Evaluation of fumigant and non-fumigant nematicides for management of Rotylenchulus reniformis on sweetpotato

Studies were conducted at the LSU AgCenter Northeast Research Station (NERS) in St. Joseph, Louisiana, United States, in a field with a history of moderate reniform nematode infestation. Soil at the site is a Bruin silt loam (31% sand; 58% silt; 11% clay; 1.1% organic matter; 6.0 pH; 5.5 meq/100g cation exchange capacity). The field was planted with cotton prior to establishing the sweetpotato field in both trial years. Small-plot field trials were conducted in 2021 and 2022 to evaluate the efficacy of various nematicide formulations for management of reniform nematode on sweetpotato. For both years, the experimental design was a randomized complete block consisting of four replicate plots of eight soil treatments; Table 1: (1) no nematicide (control); (2) 1,3-dichloropropene (Telone® II (Salt Lake Holding LLC., Midland, MI); (3) fluopyram (Velum® (Bayer CropScience LP, St. Louis, MO)); (4) oxamyl (Vydate® L (DuPont Crop Protection, Wilmington, DE)); (5) fluazaindolizine (Salibro^TM (Corteva Agriscience, Indianapolis, IN)); (6) aldicarb (AgLogic 15GG (AgLogic Chemical LLC, Chapel Hill, NC)); (7) Majestene® (heat-killed Burkholderia spp. strain A396 (Marrone Bio Innovations, Davis, CA)); and (8) fluensulfone (Nimitz (ADAMA, Raleigh, NC). Beds were fumigated with a single shank 21 days before planting (DBP). For all non-fumigant nematicides, prior to treatment application rows were opened with a plow. Aldicarb was applied in-furrow as granules 2 DBP. All other nematicides were applied as in-furrow sprays with a 187 L/ha carrier volume using a CO₂-powered backpack sprayer 2 DBP. The rows were then immediately reshaped with a plow that incorporated the treatments while injecting 853 ml/ha of Belay Insecticide (Clothianidin; Valent USA Corporation, San Ramon, CA). Rows were then rolled with a cone roller to prepare for transplanting. Prior to transplanting, a herbicide application of 219 ml/ha of Valor SX (Flumioxazin; Valent USA Corporation, San Ramon, CA) and 3.11 L/ha of Command 3ME (Clomazone; FMC Corporation, Philadelphia, PA) was applied. Plots were 9.14 m long by 2 rows wide on 1.02-m row spacing with a 1-row planted buffer between adjacent plots across the rows, and 3.05 m of planted buffer between plots along a row. The trial was planted using the ‘Orleans’ sweetpotato cultivar (cv.) with slips being obtained from generation 1 seedbeds. Slips were transplanted to a depth of 3 nodes deep on a 30-cm-in-row plant spacing on July 28, 2021 (year 1) and June 8, 2022 (year 2), utilizing separate but adjacent portions of the same field for each year of the field study. Throughout the growing season, plots were treated with a combination of group 3, 4, and 28 insecticides following standard best management practices to insure low insect damage.

Reniform nematode soil population densities were monitored in plots on four sampling dates: prior to fumigating (21 DBP), at plant (2 DBP), mid-season (56 days after planting (DAP)), and at harvest (104 DAP). On each sampling date, ten soil cores (20 to 25 cm in length, 2.5 cm in diameter) were collected at random from both rows of each plot, thoroughly mixed in a plastic bucket, then placed in a labelled plastic bag. Soil samples were stored at 10 °C for a maximum of 14 days prior to subsequent processing. Nematodes were extracted from a 250 ml subsample of soil from each plot using a NC elutriator followed by a sucrose centrifugation technique (Baker et al., 1985). After collecting the nematodes over a 25-μm sieve, the nematode samples were transferred in water into plastic vials and stored at 4 °C for a maximum of two weeks prior to counting. The abundance of plant-parasitic nematode species was determined using an inverted compound microscope. Data are presented as the number of nematodes per 500 ml of soil.

Yield data was collected from a single row of each plot using a 1-row mechanical digger on November 9, 2021 (year 1) and September 20, 2022 (year 2). Sweetpotato from each plot were sorted using modified U.S. grades and weighed. Grades consisted of U.S.#1 (5.1 – 8.9 cm diameter and 7.6 – 22.9 cm length), canner (2.5 – 5.1 cm diameter and 5.1 – 17.8 cm length), jumbo (larger than U.S.#1), and total marketable yield (the total of U.S.#1, canner, and jumbo). Data are presented as the weight of each grade per hectare after culls (rotted storage roots or roots smaller than canner grade) were removed. From each plot, a total of 10 US#1 storage roots (or as many storage roots as available for plots with fewer than 10 of them) were inspected for cracking and the percent occurrence of cracking within the 10-root subsample from each plot was recorded.

In 2022, complimentary greenhouse nematicide efficacy trials were conducted using soil collected from the field site, NERS, as well as soil collected from the LSU AgCenter Macon Ridge Research Station (MRRS) in Winnsboro, Louisiana, United States, from a field with a moderate reniform nematode infestation. Soil at the MRRS site was a Gigger silt loam (35% sand; 48% silt; 17% clay; 1.3% organic matter; 6.7 pH; 6.3 meq/100g cation exchange capacity) that was planted with cotton in 2022. Soil (~150 L) was collected from NERS on June 8, 2022, for experiment 1 and June 22, 2022, for experiment 2 as well as from MRRS on June 15, 2022, for experiment 1 and June 29, 2022, for experiment 2. The soil was placed in sealed plastic buckets and transported to the LSU AgCenter Plant Material Center in Baton Rouge, Louisiana, United States, where it was thoroughly mixed prior to subsequent use in the greenhouse experiments. Nematodes were extracted from a 250-ml subsample of soil from each field site, as described above, to determine the initial reniform nematode soil population density for each field soil during each experimental repetition. For each field site, approximately 2.5 L of soil was placed in a 19.1-cm diameter pot and planted with a single 20-cm-long ‘Orleans’ cv. sweetpotato slip prior to nematicide application. The experimental design for each field soil examined was a randomized complete block consisting of six replications of eight soil treatments: (1) no nematicide (control); (2) steam pasteurized soil (80 °C for two hours on two consecutive days); (3) fluopyram (Velum® at 1.4 μL/pot); (4) oxamyl (Vydate® L at 53.0 μL/pot); (5) fluazaindolizine (Salibro^TM 6.4 μL/pot); (6) aldicarb (AgLogic 15G 47.6 mg/pot); (7) Majestene® at 53.0 μL/pot); and (8) fluensulfone (Nimitz® at 23.2 μL/pot). All liquid nematicide formulations were applied at an equivalent label rate (Table 1) according to the calculated pot soil surface area as a 25-ml drench application. Aldicarb was applied immediately prior to planting the sweetpotato slip by mixing the granules with the field soil. After 10 weeks in a temperature-controlled greenhouse, the experiment was terminated. Plants were carefully uprooted, rinsed clean of soil under tap water, and the fresh weights of adventitious roots (>5 mm in diameter) was recorded as storage root weights. Soil from each pot was mixed thoroughly and nematodes were extracted from a 250-ml subsample, as described above, to determine the final reniform nematode soil population density in soil. The entire experiment was repeated once for each field soil examined.

For the field trials, data from each year were subjected to a one-way ANOVA in the SAS Studio (SAS University Edition; version 3.3; SAS Institute Inc., Cary, NC, USA) using the PROC GLM procedure. The experimental model was a randomized complete block with four blocks and eight nematicide treatments. Differences between treatment means were examined using Tukey's HSD test (P < 0.05). For the greenhouse trials, data from each field soil examined were subjected to a two-way ANOVA in the SAS Studio using the PROC GLM procedure. The experimental model was a randomized complete block with six blocks, eight soil treatments, and two experimental repetitions. When there was no interaction between main factors, treatment means were pooled across both experiments and data were resubjected to a one-way ANOVA.

Reniform nematode was the predominant plant-parasitic nematode found in soil samples in both growing seasons. In 2021, reniform nematode soil population densities in the field averaged 3,500 nematodes per 500 ml of soil prior to applying the soil fumigant to select plots. In 2022, initial reniform nematode soil population densities averaged 1,319 nematodes per 500 ml of soil. In 2021, southern root-knot nematode was detected from select plots during the harvest sampling date (data not shown); however, soil population densities were low (<13 juveniles per 500 ml of soil) and inconsistent among replicate plots within a treatment. Southern root-knot nematode was not detected in any sample collected during the 2022 growing season. The stunt nematode (Tylenchorhynchus sp.) was also detected from select plots during both growing seasons; however, soil population densities were similarly low, and the presence of this nematode was inconsistent among replicate plots.

In 2021, reniform nematode soil population densities in plots were not significantly different among treatments prior to applying the soil fumigant (Table 2), with soil population densities ranging from 1,625 – 5,500 nematodes per 500 ml of soil. Immediately prior to planting, reniform nematode soil population densities in plots fumigated with 1,3-dichloropropene were reduced by 84% relative to that of the untreated control. By the mid-season sampling date, many of the nematicide treatments had reduced reniform nematode soil population densities relative to that of the untreated control, including 1,3-dichloropropene (94% reduction), fluopyram (67% reduction), oxamyl (84% reduction), and aldicarb (65% reduction). By the harvest sampling date, the only nematicide treatments to maintain lower reniform nematode soil population densities relative to that of the untreated control were 1,3-dichloropropene (67% reduction), fluopyram (56% reduction), and aldicarb (63% reduction).

In 2022, reniform nematode soil population densities ranged from 975 – 1,650 nematodes per 500 ml of soil prior to applying the soil fumigant (Table 3). At planting, reniform nematode soil population densities in plots fumigated with 1,3-dichloropropene were reduced by 80% relative to that of the untreated control. By the mid-season sampling date, no soil treatment was successful in reducing reniform nematode soil population densities relative to that of the untreated control; however, the 1,3-dichloropropene treatment reduced reniform nematode abundance by 66% relative to that of the Majestene treatment. Similarly, no differences in reniform nematode soil population densities were observed during the harvest sampling date.

In 2021, nematicide treatments did not significantly increase the yield of U.S.#1, Canner, Jumbo, or the total marketable weight of sweetpotato relative to that of the untreated control (Table 4); however, the 1,3-dichloropropene and Majestene treatments resulted in significantly greater yield of US#1 sweetpotato than plots treated with oxamyl or aldicarb. Cracking of U.S.#1 sweetpotato was low (<2%) and inconsistent among replicate plots within a treatment (data not shown).

In 2022, oxamyl and aldicarb increased the yield of U.S.#1 sweetpotato relative to that of the untreated control and fluensulfone treatment (Table 5). Nematicide treatments did not significantly affect the yield of canners, jumbos, or the total marketable yield. As observed in 2021, cracking of US#1 sweetpotato in 2022 was low (<1%) and inconsistent among replicate plots within a treatment (data not shown).

In the greenhouse experiment utilizing field soil from NERS, initial reniform nematode soil population densities averaged 1,680 nematodes per 500 ml of soil in experiment 1 and 1,980 nematodes per 500 ml of soil in experiment 2. Pasteurizing field soil resulted in significantly lower final reniform nematode soil population densities relative to all other soil treatments (Table 6). Application of fluopyram and fluazaindolizine reduced final reniform nematode soil population densities relative to the untreated control and aldicarb treatment. The weight of storage roots was significantly greater in the Majestene treated pots relative to that of the untreated control, fluopyram, oxamyl, fluazaindolizine, and fluensulfone treatments. The final reniform nematode population density in soil was higher in experiment 1 than in experiment 2.

In the greenhouse experiment utilizing field soil from MRRS, initial reniform nematode soil population densities averaged 1,540 nematodes per 500 ml of soil in experiment 1 and 2,200 nematodes per 500 ml of soil in experiment 2. Pasteurizing field soil resulted in significantly lower final reniform nematode soil population densities relative to all other soil treatments (Table 7). Application of aldicarb and fluensulfone reduced final reniform nematode soil population densities relative to the untreated control. There were no differences in final reniform nematode population density and storage root weight observed between the experimental repetitions.