Belonolaimus longicaudatus management using metam potassium and fluensulfone in potato

To investigate these objectives, a field experiment was conducted at the University of Florida Hastings Agricultural Education Center near Hastings, Florida (29.692, −81.441). The soil type was Ellzey fine sand (sandy, silicaceous, hyperthermic Arenic Ochraqualf) with 95% sand, 2% silt, 3% clay; and <1% organic matter. Production was on bareground, raised beds irrigated by a subsurface tile system, which is common in the region. Cropping history was continuous spring potato followed by summer sorghum–sudan grass cover crop. The experiment was conducted twice, with the first trial occurring in early 2020 and the second trial occurring in early 2021, with some establishment activities occurring in the preceding year for each trial. Experimental units were 4-row beds 19.8 m long. Center-to-center bed spacing was 101 cm. The experiment was a randomized complete block design with six replicates and a single factor: nematicide treatment. There were five treatments, including 1) untreated control (UTC), 2) MP alone (MP-A), 3) MP mixed with fluensulfone (MP+FL), 4) fluensulfone alone (FL-A), and 5) 1,3-D. Treatment rates and products are described in Table 1.

Metam potassium (alone or as a mixture) and 1,3-D were applied by preplant fumigation using shank injection, but fumigation rig setup varied slightly for each product according to standard practices in the region. For 1,3-D, the fumigation rig had 1 shank per bed (101 cm spacing between shanks). The MP-A and MP+FL treatments were applied using 3 shanks per bed that were spaced 13 cm apart. More shanks per bed were used for MP-A and MP+FL to ensure adequate bed coverage due to relatively low volatility of MP (Gertzl et al., 1977). For MP+FL, FL and MP were combined in the containing tank of the fumigation rig in the correct proportions before application. Fumigation was conducted 25 or 26 days before planting each season (Table 2). In all cases, nematicides were applied to flattened beds that had been tilled with a rotary chopper and subsoiler after summer sorghum–sudan grass production. Fumigants were delivered approximately 15 cm deep in flattened beds and immediately followed by a second tractor equipped with angled disc implements that formed a hilled bed of loose soil in fumigant-treated plots. The fumigation rig that delivered 1,3-D was also equipped with angled discs directly on the rig. After bed formation, final fumigant placement was at least 30 cm below the tops of beds. Bed formation is the standard practice for containing fumigants in the soil—as required on fumigant labels—in potato production in the area.

For the standalone treatment, FL was sprayed on top of the soil as a broadcast application using a commercial tractor and boom sprayer. Immediately following application, fluensulfone was incorporated to 15 cm by tilling with a rotary chopper. Fluensulfone was applied on the same day as fumigant treatments, either 25 or 26 days before planting (Table 2). On the same day, following application of all treatments, individual fluensulfone and untreated control plots were angle-disced to form beds. This ensured that all plots were bedded the same number of times. Then, the entire trial was angle-disced to form uniform beds.

One to two weeks before planting (Table 2), preplant fertilizer was applied, followed by incorporation with a rotary chopper after which beds were re-formed. In each trial, “Red LaSota”, a red fresh market potato common in the area, was grown. Seed potatoes were planted in January (Table 2) using a mechanical planter. The planter was equipped with a tillage implement that opened a furrow in the beds. Seed pieces were planted at approximately 21 cm spacing in the seed furrow. After the trial was planted, a tank-mix of standard fungicide (azoxystrobin at 191 g a.i./ha) and insecticide (fipronil at 98 g a.i./ha) treatments were sprayed in furrow. The furrow was then closed, and the beds were formed again. Aside from nematicide treatments, each year, the whole trial was managed uniformly with full herbicide, fungicide, and insecticide regimes as standard for the area.

Environmental conditions can affect nematicide efficacy, and soil moisture and temperature requirements are listed on fumigant labels. Soil moisture was not quantified formally at the site but was acceptable at fumigation for both the 2020 and 2021 trials based on the USDA-established feel test (USDA-NRCS, 2005). Early season soil temperature, air temperature, and rainfall are described in Table 3 based on information from the Florida Automated Weather Network (https://fawn.ifas.ufl.edu/) station at the study site. Briefly, the 2020 trial was very dry between fumigation and planting, whereas the 2021 trial received more regular rainfall during that period (Table 3). In both years soil temperatures were acceptable for fumigation, with higher temperatures in 2020 than 2021.

Potatoes were grown for approximately 100 days before harvest in late April or early May (Table 2). From the central 2 rows of each plot, potatoes were harvested from 6.1 m of each row using a single-row mechanical harvester. Separately by plot, harvested potatoes were washed and graded using a mechanical size sorter. In 2021, the mechanical size sorter was supplemented by human intervention to remove culls with external quality defects. In 2020, quality culls were not removed or measured due to workforce restrictions during the COVID-19 pandemic. For each plot, potatoes were weighed for each grade category. Size classes were A3 (8.3–10.2 cm diameter), A2 (6.4–8.29 cm diameter), A1 (4.8–6.39 cm diameter), B (3.8–4.69 cm diameter), and C (1.3–3.79 cm diameter) based on local and USDA guidelines (USDA-AMS, 2011). No potatoes with diameter greater than 10.2 cm (class A4) were observed in either trial. All class A potatoes were considered marketable (USDA-AMS, 2011). In 2021, the categories of quality-related culls included tubers that were rotted, green (sun damage), had growth cracks, or were misshapen. In addition to weight by each cull category, the total weight of quality culls and the grand total cull weight (quality culls plus undersize tubers) was reported in 2021. For both years, total cull weight of tubers that were under marketable size (class B and C), as well as grand total yield is reported. For each plot, 20 tubers were quartered and inspected for internal defects in each trial, but there were negligible defects, so this data was not reported.

Plant stand was estimated by counting emerged shoots from 2.4 m in each of the central 2 rows of each plot and converted to stand per 3 m row for reporting. In both 2020 and 2021, plant stand was estimated twice: (1) when shoots started to emerge and (2) when all shoots had fully emerged (Table 2). Plant vigor was rated visually 3 to 4 times during the middle of the growing season (Table 2). Vigor was assessed from central 2 rows of each plot based on a 0 to 100 scale where 0 indicates all plants dead, 25 is a very poor crop, 50 is a below-average crop, 75 is an average crop, and 100 is an exceptionally excellent crop for the given time point. Vigor assessment was based primarily on the canopy height and size. Additionally, symptoms of phytotoxicity were observed in the 2020 trial and were rated at 37 days after planting (DAP). These symptoms are described further in the results.

Nematode soil abundances in each plot were quantified before nematicide application (preplant), at midseason (6 weeks after planting), and near harvest each year (Table 2). Using an Oakfield tube, 12 soil cores to 25 cm depth were collected from the central 2 beds of each plot, with samples collected near plant roots at midseason and harvest. Soil was homogenized manually, and nematodes were extracted using the sucrose-centrifugation method (Jenkins, 1964). Plant-parasitic nematodes were identified morphologically and quantified by microscope. Total free-living nematode abundances were also quantified, but taxonomic identification of free-living nematodes was not completed.

Each variable was analyzed separately for each trial and sampling date using one-way ANOVA. Before completing ANOVA, response variables were transformed, if needed, to meet assumptions of homogeneity of variance using Levene’s test (Levene, 1960) and normality of residuals based on graphing (Cook and Weisburg, 1999). Nematicide effects were considered significant at α=0.05 in ANOVA. Nematicide treatment means were separated by Fisher’s protected LSD (α=0.05) if main effects were significant in ANOVA. Analyses were conducted in R statistical software (version 3.4.4, The R Foundation for Statistical Computing, Vienna, Austria).

In both 2020 and 2021, MP-A or MP+FL generally significantly reduced plant stand, particularly for the earlier assessment conducted at seedling emergence (Fig. 1). In 2020, MP-A or MP+FL significantly decreased plant stand at 30 DAP relative to all other treatments. In 2020, stand at 37 DAP—when plants had fully emerged—was significantly less for MP-A than any other treatment and less for MP+FL than FL-A. At 36 DAP in 2021, MP+FL significantly reduced stand relative to all other treatments and MP-A reduced stand relative to 1,3-D. By 44 DAP, MP+FL had less stand than 1,3-D or FL-A, but there were no other significant differences among treatments.

Figure 1:

In 2020, MP-A and MP+FL significantly reduced plant vigor compared with any other treatment at each assessment date (Figs. 2, 3). Qualitative, visual symptoms of phytotoxicity—particularly browning of shoot tips and delayed leaf emergence producing a burnt appearance—were observed for those treatments in 2020 beginning at 30 DAP (Fig. 4). When phytotoxicity was rated at 37 DAP in 2020, symptoms were only observed for MP-A and MP+FL (Fig. 2). By the next assessment date (50 DAP), no browning was observed on leaves.

Figure 2:

Figure 3:

Figure 4:

In 2021, for all assessment dates, plant vigor was significantly less for MP+FL than any other treatment, except that it was similar to MP-A at 59 DAP (Fig. 5). Plant vigor was also significantly less for MP-A than 1,3-D at each assessment date and less than FL-A at 36 and 59 DAP. Later in the season (50 and 59 DAP), plant vigor was significantly less for UTC than 1,3-D.

Figure 5:

In 2020, total and marketable yields were significantly greater for 1,3-D than MP-A and UTC (Fig. 6). Total and marketable yields were also significantly greater for FL-A than MP-A in 2020. Class A1 and A2 yields were significantly affected by nematicides in 2020 and followed the same trends as marketable yield (Table 4). No other size category was significantly affected by nematicide treatments in 2020.

Figure 6:

In 2021, total yield was significantly greater for MP-A and 1,3-D than any other treatment. Marketable yield in 2021 followed a similar trend except that MP+FL had similar yield to 1,3-D. In 2021, there were no size A3 potatoes and A1 yield was not significantly affected by nematicide treatments (Table 4). Class A2 yield was significantly affected by nematicides in 2020, with yield significantly greater for MP-A than each treatment except 1,3-D and significantly greater for 1,3-D than UTC or FL-A. In 2021, the only year quality-related culls were reported, rotted and grand total cull weights were significantly affected by nematicides and generally followed the same trends as marketable yield (Table 5). In contrast, green cull weight was significantly greater for MP-A than UTC, FL, or 1,3-D and greater for MP+FL than UTC or FL-A. Growth crack, misshapen, or total quality culls were not significantly affected by nematicide treatments in 2021.

Each year, sting nematode abundances were relatively low before fumigation and did not vary significantly based on upcoming treatments (Fig. 7). Sting nematode abundances were generally greater for UTC or FL-A than treatments including MP or 1,3-D at midseason in 2020 and 2021 as well as harvest in 2021, but exact significance of treatment separation varied by season (Fig. 7). Sting nematode abundances were low and unaffected by nematicides at harvest in 2020.

Figure 7:

At harvest in 2020, stunt nematode abundances were significantly greater for UTC, FL-A, or 1,3-D than MP-A or MP+FL (Fig. 8). At midseason and harvest in 2021, stunt nematode abundances were significantly greater for UTC or FL-A than any other nematicide treatment. Stunt nematode abundances were not significantly affected by nematicides at midseason in 2020 or preplant in either year.

Figure 8:

Free-living nematode soil abundances were significantly affected by nematicides only at harvest in 2020 (Fig. 9). In that season, free-living nematode abundances were significantly greater following 1,3-D than MP-A or MP+FL and greater following FL-A than MP+FL.

Figure 9: