Fluensulfone and 1,3-dichloroprene for plant-parasitic nematode management in potato production

The field trials were located at the University of Florida Hastings Agricultural Education Center in Hastings, Florida (29.692, −81.441). Soil at the field site was an Ellzey fine sand (sandy, silicaceous, hyperthermic Arenic Ochraqualf) with 95% sand, 2% silt, 3% clay, and <1% organic matter. The experimental units were field plots of four rows spaced 102 cm apart and 26 m long. The study was a randomized complete block design with six replicates and a single factor – nematicide application. The study was conducted in 2016, 2017, and 2018 at the same field site and treatments were not re-randomized each year. There were six nematicide application treatments as describe in Table 1: (i) fluensulfone at 1.40 kg a.i./ha, (ii) fluensulfone at 1.96 kg a.i./ha, (iii) fluensulfone at 2.80 kg a.i./ha, (iv) fluensulfone at 3.92 kg/ha, (v) 1,3-D at 60.90 l/ha, and (vi) untreated control.

Nematicide applications were made in early January, two to four weeks before planting. In 2016, 2017, and 2018, nematicides were applied 21, 15, and 27 d before planting, respectively. The time between nematicide application and planting varied by year based on weather conditions, which limited the ability to access the field for planting. See Table 2 for specific dates of nematicide application and other trial maintenance tasks. Before nematicide application, the field was chopped using a rotary-type spade to a depth of 15 cm. Fluensulfone treatments were applied on a broadcast basis across all four rows of the selected plots using tractor-driven spray boom delivered via flat fan nozzles. Nozzles were located 51 cm or less above ground, according to label instructions. Following fluensulfone application, the field was tilled to a depth of 15 cm again using a rotary spade, immediately followed by constructing hilled beds using two bedding discs per bed, and shaping beds with a bed shaper to press and flatten beds.

In plots that received 1,3-D treatment, immediately following pre-plant tillage, 1,3-D fumigant was injected into the soil to 25 cm via shanks spaced at 102-cm intervals to achieve in-row treatments. Immediately following fumigant injection, the beds were formed and shaped. Rainfall, soil, and air temperature for the four weeks before and after nematicide application in 2016 to 2018 is provided in Table 3 as moisture and temperature may affect pesticide efficacy. Rainfall and temperature information were collected from the Florida Automated Weather Network weather station at the Hastings Agricultural Extension Center where this study was conducted. Conditions were moist when nematicides were applied in both 2016 and 2018, but somewhat drier in 2017. In all three years, 2 cm or more rainfall occurred within 3 d after nematicide application – a factor that is important because irrigation or rainfall is recommended after fluensulfone application to help incorporate the product into soil.

Potato seed pieces (Red LaSoda cultivar) were planted approximately three weeks later (Table 2). The trial was maintained uniformly with standard fertilizer and pest management practices for the region (Zotarelli et al., 2017). Plots were irrigated by subsurface tile drainage as needed throughout potato production. Sorghum-sudangrass (Sorghum bicolor) was grown as a summer cover crop before each season. This is the standard cover crop for the region and is susceptible to most of the nematodes present in this study, so growing it helped maintain nematode populations. Potatoes were harvested in late May or early June (Table 2) from 6.1 m of the center two rows of each plot using a single-row digger. All potatoes harvested from these sections were graded and weighed by size class for marketable tubers and damage category for cull, unmarketable tubers. The marketable size classes were A1 (8.4-10.2 cm diameter), A2 (6.4-8.3 cm), A3 (4.8-6.3 cm), B (3.8-4.7 cm), and C (1.3-3.7 cm). Total yield for all grade A potatoes was also calculated and abbreviated as A123. Any potatoes that were smaller than size class C, rotten, green from sunburn, had growth cracks, or were misshapen were considered culls and cull weights in each category were weighed. A 20-potatoes subsample from each plot was also assessed for internal defects including corky ringspot, internal heat necrosis, and hollow heart.

Soil samples were collected for nematode analysis before nematicide application and around harvest (Table 2). In each plot, 12 soil cores to a depth of 30 cm and 2 cm diameter were collected using an Oakfield tube. Cores were collected from the center two hilled beds in each plot. At harvest, cores were collected close to potato plants and cores were collected from bare ground before planting since beds were fallow. Samples were homogenized in plastic bags and nematodes were extracted from 100 cm³ soil. In 2016 and 2017, nematodes were extracted by Baermann funnel (Southey, 1986) whereas in 2018, nematodes were extracted by sucrose floatation and centrifugation (Byrd et al., 1976). After nematodes were extracted, plant-parasitic nematodes were identified to genera and quantified using an inverted, light microscope. Sting nematode, lesion nematode (Pratylenchus sp.), and stubby-root nematode were present across the sites in each year of the study. Stunt nematodes (Tylenchorhynchus sp.) were present across the site in 2018.

Response variables (nematode abundances and potato yield or culls by category) were analyzed by separately by year and season because nematode extraction methods differed by year and treatment effects varied by year and season. Response variables were subjected to ANOVA using SAS. Response variables were not transformed before analysis. For variables with significant (P < 0.05) treatment effects in ANOVA, means were separated by Fisher’s least significant differences at α = 0.05.

Sting nematode abundance before planting was not significantly different among nematicide treatments in any year (Fig. 1). In all three years, sting nematode abundance at harvest were significantly greater in the untreated control than any of the 1,3-D or fluensulfone treatments. In 2017 and 2018, at harvest there were no significant differences in sting nematode abundance among fluensulfone rates or the 1,3-D treatment (Fig. 1). At harvest in 2016, there were more sting nematodes at the lowest rate of fluensulfone than the 1,3-D treatment. Nematicide treatments decreased sting nematode abundance at harvest by 52 to 91%, 66 to 100%, and 55 to 84% in 2016, 2017, and 2018, respectively.

Figure 1:

Stubby-root nematodes were present at very low abundance in 2016 and 2017 and were not affected by nematicide treatments except before planting in 2017 (Fig. 2). In 2018, stubby-root nematode abundances varied somewhat across treatments before planting. Fewer stubby-root nematodes were present at harvest than before planting, but were not affected by nematicide treatments at harvest.

Figure 2:

Lesion nematode abundances were not significantly different among nematicide treatments before planting in any year (Fig. 3). Nematicide impacts on final lesion nematode abundances varied among years. There were no significant effects in 2016, all nematicides effectively reduced lesion nematodes compared with the untreated control in 2017, and 1,3-D reduced lesion nematode abundances compared with any other treatment in 2018. Stunt nematode were present in large numbers in 2018 (219 and 888 nematodes/100 cm³ soil before planting and at harvest, respectively), but their numbers were not significantly affected by nematicide treatments.

Figure 3:

Total marketable yield of potato was significantly affected by nematicide treatments each year, but trends varied by year (Fig. 4). In 2016 and 2018, marketable yields were generally greater for fluensulfone or 1,3-D treatments than the untreated control, but did not vary among fluensulfone or 1,3-D treatments. In 2016, lower rates of fluensulfone (3 and 4 l/ha) had statistically similar yields to the untreated control and, in 2018, fluensulfone at 8 l/ha was not different from the untreated control. In 2016, fluensulfone (6 and 8 l/ha) and 1,3-D increased marketable potato yield 5,800 to 6,000 kg/ha or 49 to 66% relative to the untreated control. In 2018, fluensulfone (3, 4, and 6 l/ha) and 1,3-D increased marketable potato yield 4,400 to 7400 kg/ha or 33 to 55%. The trend was different in 2017 as marketable yields were greater at low fluensulfone rates (3, 4, and 6 l/ha) than the untreated control, but fluensulfone at 8 l/ha and 1,3-D treatments had similar yields to control and significantly lesser yield than 3 and 4 l/ha fluensulfone. In 2017, fluensulfone (3, 4, and 6 l/ha) increased marketable potato yields 5,300 to 7,800 kg/ha or 41 to 61% relative to the untreated control.

Figure 4:

When potato yield was analyzed by size grade, nematicides affected primarily grade A yield and trends were generally similar to trends for total marketable yield (Table 4). In 2016, nematicides significantly affected A1, A2, and A123 with yields generally significantly lesser for control than 1,3-D and greater fluensulfone rates. This was similar to overall trends for total marketable yield. In 2017, nematicides affected all grade A yield categories. Yields were generally lesser for 1,3-D and greater fluensulfone rates than lesser fluensulfone rates which was similar to overall trends for total marketable yield. In 2018, nematicides affected grades A1 and A123 yields with yields greater for fluensulfone at 3 and 4 l/ha and 1,3-D than the untreated control. Grades A1 and A123 yields for fluensulfone at 6 and 8 l/ha were not significantly different than the untreated control. Nematicides did not significantly affect grade C yields in any year or grade B yields in 2016 or 2017. In 2018, grade B yields were significantly greater for fluensulfone at 3 l/ha than the untreated control, but not different among other treatments.

Nematicide application did not significantly affect cull weight in any category in 2016 or 2017 (Table 5). In 2018, cull weight of rotted tubers was significantly greater for 1,3-D than fluensulfone at 4 l/ha or the untreated control and misshapen cull weight was significantly greater for 1,3-D than any treatment except fluensulfone at 6 l/ha. Incidence of internal defects was low (generally less than 1%) and nematicide application did not affect incidence of internal defects consistent with CRS, internal heat necrosis, or hollow heart in any year.