco (Nicotiana tabacum L.) on sweet potato (Ipomoea batatas L. Lam. storage root yield and quality) planted in the following year. Treatments in these studies consisted of flumetralin at 0, 0.67, 1.01, 1.34, 2.02, and 2.69 kg active ingredient (a.i.)/ha applied for sucker control to tobacco 2 weeks after topping (WAT) using a ground sprayer equipped with 3 nozzles per row. In 2012, 3 additional treatments included flumetralin at 1.01 and 2.02 kg/ha applied 2 WAT with a dropline method, and a lay-by treatment of 532 g a.i./ha pendimethalin at 4 to 5 weeks after tobacco transplanting followed by (fb) 1.01 kg/ha flumetralin at 2 WAT (using a 3-nozzle boom sprayer). Sweet potato planted the following year (2011) after tobacco had reduction in no. 1 sweet potato storage root yield in the 2.69 and 2.02 kg/ha flumetralin treatments applied with a 3-nozzle boom sprayer in 2011 and with both application methods (3-nozzle boom sprayer, dropline) in 2013, respectively. Flumetralin application rate (both years) or application method (2013) did not affect jumbo and marketable sweet potato yields. The lay-by treatment of pendimethalin followed by flumetralin in tobacco did not reduce yield (across all grades) of sweet potato planted the following year after tobacco. Overall, these results showed that no concern should exist for sweet potato planted the following year if flumetralin is applied at ≤1.34 kg a.i./ha (label-recommended dose) in tobacco.
Sweet potato (Ipomoea batatas L. Lam.) and tobacco (Nicotiana tabacum L.) are important agricultural commodities in North Carolina as the state ranked first in the production of these crops in the United States (6). In 2017, North Carolina farmers grew 66,350 and 36,400 ha of tobacco and sweet potato with an estimated value of $724 and $346 million, respectively (6). In North Carolina, tobacco is often grown in rotation with sweet potato, as this crop rotation system is beneficial to farmers since similar equipment and shared labor can be used for both crops (2).
Flumetralin is a plant growth regulator in the herbicide class dinitroaniline (group 2, Weed Science Society of America; 1) and is registered in tobacco to control axillary buds (suckers) after the removal (topping) of the terminal inflorescence, which is applied using hand or ground spray equipment. Flumetralin is absorbed by the tobacco plant within a few hours after application and provides residual sucker control through the growing season.
Flumetralin used for sucker control in tobacco has potential to impede growth of the rotational crop planted the following year. If tobacco is planted the next season instead of a rotational crop and a dinitroanaline herbicide is used, then early tobacco stunting may occur (8). The flumetralin label allows tobacco to be planted the following year after application of flumetralin; however, a 12-month and 45-day restriction is required for planting cucurbit vegetables and any crop for food, respectively (1). Flumetralin rate should not exceed 1.34 kg a.i./ha per year according to the label (1). Flumetralin half-life was reported >180 days depending upon soil type (4). As flumetralin is normally applied later in the season and because it has a relatively long half-life, it may persist and injure rotational crops. Limited research has occurred on the potential for flumetralin to carry over to subsequent crops. Shelby et al. (8) reported a reduction in the establishment of small grains in the fall after application of flumetralin alone or in combination with pendimethalin in tobacco. Carryover potential to sweet potato is unknown, and this is especially important for North Carolina, where tobacco and sweet potato are grown in rotation. Thus, the objective of this study was to determine the effect of flumetralin applied as a sucker control treatment in tobacco on sweet potato planted the following year.
Materials and Methods
Two-year rotational studies were conducted from 2010 to 2011 and 2012 to 2013 at a commercial field in Newton Grove (35.2853 N, 78.4351 W) and Benson, NC (35.3354 N, 78.4635 W). Soils for the studies were an Autryville sand (loamy, siliceous, subactive, thermic Arenic Paleudults) at Newton Grove and a Norfolk loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudults) at Benson. Plot size was 1.07 m (2 bedded rows) wide by 30 m (2010/2011) or 15 m (2012/2013) long with 1 border row on either side. Tobacco cultivars PVH 2110 and NC 71 were transplanted at a 0.62-m in-row spacing on April 20, 2010 and April 26, 2012, respectively. Sucker control treatments consisted of flumetralin (Prime+ EC, Syngenta Crop Protection, Inc., Greensboro, NC) at 0, 0.67, 1.01, 1.34 (maximum labeled rate; 1), 2.02, and 2.69 kg a.i./ha applied 2 weeks after removal of the tobacco flower or approximately 80 days after transplanting. Timing of application was based on North Carolina Cooperative Extension Service recommendation for sucker control products (3). All treatments were applied using a high-clearance sprayer equipped with a 3-nozzle boom (26-cm nozzle spacing) calibrated to apply 467 L/ha spray solution. The center nozzle was a TG-5 (TeeJet: Spraying Systems Co., Wheaton, IL) oriented 46 cm above tobacco buds and the outside nozzles were TG-3 (TeeJet) angled 45° toward the boom center. In 2012, the study was expanded by 3 treatments to include flumetralin at 1.01 and 2.02 kg a.i./ha applied with a dropline at 2 weeks after topping (WAT) and a lay-by application of 532 g a.i./ha pendimethalin at 4 to 5 weeks after tobacco transplanting followed by 1.01 kg a.i./ha flumetralin at 2 WAT (using 3-nozzle boom sprayer). The dropline treatments were applied using a CO2-pressurized backpack sprayer and a single TG-5 nozzle boom calibrated to deliver 30 mL of solution per plant using a hand-operated gate valve. The solution was delivered directly to the top center of the plant. The lay-by application of pendimethalin was applied using a CO2-pressurized backpack sprayer fitted with a 2-nozzle boom equipped with TeeJet XR 8003VS flat fan nozzles and calibrated to deliver 187 L/ha spray solution at 173 kPa. The application of pendimethalin was soil applied to the row middles.
The study design each year was a randomized complete block with 4 replications. The sucker control treatments in tobacco were applied 340 ± 10 days before transplanting sweet potato. The same year of treatment application, tobacco was harvested without recording yield and the stubble remained undisturbed over the winter. The following spring the field site was tilled, and then bedded rows were prepared to transplant sweet potato. Covington sweet potato cultivar cuttings (no roots) from a field propagation bed were mechanically transplanted to a 30-cm in-row spacing on June 4, 2011 and June 28, 2013. All plots were maintained weed free with 532 g a.i./ha pendimethalin (Prowl H20, BASF Corporation, Research Triangle Park, NC) preplant incorporated in tobacco and 980 g a.i./ha S-ethyl dipropylthiocarbamate (Eptam 7E, Gowan Company, Yuma, AZ) in sweet potato, between-row cultivation, and hand removal of weeds as needed. Production and pest management practices other than specific study treatments were held constant over the entire experiment and were done in compliance with recommendations for tobacco (3) and sweet potato grown in the southeastern United States (5). Sweet potato storage roots were harvested 130 (2011) and 120 (2013) days after planting using a tractor-mounted disc turn plow from the center 15- and 7.5-m-long portion of 1 row in 2011 and 2013, respectively. Harvested sweet potato roots were hand graded into jumbo (>8.9-cm diameter), no. 1 (>4.4-cm but <8.9-cm diameter), and canner (>2.5-cm but <4.4-cm diameter) grades (9). Total marketable yield was calculated as the sum of jumbo, no. 1, and canner grades.
Data were subjected to analysis of variance using SAS (SAS 9.3, SAS Institute Inc., Cary, NC) PROC MIXED with the fixed effects of application rate and random effects of replication. Data were analyzed separately because of the limitation of the experimental design; that is, three additional treatments included in the second year. Treatment means were separated by Fisher's Protected LSD test at a significance level of 0.05.
Results and Discussion
Flumetralin rate did not affect sweet potato storage root yield except for no. 1 grade (Table 1). Crop yield of no. 1 grade was similar among 0.67, 1.01, 1.34, and 2.02 kg a.i./ha and ranged from 17.5 to 24.3 × 103 kg/ha. However, a 36% reduction in no.1 yield was observed from flumetralin 2.69 kg a.i./ha compared with the nontreated control. Regardless of flumetralin application rate, canner, jumbo, and marketable yields ranged from 3.8 to 5.5, 5.2 to 10.9, and 31.6 to 36.4 ×103 kg/ha, respectively.
The effect of treatments was significant for no. 1 and canner yields (Table 2). No. 1 yield was similar among 0.67, 1.01, 1.34, and 2.69 kg a.i./ha and ranged from 17.5 to 24.3 × 103 kg/ha. Flumetralin at 2.02 kg a.i./ha reduced no. 1 root yield 23 to 29% when applied with either method (3-nozzle boom sprayer or dropline), respectively. Regardless of flumetralin application rate and method, jumbo and marketable yields ranged from 0.1 to 2.1 and 16.9 to 20.7 × 103 kg/ha, respectively. The lay-by treatment of pendimethalin fb flumetralin did not reduce sweet potato storage root yield.
Previously researchers have reported that flumetralin has potential to damage succeeding crops including tobacco because of its persistence in soil. Shelby and Fowlkes (7) surveyed farmers in Tennessee and reported a strong correlation between flumetralin use and carryover damage to fall- and spring-planted crops such as winter rye (Secale cereale L.) and spring wheat (Triticum aestivum L.). Shelby et al. (8) reported up to 55 and 18% injury to winter wheat cover crop and subsequent burley tobacco crops, respectively, after burley tobacco crop where plants were sprayed with flumetralin at 86 mg a.i. per plant for sucker control. However, this study's results indicate that there is no concern for sweet potato after tobacco in which flumetralin was applied at 1.34 kg a.i./ha (maximum labeled rate) or less the previous year. Flumetralin did not carry over to adversely affect sweet potato when used in accordance with the label recommendations in tobacco. The dropline application method may have the potential to promote greater stalk rundown, thus having more product reach the soil under the plant. This could be the reason for reduction of no. 1 grade yield with flumetralin 2.02 kg a.i./ha application with dropline method. However, the marketable yield was not affected by flumetralin application method. Therefore, future research needs to be performed under different soil or environmental conditions to understand the potential for flumetralin carryover with respect to different application methods.
The authors thank the sweet potato growers for the cultural management of sweet potato and tobacco research plots throughout the studies. The authors also thank North Carolina State Extension, North Carolina Agricultural Research Service, and Syngenta Crop Protection Company for supporting this project.