Sweet potato (
Currently, there are few SRKN management methods available for sweet potato growers, with options including crop rotation, resistant cultivars, and nematicide application. Among nematicides, there are both fumigants and non-fumigants registered for nematode management on sweet potato. Since nematodes are soil-borne pathogens, nematode management before planting is crucial. Fumigants applied before planting have proven to be effective in the control of a wide spectrum of nematodes. In the United States, commonly used fumigants registered use on sweet potato include 1.3-dichloropropene (1,3-D), metam sodium, and metam potassium. With the growing concerns for environmental safety, in the past decades, several major nematicides (e.g., methyl bromide) have been phased out, and increasing regulatory pressure on older chemistries has increased emphasis on discoveries of new non-fumigant nematicides (Desaeger
In recent years, several new fluorinated non-fumigant nematicides became available, including fluensulfone, fluopyram, and fluazaindolizine, and have been found to be effective in managing root-knot nematodes (RKN) on several vegetable crops in greenhouses and field studies (Becker
Fluazaindolizine is a novel sulfonamide nematicide (Lahm
Free-living nematodes are important indicators of soil health since they can contribute to improving soil nutrient cycling (Holajjer
The objectives of this study are to investigate the effect of fluazaindolizine at different rates and in combination with older non-fumigant nematicides on management of SRKN on sweet potato and non-target free-living nematodes, relative to other fumigant and non-fumigant nematicides.
To assess these objectives, three field trials were carried out at the North Florida Research and Education Center-Suwannee Valley in Live Oak, Florida (30°18¢11.62N 82°53¢48.62W). Trials were conducted in 2018, 2019, and 2020 in three different fields at the center. Soil at the sites was Chipley sand (91% sand, 6.8% silt, 2.4% clay, and 3.1% organic matter). In all experiments, fertilization, irrigation (supplied by overhead lateral line or center pivot irrigation), and herbicide applications were uniform across the trial and based on University of Florida recommendations (Mylavarapu
This study was a randomized complete block design with five replicates and nematicides as the single factor (Table 1). Each field plot consisted of two beds of sweet potatoes with 1 m space from row center to center and 0.6 m bed tops. Within beds, plant spacing was 30 cm. Plots were 9.1 m long. In 2018, 2019, and 2020, 1,3-D was applied 3–4 wk before planting (Table 2) via a fumigation rig with shanks spaced 30 cm apart. The rig was configured with five coulters to open traces immediately in front of the five shanks with press wheels behind each shank to seal traces, and 1,3-D was released at 25 cm deep in the soil profile. The area fumigated was 1.83 m wide, which covered the area where soil was pulled into beds. Non-fumigant nematicides were applied as a broadcast application 8–11 d before planting via CO2 powered backpack sprayer except that fluopyram was applied via drench at planting in 2020 (Table 1). For broadcast sprays, non-fumigant nematicides were applied using a wand with three nozzles spaced 0.6 m apart to produce a 2-m spray band at a solution application rate of 150 L/ ha. Following non-fumigant nematicide broadcast spray applications, all plots were rototilled at a 15-cm depth to incorporate nematicides then irrigated for further incorporation. In 2020, slips were transplanted, then later in the day, fluopyram was applied as a drench. For 2020 drench application, fluopyram was mixed in water and applied as a broadcast treatment manually with watering cans to selected plots to mimic application via overhead irrigation. The drench volume was 36.1 L/plot or 19,482 L/ha, which is equivalent to 1.96 cm of rain or irrigation. Before planting, 0.6 m wide hills were formed by a hill-forming mechanical discer, and sweet potato slips were planted in bare ground hills. In 2018, a sweet potato cultivar moderately resistant to SRKN (“Covington”) was used, whereas in the SRKN-susceptible cultivars “Beauregard” and “Orleans” were used in 2019 and 2020, respectively. A moderately resistant cultivar was used in 2018 since it is the primary cultivar growers use in this region, but the switch to susceptible cultivars was made in 2019 to evaluate the nematicides under higher SRKN pressure that accompanies the use of a susceptible cultivar.
Nematicide application treatment rates and application methods in 2018–2020 trials.
Treatment | Product | Active ingredient | Product application rate | Application rate (a.i.) | Application method |
---|---|---|---|---|---|
1 | Untreated control | ||||
2 | Salibroa | Fluazaindolizine | 2.24 L/ha | 1.12 kg/ha | Broadcast spray |
3 | Salibro | Fluazaindolizine | 4.48 L/ha | 2.24 kg/ha | Broadcast spray |
4 | Salibro + Vydate Lb | Fluazaindolizine + oxamyl | 2.24 L/ha + 9.35 L/ ha | 1.12 kg/ha + 2.14 kg/ha | Broadcast spray |
5 | Telone IIb | 1,3-D | 74 L/ha | 84 kg/ha | Broadcast shank fumigation |
6 | Velum Primec | Fluopyram | 499 mL/ha | 238 g/ha | Broadcast spray (2018–2019) Drench (2020) |
7 | Vydate L | Oxamyl | 9.35 L/ha | 2.14 kg/ha | Broadcast spray |
aSalibro was from Corteva Agrisciences (Indianapolis, IN).
bVydate L and Telone II were from Dow Agrisciences (Indianapolis, IN).
cVelum Prime was from Bayer CropScience LP (St. Louis, MO).
Schedule for data collection and trial maintenance in 2018–2020.
Task | 2018 | 2019 | 2020 |
---|---|---|---|
Preplant soil sample | May 16 (37 DBP)a | May 4 (40 DBP) | May 18 (24 DBP) |
Fumigation | May 22 (31 DBP) | May 13 (31 DBP) | May 22 (20 DBP) |
Nematicide broadcast spray applicationsb | June 11 (11 DBP) | June 4 (9 DBP) | June 3 (8 DBP) |
Sweet potato planted | June 22 | June 13 | June 11 |
Midseason soil sample | August 23 (64 DAP) | August 16 (64 DAP) | August 3 (53 DAP) |
Harvest soil sample | October 4 (106 DAP) | October 31 (139 DAP) | November 6 (149 DAP) |
Sweet potato harvest | October 23 (127 DAP) | November 1 (140 DAP) | November 11 (154 DAP) |
aNumbers in parentheses are DBP or DAP.
bIn 2020, fluopyram nematicide was applied by drench at planting.
DAP, days after planting; DBP, days before planting.
Sweet potato vines were mowed approximately 5 mon after planting (Table 2), tubers were inverted mechanically the next day, and tubers were picked by hand from both rows of the entire length of the plot. Total sweet potato tuber yield, in weight, was measured. To estimate marketable yield, a random subsample of sweet potato tubers from each plot was collected by filling a 19-L bucket with tubers, because it was not feasible to grade all tubers in each plot. Each tuber in the subsample was graded manually and sorted into the various marketable or unmarketable categories described below. Total weight for each category was calculated based on total yield and proportion weight of each grade category in the subsample on a per plot basis. In 2018, sweet potato subsamples were sorted into marketable and unmarketable categories and weighed. Any tubers <7.6 cm long, <3.8 cm diam., or with quality defects, described below, were considered unmarketable. In 2019 and 2020, tubers were graded to USDA standards (USDA AMS, 2005) with both USDA #1 and USDA #2 considered marketable, but USDA 1 representing the highest quality grade. Unmarketable categories included size outliers and defects due to quality issues. Requirements for USDA 1 were 4.4–8.9 cm diam., <0.5 kg, 7.6–22.9 cm length, free from damage, firm, fairly smooth, fairly clean, and fairly well-shaped. Requirements for USDA 2 were >3.8 cm diameter., <1 kg, firm and free from damage. Any tuber with damage as defined in USDA standards was considered a defect with the vast majority of defects due to damage from wireworms (Coleoptera: Elateridae), juvenile stages of click beetles. Remaining defects were primarily due to mechanical damage from digging or collecting soil samples, and a few miscellaneous defects such as rot. In 2019 and 2020, percent tuber surface galling (0%–100%) was estimated for each of a subsample of 50 harvested tubers. This was estimated visually by a single researcher throughout each trial to increase rating consistency. In 2018, when the resistant cultivar was grown, there was minimal tuber galling at harvest, so formal assessment was not conducted.
Soil samples were taken at preplant, midseason (approximately 2 mon after planting), and harvest each year, with precise sampling dates listed in Table 2. Twelve soil cores were collected in the root zone to 30 cm deep with a probe in each plot and homogenized. Soil samples were stored in plastic bags at 4°C for 48 hr maximum before subsequence processing. A 100 cm3 subsample soil was taken from each sample and used for nematode extraction by centrifugal-floatation method (Jenkins, 1964). Nematodes were identified to genera for plant-parasitic nematodes, and the quantity of each plant-parasitic nematode and total free-living nematodes were determined immediately after extraction with an inverted light microscope (Zeiss, Primovert) at 400× magnification.
Since the cultivar used varied by year, the data was analyzed separately by year. Southern root-knot nematode abundance, free-living nematode abundance, sweet potato tuber yield by categories, and root galling were used as response variables. Response variables were not transformed since all data met assumptions of homogeneity of variance using Levene’s test (Levene, 1960) and normality of residuals based on graphing (Cook and Weisburg, 1999). Data were statistically analyzed by ANOVA using RStudio (Version 1.2.5019, Boston, MA) and treatment means were separated using Fischer’s LSD (
Southern root-knot nematode was the major plant-parasitic nematode found in soil samples. Ring nematode (
In 2019, the initial nematode pressure was low with four SRKN juveniles/100 cm3 soil, but increased rapidly during the growing season. At midseason in 2019, all nematicide treatments decreased SRKN soil abundances significantly compared with untreated control, except for fluazaindolizine at 1.12 kg/ha and fluopyram (Fig. 2). Fluazaindolizine at 2.24 kg/ha, fluazaindolizine + oxamyl, 1,3-D, and oxamyl reduced RKN by 85%, 58%, 60%, and 81%, respectively, compared to untreated control. There were no treatment differences on SRKN populations at harvest in 2019.
In 2020, the initial nematode pressure was low with 14 SRKN juveniles/100 cm3 soil. There was a substantial increase in RKN population throughout the 2020 growing season. At midseason in 2020, 1,3-D significantly decreased RKN population by 95% compared to untreated control (Fig. 3) and was the only treatment that affected SRKN population at that time. At harvest in 2020, none of the nematicide treatments significantly reduced SRKN soil abundances relative to untreated, but 1,3-D significantly reduced SRKN soil populations compared with fluazaindolizine at 1.12 kg/ha and 2.24 kg/ha and fluopyram.
In 2018, free-living nematode soil abundances did not significantly differ by treatments at midseason. At harvest, 1,3-D reduced free-living nematode abundances by 52% compared with untreated control, and for other nematicide treatments, abundances were not significantly different from untreated control (Fig. 4). Treatments showed no significant effects on free-living nematodes (Fig. 5) throughout the 2019 trial. In 2020, all treatments decreased free-living nematode population at midseason except for oxamylalone and fluazaindolizine alone at the higher rate (Fig. 6). No treatment effects were found at harvest in 2020 (Fig. 6).
In 2018, 1,3-D significantly increased total tuber yield compared with control, fluazaindolizine at 1.12 kg/ha, and fluopyram with a 30% yield increase compared to untreated control (Fig. 7). Treatment with 1,3-D as well as fluazaindolizine at 2.24 kg/ha increased marketable yield by 55% and 31%, respectively in 2018 (Fig. 8).
Treatments did not significantly affect total tuber yield in 2019 (Fig. 7), but significantly greater marketable tuber yield was observed with fluazaindolizine + oxamyl as well as oxamyl alone compared with untreated control (Fig. 8). Compared to untreated control, fluazaindolizine + oxamyl and oxamyl increased marketable yield by 177% and 126%, respectively. No significant treatment effects on tuber galling were observed in 2019 (Table 3). Among individual yield categories, only USDA 1 yield and shape outlier cull weight were significantly affected by treatments, but USDA 2, defects, and proportion defects were not (Table 3). Tuber yield of USDA 1 grade was significantly greater for fluazaindolizine + oxamyl than any other treatment, except it was not significantly different from fluazaindolizine at 1.12 kg/ ha. Similarly, shape outlier cull weight was greater for fluazaindolizine + oxamyl than untreated control or oxamyl alone with other treatments intermediate (Table 3).
Sweet potato tuber yield (Mg/ha) by grade category and tuber gall rating as affected by nematicide treatments in 2019 and 2020.a
Treatment | USDA 1 (Mg/ha)b | USDA 2 (Mg/ha) | Defects (Mg/ha) | Size outliers (Mg/ha) | Percent defectsc | Tuber galling (%)d |
---|---|---|---|---|---|---|
2019 | ||||||
Control | 1.03 b | 2.31 | 16.2 | 0.47 b | 84 | 0.41 |
Fluazaindolizine | 2.76 ab | 2.60 | 16.29 | 0.87 ab | 74 | 1.06 |
1.12 kg/ha | ||||||
Fluazaindolizine | 2.40 b | 4.84 | 17.56 | 0.94 ab | 68 | 0.03 |
2.24 kg/ha | ||||||
Fluazaindolizine | 4.46 a | 4.70 | 16.82 | 2.49 a | 60 | 0.21 |
1.12 kg/ha + oxamyl | ||||||
1,3-D | 1.59 b | 4.17 | 20.99 | 0.85 ab | 76 | 0.67 |
Fluopyram | 0.85 b | 4.30 | 16.06 | 1.03 ab | 73 | 1.14 |
Oxamyl | 2.51 b | 4.97 | 14.92 | 0.27 b | 67 | 0.57 |
2020 | ||||||
Control | 1.73 b | 1.93 b | 11.02 | 0.27 | 73 a | 7.14 ab |
Fluazaindolizine | 1.57 b | 2.80 b | 13.71 | 0.25 | 75 a | 8.51 a |
1.12 kg/ha | ||||||
Fluazaindolizine | 1.84 b | 2.24 b | 12.84 | 0.34 | 76 a | 6.18 ab |
2.24 kg/ha | ||||||
Fluazaindolizine | 1.99 b | 2.62 b | 11.45 | 0.29 | 69 ab | 8.22 a |
1.12 kg/ha + oxamyl | ||||||
1,3-D | 7.64 a | 8.33 a | 20.16 | 0.20 | 55 b | 0.60 c |
Fluopyram | 1.84 b | 2.04 b | 17.03 | 0.27 | 81 a | 3.93 bc |
Oxamyl | 1.99 b | 1.86 b | 13.35 | 0.27 | 78 a | 6.50 ab |
aGrade categories are based on USDA grade standards (USDA AMS, 2005) as summarized in the materials and methods. USDA 1 and USDA 2 are marketable grades. Defects are unmarketable due to poor quality tubers, primarily from wireworm damage. Size outliers are unmarketable, being outside USDA 2 requirements.
bTreatments with the same letters within the same column and year are not significantly different (Fisher’s protected LSD, a = 0.05).
cPercent culls (defects and size outliers) by weight relative to total yield.
dPercent tuber surface galled, average of 50 tubers assessed at harvest.
In 2020, 1,3-D significantly increased total tuber yield by 149% compared with untreated control (Fig. 7), while no other treatments performed differently from untreated control. There was a similar trend for total marketable tuber yield (Fig. 8), USDA 1 yield, and USDA 2 yield (Table 3). Although no significant treatment effects were found on defect and outlier cull weight, 1,3-D had a significantly lower proportion of defects compared with any treatment except fluazaindoline + oxamyl in 2020 (Table 3). Significantly lower tuber galling was observed with 1,3-D with 92% gall reduction compared to control (Table 3), while no other treatment reduced galling significantly.
Many field studies have demonstrated the efficacy of fluazaindolizine against RKN on a series of crops including carrots, cucumber, tomato, and squash (Becker
Fumigation with 1,3-D was the most consistently effective nematicide at managing SRKN soil populations with efficacy in all 3 yr of testing. Past research also indicated that 1,3-D effectively managed RKN in sweet potato (Averre
Aside from which chemistries are most effective, this study provided other insights into SRKN management in sweet potato. Study results indicated that even when a moderately resistant cultivar (“Covington”) is grown, applying nematicides could improve SRKN population control, particularly at the end of the season when SRKN populations were increased. Even with a resistant cultivar, it is still valuable for growers to include a nematicide in their control program. With susceptible cultivars (2019 and 2020), nematicide efficacy was most prominent at midseason, which is common. By harvest, SRKN soil populations were substantial and nematicides did not differ from control by the end of the growing season. This shows that with a susceptible cultivar, even if SRKN population was suppressed by nematicides at the beginning of the season, the protection could not last throughout the whole growing season, and SRKN reinfestation is occurring in the soil. While nematicides can still be effective at protecting sweet potato yield, they would not be effective for managing SRKN for a subsequent crop, meaning that nematicide application must be done each growing season unless combined with other management practices.
Nematicides also varied in their efficacy at increasing sweet potato yield. Fumigation with 1,3-D was the most effective nematicide, increasing total and marketable yield relative to untreated in 2 of 3 yr (2018 and 2020). No other nematicide significantly increased tuber yield, although oxamyl alone, fluazaindolizine at 2.24 kg/ha, and fluazaindolizine + oxamyl each increased marketable yield in 1 of 3 yr. Neither the low fluazindolizine rate (1.12 kg/ha) nor fluopyram ever significantly increased sweet potato yield. Similar to SRKN soil population results, 1,3-D was the most consistently effective nematicide for increasing yield among those tested, but fluazaindolizine (at 2.24 kg/ha), oxamyl, or combinations of the two can also be effective, albeit less consistently. In general, yield benefits of nematicide application roughly corresponded to SRKN population control, in that for a given year, the nematicides that best managed SRKN soil abundances also yielded the best. This suggests that most of the yield benefits of nematicide application were due to SRKN management.
Other factors, namely wireworms (
All nematicides tested had non-target effects on free-living nematodes, although not in every sampling date or year. Fumigation with 1,3-D had the most consistent negative impacts on free-living nematodes (2 of 3 yr), which agrees with many previous studies (Collins
Various results have been reported on effects of fluopyram on free-living nematodes. Grabau
In summary, 1,3-D was the most consistent nematicide for SRKN management in sweet potato. Fluazaindolizine – particularly at a higher rate of 2.24 kg/ha – and oxamyl can be effective non-fumigant nematicides for SRKN sweet potato production, but are not as consistent as 1,3-D fumigation. Fluopyram did not show much efficacy, but more testing with drench application methods is needed. With the increasing concerns for environment protection, searching for new nematicides that can cause less harm to soil, water, and microbes are in urgent need. Our study showed that fluazaindolizine can be effective for reducing SRKN and can serve as a potential alternative to soil fumigation for sweet potato growers.