3.1. Peanut Yield and Plant-Parasitic Nematode Population in Soil in 2013
The interaction of crop sequence and tillage system was not significant for peanut yield in 2013 at either location (
Supplemental Tables S2 and S3). However, the main effect of the tillage system was significant at both locations. When pooled over the rotation sequence, peanut yield was lower in strip tillage compared to conventional tillage at both locations (
Table 3). The results for Rocky Mount in 2013 were consistent with previous findings that peanut yield in reduced tillage can be lower than the yield in conventional tillage systems on fine-textured soils [
35]. Lower peanut yield can also occur on coarse-textured soils in reduced tillage compared with conventional tillage [
35]. However, the likelihood of peanut yield being similar in reduced tillage and conventional tillage is greater when peanut is grown on coarser-textured soil [
35]. The population of stunt nematodes was greater in strip tillage compared with conventional tillage at Rocky Mount (
Table 3). However, it is unlikely that this population of nematodes in soil was the primary reason for the lower yield in strip tillage compared with conventional tillage. It is postulated that digging peanut in reduced tillage on finer-textured soils where soils are relatively flat compared with conventional tillage on these soils with raised seedbeds results in greater pod loss at digging [
35].
The peanut yield at Lewiston-Woodville in 2013 was lower when peanut was planted in two of seven years compared with the yield when peanut followed six years of cotton and corn (
Table 4). The lower yield with the additional year of peanut in the crop sequence may have been a result of injury from root-knot nematodes. A significantly higher population of root-knot nematodes was observed when the rotation had one more year of peanut (
Table 4). The lack of a response to crop sequence at Rocky Mount may have been a result of overall lower populations of dagger and ring nematodes (
Table 5). Additionally, the dagger nematode population in soil was higher when peanut was only planted in 2013. This result is unexpected because corn is a host of dagger nematodes, while cotton and peanut are not and each rotation sequence had the same number of years of corn [
35,
54]. In contrast, the population of ring nematodes was lower when peanut was planted in one year compared to two years. This is expected given peanut is a host for ring nematodes [
35].
3.2. Peanut Yield and Plant-Parasitic Nematode Population in Soil in 2019 and 2022
The interaction of the tillage system, the length of rotation relative to peanut, previous crops, and nematicide was not significant for peanut yield at either location during either year (
Supplemental Tables S3 and S4). Two-way and three-way interactions were also not significant for peanut yield. Rotation length was significant for peanut yield at Lewiston-Woodville in 2019 and 2022. No other main effects were significant for peanut yield. In 2019, peanut yield was greater when cotton or corn were the only crops in the rotation between peanut in 2013 and 2019 (
Table 6). This difference in peanut yield due to crop sequence was also noted in 2022 after two years of corn was planted in all plots during 2020 and 2021.
Plant condition, a reflection of plant health, was affected by the interaction of rotation length, previous crops, and the tillage system in 2019 at Lewiston-Woodville. When pooled over nematicide treatments, the plant-condition rating was higher when peanut was strip-tilled compared with conventional tillage and when fewer years of cotton separated peanut plantings (
Table 7). No difference in plant condition was noted between conventional and strip tillage systems for the other combinations of rotation length and previous crops. In 2022, at this location, plant condition was rated higher when peanut was strip-tilled (4.2 on a scale of 0–5) compared with conventional tillage (3.9).
At Rocky Mount, peanut yield was affected by the main effect of the tillage system during both years but not by crop sequence. When pooled over crop sequence and nematicide treatment, peanut yield was 810 kg/ha and 770 kg/ha higher in 2019 and 2022, respectively, when peanut was planted in conventional tillage compared with strip tillage. In 2019, plant condition was lower when peanut was planted into the strip tillage system compared with conventional tillage and when peanut was planted in fewer years. No difference in plant condition was noted between tillage systems when peanut was planted one more year in the crop sequence (
Table 8). However, plant condition was lower in strip tillage when peanut was not planted from 2014–2018. Plant condition did not differ when comparing rotation length relative to peanut within a tillage system.
The population of dagger nematodes was 5 individuals/500 cm
3 when comparing response to crop rotation length and previous crops at Lewiston-Woodville in 2019 (
Table 9). While differences in the population of this nematode were noted when comparing rotations, these differences are likely not biologically significant. The highest population of lesion nematodes in soil was noted when peanut was planted for fewer years and when corn was the previous rotation crop (
Table 9).
The interaction of rotation sequence, strip tillage, and nematicide was significant for spiral nematode populations in soil (
Table 10). Although spiral nematode populations were relatively low, the interaction was caused primarily by differences in the population when fluopyram was applied due to the tillage system and when peanut was planted in the shorter rotation relative to peanut. In the longer rotation, no difference in spiral populations in soil was noted when comparing combinations of rotation, tillage, and fluopyram. The cause of the change in response to fluopyram as a result of tillage in the shorter rotation is not known. The lack of difference in fluopyram treatment in the longer rotation was expected because fewer years of peanut would likely result in lower populations of this nematode in soil and there is a greater chance that no response to fluopyram would be observed.
Though corn, cotton, and peanut are non-hosts for soybean cyst nematodes [
55], relatively low populations were found in the soil. Differences in soybean cyst nematode populations due to the tillage system in 2019 and 2022 were observed at Lewiston-Woodville. When pooled over levels of other treatment factors, the population of this nematode was lower in conventional tillage in 2019 but higher in this tillage system in 2022. The apparent change in response between the two years could not be explained. Rotation sequence and fluopyram did not affect the soybean cyst nematode population in 2019 but did affect this nematode in 2022. The rotation sequence with fewer years of peanut had lower populations of soybean cyst nematode compared to the short rotation relative to peanut. This is unexpected because corn and cotton are non-hosts of soybean cyst nematodes.
In 2019 at Lewiston-Woodville, the population of root-knot nematodes in soil was affected by the main effect of rotation. When pooled over previous crops, the tillage system, and fluopyram treatment, the population of this nematode in soil was higher when peanut was planted in three years from 2013 to 2019 (1091 nematodes/cm
3) compared with planting only in 2013 and 2019 (29 nematodes/500 cm
3). This response was expected because peanut is an effective host for this nematode [
56]. Jordan et al. [
35] reported higher populations of root-knot nematodes at these locations when peanut was present in the rotation more frequently compared with corn or cotton.
The stunt nematode population was affected by the interaction of the rotation sequence and the tillage system. When pooled over previous crops and fluopyram treatment, no difference in population was observed based on the rotation when peanut was planted in conventional tillage (30 to 40 nematodes/500 cm
3. However, in strip tillage, with the longer rotation relative to peanut, a higher population of this nematode was observed in comparison to the longer rotation in conventional tillage (65 vs. 40 nematodes/cm
3 or when comparing the long rotation with the shorter rotation within the strip tillage system (65 vs. 26 nematodes/500 cm
3. Cotton is not a host of stunt nematodes [
57]; however, corn is [
58]. This could explain the higher populations of this nematode in the longer rotation sequence.
In 2022, at Lewiston-Woodville, the root-knot nematode population was affected by the interaction of rotation length relative to peanut, the previous crop in the rotation, the tillage system, and fluopyram treatment (
Table 11). No difference in the nematode population was observed in the shorter rotation relative to peanut in the absence of fluopyram, regardless of the previous crop in the rotation or the tillage system (
Table 11). In the longer rotation sequence without fluopyram, a higher population of root-knot nematodes was observed when cotton was the previous crop in the rotation in conventional tillage compared with corn in strip tillage. When fluopyram was applied, fewer differences in population were observed as a result of the rotation sequence, the previous crop, and the tillage system. When comparing fluopyram treatments based on combinations of the rotation sequence, the previous crop, and the tillage system, fluopyram lowered the nematode population when the rotation sequence was short relative to peanut and when corn was the previous crop in continuous conventional tillage or when cotton was the previous crop in continuous strip tillage. It is important to note that these differences were observed when the population of nematodes was considerably higher in the no-fluopyram control compared with the populations in the absence of fluopyram for other rotation sequence, previous crop, and tillage system combinations.
The populations of soybean cyst nematodes were lower when cotton was in the rotation rather than corn in 2019 (11 vs. 5 nematodes/500 cm
3 and 2020 (9 vs. 2 nematodes/500 cm
3 at Lewiston-Woodville. Similarly, in 2019, at this location, spiral and stunt nematode populations in soil were lower when cotton was planted compared with corn (11 vs. 2 nematodes/500 cm
3 and 57 vs. 24 nematodes/500 cm
3, respectively. This is probably due to the fact that cotton is not a host of stunt or spiral nematodes [
59].
At Rocky Mount, in 2019, the population of dagger nematodes in soil was affected by the interaction of crop rotation and fluopyram treatment. When pooled over tillage systems, the population of this nematode was highest with the longer rotation when fluopyram was applied (11 nematodes/500 cm3 compared with this rotation sequence in the absence of fluopyram (1 nematode/500 cm3 or when one more year of peanut was included in the rotation irrespective of fluopyram treatment (2 nematodes/500 cm3. These results could not be easily explained as a lower population of nematode in soil would be expected due to fluopyram treatment. A higher population of ring nematodes was noted when fewer years of peanut were present in the rotation (4723 vs. 2575 nematodes/500 cm3. Similarly, a greater number of soybean cyst nematodes were noted when peanut was included more frequently in the rotation sequence (13 vs. 2 nematodes/500 cm3. The stubby root nematode population was affected by the interaction of the tillage system and fluopyram treatment. When pooled over crop rotation, the population in soil was lower when fluopyram was applied in conventional tillage (1 nematode/500 cm3 compared with no treatment of fluopyram (6 nematodes/500 cm3 or with conventional tillage when fluopyram was applied (6 nematodes/500 cm3) compared to strip tillage when fluopyram was applied (2 nematodes/500 cm3). Fluopyram did not affect the population of this nematode in strip tillage.