Meloidogyne incognita Management Using Fumigant and Non-fumigant Nematicides on Sweet Potato

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 et al., 2017; Beuzelin et al., 2021). No fungicides or insecticides were applied in any year. Trial sites at the center were selected for presence of SRKN. The trial sites in 2018 and 2020 were grown to sorghum (Sorghum bicolor) the previous year and peanut (Arachis hypogea), field corn (Zea mays), and winter rye (Secale cereale) cover crop were among previous crops. The 2019 site was grown to cotton (Gossypium hirsutum) the previous year and had a history of both vegetable and agronomic crops.

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.

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 (P = 0.05) when treatment effects were statistically significant in ANOVA (P < 0.05).

Southern root-knot nematode was the major plant-parasitic nematode found in soil samples. Ring nematode (Mesocriconema sp.) and stubby-root nematode (Paratrichodorus sp.) were also found at the experimental sites. Ring and stubby-root nematode soil abundances found at the site were not formally analyzed since their presence was inconsistent and neither nematode is reported to be damaging on sweet potato. In 2018, nematode abundance at the site was 180 SRKN J2/100 cm³ soil before planting. Southern root-knot nematode abundance was not affected by treatments at midseason but was affected at harvest in 2018 (Fig. 1). Fluazaindolizine at 1.12 kg/ha, 2.24 kg/ ha and fluazaindolizine + oxamyl significantly reduced nematode abundance by 73%, 62%, and 69%, respectively, compared to untreated control. Among treatments that included fluazaindolizine, there was no significant variation in level of suppressing SRKN abundances in 2018. None of the other nematicide treatments significantly reduced SRKN abundances relative to control in 2018.

Figure 1

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.

Figure 2

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.

Figure 3

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).

Figure 4

Figure 5

Figure 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).

Figure 7

Figure 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).

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.