Sweetpotato (
Root-knot nematodes (RKN:
Some cultivars (e.g. Covington, Murasaki) have good RKN resistance, but under high soil temperatures, even resistant cultivars can still result in a large RKN population increase during one crop cycle (Roberts and Scheuerman, 1984). Furthermore, although storage root quality of resistant cultivars was not affected by RKN, yield losses resulting from RKN were still considerable, and additional management strategies are needed in RKN infested fields, even when growing RKN-resistant cultivars (Roberts and Scheuerman, 1984).
Typically soil fumigants are used to control RKN both in nursery hotbeds and in production fields. According to 2015 data (CA-DPR), sweetpotato was among the five crops in California with the highest use of the fumigant 1,3-dichloropropene (2,999 ha). Other fumigants used in sweetpotato in California are metam-potassium (809 ha) and metam-sodium (33 ha). As they are potential environmental and health hazards, they are limited by regulatory restrictions related to the emission of volatile organic compounds (VOC) and their toxicity. Until recently, effective, environmentally acceptable, and economically viable alternatives were not available, and this has been an important factor in the continued use of soil fumigants (Noling and Becker, 1994; Becker, 2014). Fluensulfone (tradename: Nimitz, ADAMA Agricultural Solutions Ltd., Raleigh, NC) is a non-fumigant nematicide that is registered for use in fruiting vegetable crops in California. It has a ‘caution’ label and no re-entry interval (0 hr REI) after application. The product is applied pre-plant, either by chemigation through the drip tubing, or by soil incorporation at rates between 4.1 and 5.8 liter/ha (
The goal of this two-year field study was to evaluate the effectiveness of fluensulfone in comparison to an untreated control and to metam-sodium in sweetpotato grown on an uniformly
The trials were located on a field with sandy-loam soil (70% sand, 18% silt, 12% clay, 0.1% organic matter, pH 7.3) at the University of California South Coast Research and Extension Center, Irvine, CA. The field had been inoculated five years previously with an egg suspension of a
The trials were conducted in 2016 and 2017 on different, but nearby areas of the field. In both years, 152 cm wide (center to center) beds were prepared in May and plots were laid out. Individual plots were 6.1 m long sections of bed, separated along the beds by a 91 cm border section. The experiment was designed according to a completely randomized block design with five replicates and four treatments. In both years treatments included an untreated control, a Vapam (a.i. metam-sodium) treatment at 701 liter/ha (294 liter a.i./ha), and two fluensulfone treatments. In 2016, the fluensulfone treatments were (i) Nimitz at 7 liter/ha (3.36 kg a.i./ha, pre-plant incorporated) and (ii) Nimitz at 7 liter/ha (3.36 kg a.i./ha, pre-plant incorporated) followed by two post-plant spray applications of 3.5 liter/ha (1.68 kg a.i./ha) at 26 and 58 d after planting. In 2017, fluensulfone treatments were (i) Nimitz at 5.8 liter/ha (2.8 kg a.i./ha) and (ii) Nimitz at 4.1 liter/ha (1.96 kg a.i./ha) both pre-plant incorporated. Vapam was applied 21 and 26 d before planting in 2016 and 2017, respectively. Pre-plant Nimitz applications were 2 and 7 d before planting in 2016 and 2017, respectively. Amounts applied per plot were based on the bed surface area of each plot (5.88 m2). All plots were pre-irrigated for 1 hr with overhead sprinklers the day prior to any pre-plant application to achieve adequate soil moisture. For each plot, Vapam and pre-plant Nimitz were suspended in 7.6 liter of water and watered evenly over the plot surface with a watering can. An additional 45.4 liter of water was applied over each plot, and the plots were tilled with a rototiller to a depth of 10 to 13 cm. Post-plant Nimitz applications were applied in 7.6 liter of water with a backpack sprayer over the crop foliage.
For RKN analysis, a composite sample consisting of six cores of soil (1.5 cm diameter, 5–30 cm depth) was collected from each plot just before applying Vapam (initial population: Pi) and just before harvest (final population: Pf). Nematodes were extracted from 100 g soil subsamples in a modified Baerman-funnel technique (Rodriguez-Kabana and Pope, 1981), and RKN J2 were counted at ×40 magnification.
Rootless slips of the RKN-susceptible cultivars O’Henry and Beauregard were planted on June 10, 2016 and May 18, 2017, respectively. The slips were planted in pre-wetted beds at 41 cm within-row spacing, with two rows per bed, resulting in 30 slips per plot. At planting, approximately 0.5 liter water was added to each cutting, and irrigation was through drip tubing (drip emitters 2 liter/hr, 30.5 cm spacing) on top and in the center of the beds. Fertilization was according to standard practices, applied pre-plant incorporated and post-plant through the drip tubing. Weeds were removed by hand, and no fungicides or insecticides were required. In total, 20 and 50 d after planting, the general vigor of each plot was visually examined and indexed (1–10 scale). Plots were harvested mechanically on October 9, 2016 and September 22, 2017. For each plot, total yields (weight and number of roots) were determined. In total, 20 roots were randomly collected from each plot, and assigned to one of three categories: marketable, non-marketable because of RKN damage, and non-marketable because of defects not related to RKN. The weight of these roots in each category was determined. In addition, 10 randomly selected roots from each plot were taken to the laboratory and cut in half cross-wise. One half was discarded. The 10 remaining half roots were weighed and then peeled with a potato peeler. Nematode eggs were extracted from both the peels and the peeled roots by shaking for 3 min in a 0.5% NaOCl solution (Hussey and Barker, 1973) and collected by washing over two stacked 25 μm pore-size sieves. The eggs were counted at ×40 magnification.
Treatment effects on nematode counts, crop vigor, sweetpotato yield, and sweetpotato quality were analyzed using an analysis of variance (ANOVA) procedure, and means were compared using Fisher’s protected least significant difference (LSD) test (
General growing conditions for the trial were excellent in both years, and nearly 100% of planted slips survived. In both trial years, crop vigor was not affected by the treatments (Table 1). In 2016, effects of the two fluensulfone treatments on sweetpotato yields (kg) were highly significant. Both fluensulfone treatments more than doubled the overall yield relative to the untreated control (Table 2). In 2017, the fluensulfone treatments yielded about 9 kg/plot more than the untreated controls, but these differences were not significant. In both years, the fluensulfone treatments dramatically increased the marketable yield compared to the untreated control. The metam-sodium treatment failed to improve sweetpotato yields (quantity, quality) and was not significantly better than the untreated control. When examining the yields as percentages from the total yield, the same general effects exist (Table 3). Compared to the untreated control, the percentage of harvested roots culled because of obvious RKN symptoms (bumpiness, cracking) was reduced by the fluensulfone treatments in both years. In 2016, the percentage of roots culled because of other reasons (insect damage, too small, misshapen) was significantly higher in both fluensulfone treatments, but this was not the case in 2017. Metam-sodium treatments did not significantly affect the relative tuber yields in the three different quality classes (marketable, cull RKN, cull other) compared to the untreated control in either year.
Average (
Vigor rating (days after planting) | ||
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Treatment | 20 | 50 |
|
||
1. Untreated Control | 7.4 ± 0.89 | 7.2 ± 0.84 |
2. Metam-sodium (294 liter/ha) | 8.0 ± 0.71 | 7.8 ± 0.45 |
3. Fluensulfone pre-plant (3.36 kg/ha) | 7.6 ± 0.89 | 7.6 ± 0.55 |
4. Fluensulfone pre-plant (3.36 kg/ha) and 2× post (1.68 kg/ha + 1.68 kg/ha) | 7.8 ± 0.45 | 7.6 ± 0.55 |
treatment |
0.62 | 0.56 |
|
||
1. Untreated Control | 4.8 ± 0.49 | 6.0 ± 0.32 |
2. Metam-sodium (294 liter/ha) | 6.0 ± 0.89 | 6.2 ± 0.37 |
3. Fluensulfone pre-plant (1.96 kg/ha) | 7.2 ± 0.66 | 7.2 ± 0.37 |
4. Fluensulfone pre-plant (2.8 kg/ha) | 6.4 ± 0.81 | 6.4 ± 0.40 |
Treatment |
0.19 | 0.20 |
Average yield (
Sweetpotato Yield (kg/plota) | ||||||||
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Treatment | Total | Market | Cull RKN | Cull other | ||||
|
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1. Untreated Control | 14.9 ± 1.5 | bb | 0.8 ± 0.4 | b | 10.5 ± 1.5 | a | 3.6 ± 1.2 | b |
2. Metam-sodium (294 liter/ha) | 19.7 ± 5.0 | b | 0.9 ± 0.3 | b | 11.7 ± 1.5 | a | 7.0 ± 3.5 | b |
3. Fluensulfone pre-plant (3.36 kg/ha) | 29.6 ± 3.5 | a | 8.2 ± 0.2 | a | 4.6 ± 1.3 | b | 16.8 ± 3.0 | a |
4. Fluensulfone pre-plant (3.36 kg/ha) and 2× post (1.68 kg/ha + 1.68 kg/ha) | 29.8 ± 3.0 | a | 10.1 ± 0.4 | a | 3.6 ± 0.7 | b | 16.1 ± 2.6 | a |
Treatment |
0.01 | 0.0001 | 0.0003 | 0.006 | ||||
|
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1. Untreated Control | 24.8 ± 2.7 | a | 6.7 ± 1.9 | b | 15.0 ± 3.9 | a | 3.1 ± 0.6 | a |
2. Metam-sodium (294 liter/ha) | 27.7 ± 2.8 | a | 9.9 ± 1.0 | b | 12.5 ± 2.7 | a | 5.3 ± 1.2 | a |
3. Fluensulfone pre-plant (1.96 kg/ha) | 34.0 ± 2.4 | a | 18.4 ± 2.6 | a | 12.0 ± 2.6 | a | 3.5 ± 1.4 | a |
4. Fluensulfone pre-plant (2.8 kg/ha) | 33.0 ± 3.6 | a | 23.3 ± 3.4 | a | 7.1 ± 1.4 | a | 2.6 ± 0.8 | a |
Treatment |
0.13 | 0.002 | 0.32 | 0.30 |
Average percentage (
Sweetpotato yield (%) | ||||||
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Treatment | Market | Cull RKN | Cull other | |||
|
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1. Untreated Control | 6.6 ± 4.3 | ba | 70.3 ± 8.1 | a | 23.3 ± 6.0 | b |
2. Metam-sodium (294 liter/ha) | 5.3 ± 2.3 | b | 66.3 ± 6.9 | a | 28.5 ± 7.4 | b |
3. Fluensulfone pre-plant (3.36 kg/ha) | 28.3 ± 5.3 | a | 16.4 ± 5.5 | b | 55.4 ± 3.8 | a |
4. Fluensulfone pre-plant (3.36 kg/ha) and 2× post (1.68 kg/ha + 1.68 kg/ha) | 35.1 ± 2.9 | a | 12.0 ± 2.1 | b | 53.0 ± 4.3 | a |
Treatment |
0.0001 | 0.0043 | 0.003 | |||
|
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1. Untreated Control | 29 ± 8.0 | c | 58 ± 9.4 | a | 13 ± 3.0 | a |
2. Metam-sodium (294 liter/ha) | 37 ± 4.0 | bc | 44 ± 7.0 | ab | 19 ± 4.3 | a |
3. Fluensulfone pre-plant (1.96 kg/ha) | 54 ± 6.6 | ab | 35 ± 7.6 | bc | 11 ± 5.1 | a |
4. Fluensulfone pre-plant (2.8 kg/ha) | 70 ± 3.2 | a | 21 ± 2.9 | c | 9 ± 2.9 | a |
Treatment |
0.005 | 0.02 | 0.27 |
The average RKN J2 levels at the start of the trial were 15.8 and 47.5 J2 per 100 g soil in 2016 and 2017, respectively (Table 4). In both years, these pre-treatment nematode levels were not significantly different among the treatments. At harvest, nematode levels had increased about 13-fold in 2016 and 8-fold in 2017 and were not significantly different among the four treatments. In both years, the level of nematode infestation of the harvested sweetpotato roots however was significantly lowered by the fluensulfone treatments resulting in a reduction of the egg load of the roots by over 80% compared to the untreated control. The metam-sodium treatment did not result in a significant reduction in sweetpotato root infestation levels at harvest relative to the untreated control.
Average root-knot nematode levels (
J2 per 100 g soil | ||||||
---|---|---|---|---|---|---|
Treatment | Pre-plant (Pi) | Post-plant (Pf) | Eggs per g sweetpotato | |||
|
||||||
1. Untreated Control | 23 ± 16 | aa | 198 ± 42 | a | 536 ± 38 | a |
2. Metam-sodium (294 liter/ha) | 12 ± 5 | a | 300 ± 61 | a | 573 ± 133 | a |
3. Fluensulfone pre-plant (3.36 kg/ha) | 14 ± 6 | 173 ± 51 | a | 79 ± 17 | b | |
4. Fluensulfone pre-plant (3.36 kg/ha) and 2x post (1.68 kg/ha + 1.68 kg/ha) | 14 ± 8 | a | 156 ± 33 | a | 98 ± 34 | b |
Treatment |
0.95 | 0.29 | 0.0001 | |||
|
||||||
1. Untreated Control | 21.2 ± 9.1 | a | 360 ± 107 | a | 304 ± 46 | a |
2. Metam-sodium (294 liter/ha) | 25.0 ± 12.3 | a | 261 ± 52 | a | 228 ± 87 | a |
3. Fluensulfone pre-plant (1.96 kg/ha) | 34.4 ± 14.4 | a | 396 ± 80 | a | 37 ± 16 | b |
4. Fluensulfone pre-plant (2.8 kg/ha) | 49.0 ± 17.8 | a | 532 ± 132 | a | 21 ± 5 | b |
Treatment |
0.65 | 0.54 | 0.0005 |
The earliest report of fluensulfone use against nematodes was from 2010 showing that the nematicide reduced root-galling and increased yield of tobacco grown in a
In both years, the RKN J2 soil populations at harvest were similar among the treatments. This was true also in previous field trials on carrot (Ploeg et al., 2013) and tomato (Becker et al., 2013), although others did find that fluensulfone resulted in significant reductions in RKN J2 populations at harvest in field or microplot trials with tobacco (Csinos et al., 2010), tomato (Morris et al., 2015), and lima bean (Jones et al., 2017). In our trials, the metam-sodium treatment did not differ from the untreated control. This may have been due to the relatively superficial incorporation of the product and the failure to provide an adequate seal post application. The positive effect of fluensulfone on the marketable root yield was reflected in its ability to strongly reduce nematode infestation of the harvested storage roots, even though soil RKN populations were not lowered. This suggests that nematode infestation of the storage roots was more effectively controlled than of the feeder roots and that the increase in nematode soil levels in the fluensulfone treatments was mostly the result of nematode multiplication in the feeder roots. Possibly, the developing young storage roots are most susceptible to RKN infestation, when the activity of fluensulfone is still high, and lose their susceptibility as they develop, while the feeder roots remain susceptible throughout the crop cycle. The observed outcome is similar to what Roberts and Scheuerman (1984) observed after growing nematode resistant sweetpotato cultivars: the storage roots remained virtually free of nematode symptoms while post-harvest soil RKN populations increased. Villordon et al. (2009) noted that storage root development in sweetpotato is largely determined in the first 17 d after transplanting.
These trials show that fluensulfone when applied as an incorporated soil drench at least 2 d before planting a nematode-susceptible sweetpotato cultivar in RKN (