Collectively, SAP increased peanut emergence compared to treatments without SAP in 2017 at EREC (P = 0.0085) by 0.9 plants/m, but this was not observed the subsequent year where SAP tended to weakly decrease emergence by 0.3 plants/m (P = 0.0854) (Table 2). The difference in emergence may have been associated with soil conditions when comparing the two years. Weather conditions were relatively dryer between planting and emergence counts in April and May in 2018 than in 2017 at EREC (19 mm in 2017 and 0 mm in 2018), whereas rainfall during that time in 2018 at PREC was increased by 5.3 mm compared to 2017 (Weather Underground 2019). Lack of precipitation at EREC during the emergence period would plausibly prevent SAP's water-holding capacity from providing those peanut seeds with greater stored moisture compared to peanut seed planted without SAP, and so it is not unexpected that emergence under those conditions did not differ among SAP treatments. The only significant interaction of SAP for emergence was at PREC in 2018 where its interaction with insecticide was at P = 0.0329. From this interaction, imidacloprid without SAP had greater emergence (by 1.4 plant/m) than imidacloprid with SAP, whereas phorate with SAP did not increase emergence compared to phorate without SAP (Table 2).

Cultivar susceptibility affected stand counts across the locations and years examined (P < 0.007), with the moderately susceptible cultivar (Georgia 06G) having less emerged plants than resistant and susceptible cultivars. Insecticide (P = 0.0242) and its interaction with cultivar susceptibility (P = 0.003) affected stand counts in 2017 at EREC. Greater counts were observed in moderately susceptible and resistant cultivars treated with imidacloprid compared to phorate, whereas susceptible peanuts did not exhibit a difference between the two insecticides overall (Table 2). At EREC in 2017, imidacloprid-treated plots had more emerged peanuts than phorate-treated plots, but this was reversed in 2018 at EREC (P = 0.0041) and not significant at PREC or in the pooled analysis (P > 0.22).

For simplicity, only phytotoxicity results from 24 to 29 DAP are presented. Across both years and locations (EREC and PREC) and consistent with previous research demonstrating that phorate can injure peanut seedlings (Culbreath et al., 2003; Culbreath et al., 2016; Culbreath and Srinivasan 2011; Herbert et al., 2007; Hurt et al., 2005; Tubbs et al., 2015), peanut treated with phorate generally expressed higher levels of phytotoxicity than those treated with imidacloprid or not treated with insecticide (Table 3). Insecticide was consistently significant at P < 0.01. Over both locations in 2017 as well as in the combined analysis, phytotoxicity varied among cultivar susceptibility (P < 0.001), where Georgia 06G, the only moderate-susceptibility cultivar in the study, treated with phorate with or without SAP either exhibited the greatest phytotoxicity or was within 2% of the greatest phytotoxicity observed (Table 3). While the mechanistic elucidation of this general observation was outside the scope of this study, it is plausible the increased phytotoxicity observed in Georgia 06G was related to how this cultivar was noted to grow slower overall compared to the other examined cultivars (Grey et al., 2011; Holbrook et al., 2017; Tillman and Gorbet 2017). This possibility is corroborated by the (P < 0.0001) lower emergence observed from the combined analysis (9 plants/m compared to 10.5 and 10.6 for resistant and susceptible cultivars, respectively), though formal canopy measurements were not conducted in this study. Slower growth rates would correspond to less total foliage available to absorb or metabolize the same applied rate of phorate (Wehtje et al., 1991), which reasonably could have contributed to a greater proportion of apparent phytotoxicity. The effect of susceptibility by insecticide was also significant (P < 0.02) across all datasets and the combined data, with the exception of PREC in 2018, with observed results generally following the observations noted with Georgia 06G. In addition, imidacloprid was not generally associated with phytotoxicity across cultivars. While neither SAP alone nor its interactions affected phytotoxicity in most cases, a significant interaction with insecticide (P = 0.033) was observed in PREC in 2017. In that location-year, phorate treated with SAP had 4.2% less phytotoxicity than phorate without SAP (P = 0.0005). Phytotoxicity was not affected by SAP rate in either year (Table 4).

Insecticide treatment affected thrips injury (P < 0.0001, Table 5) across locations and years. The interaction of susceptibility and insecticide was significant (P = 0.0004) in 2017 at CPES, where untreated susceptible peanut (FloRun 157) had more thrips injury than other cultivars (5.6 compared to approximately 4.8, Table 5), as well as in the combined analysis (P = 0.0253). Marasigan et al., (2016, 2018) has previously reported imidacloprid to be as effective as phorate in managing thrips injury, which was reflected in this study. Research from other studies indicate that insecticide treatments tend to provide good management of larval feeding damage (Chamberlin et al., 1992b; Chamberlin et al., 1993; Culbreath and Srinivasan 2011) but were ineffective in killing adults that migrate from outside the field (Chamberlin et al., 1992a; Funderburk et al., 1998; Herbert Jr. et al., 1991; Hurt et al., 2005). In this study, injury was not categorized by the stage of thrips causing the injury. Regardless, both insecticides beneficially contributed to overall thrips management. With the two following exceptions, thrips injury was not affected by SAP or its interactions. The insecticide × SAP interaction was significant in 2017 at PREC (P = 0.0028). Treatments with SAP generally decreased injury compared to those without SAP in both insecticide groups, with this difference being significant for imidacloprid (-0.7, P = 0.0143) but not for phorate (-0.3, P = 0.3429). Treatment with SAP also affected thrips injury (P = 0.0092) at CPES in 2017, where treatments with SAP had less injury, albeit by a small margin: 3.1 compared to 3.4. In 2018, cultivar susceptibility affected thrips injury at EREC and at CPES (P < 0.03). Thrips injury was not affected by SAP rate by itself or its interaction with insecticide (P > 0.5).

ImmunoStrip Kits used in 2018 confirmed presence of Tomato spotted wilt virus at all testing sites. Cultivar susceptibility affected SWP incidence (P < 0.01), and characteristically, resistant cultivars had mostly lower incidence throughout the study compared to other cultivars (Table 6). Insecticide treatment was also significant (P < 0.02), with the exception of PREC in 2017 where insecticide was marginally significant at P = 0.091. In combined data, peanut with phorate had statistically lower incidence compared to imidacloprid (Table 6) (9.9 vs. 16.5%, respectively), which reflected previous studies (Culbreath et al., 2003; Culbreath et al., 2008; Culbreath et al., 2016; Culbreath and Srinivasan 2011; Hurt et al., 2005). Susceptibility × insecticide affected incidence of SWP at EREC and CPES in 2017, as well as at PREC and CPES in 2018, and this interaction was moderately significant in the combined data (P = 0.0547). Overall, the difference in SWP incidence that developed following imidacloprid compared to phorate treatment was significant for moderate and susceptible cultivars but was not for resistant cultivars (7.4 at P < 0.0001, 9.4 at P < 0.0001, and 3.1% at P = 0.0710, respectively). This corroborates previous reports describing cultivar phenotypes during spotted wilt epidemics (Anco et al., 2018; Branch 2007; Kemerait et al., 2018; McKinney and Tillman 2017; Tillman et al., 2006) and that phorate is more effective in managing this disease. With a few localized exceptions, SAP-related effects generally did not (P > 0.07) affect SWP incidence across location-years. In 2017 at PREC the interaction of cultivar susceptibility by insecticide by SAP was significant at P = 0.0542, within which SWP varied for the susceptible cultivar (P = 0.0397) but not for the moderate or resistant cultivars (P = 0.29). However, in addition to the lack of this observation being significant in other location-years, it is further interesting to note that at PREC in 2017 the susceptible cultivar treated with phorate in the absence of SAP uncharacteristically exhibited higher SWP incidence compared to the corresponding imidacloprid treatment (18.0 compared to 12.7%, respectively). As a result, the phorate + SAP treatment (7.8%) had less SWP incidence than phorate treatment without SAP. Accordingly, it is unsurprising that this three-way interaction was not observed across location-years (i.e., SAP effects elsewhere and overall were marginal, whereas SWP incidence more frequently varied according to the interaction of susceptibility by insecticide). At CPES in 2018 treatment with SAP was associated with a 2.3% decrease in SWP incidence at a significance level of P = 0.0687, but again, this was not observed in other location-years nor in the combined analysis. At PREC in 2018, the effect of SAP was also marginal at P = 0.0889, with a comparable magnitude, 2.0%, observed compared to the 2018 CPES data. When estimated SWP incidence from the SAP rate trial was regressed against SAP rates, significant relationships were observed. Parameter estimates are listed in Figure 1. Corresponding estimated parameter P-values were as such: imidacloprid 2017 intercept P <0.0001, SAP rate slope P = 0.0233; imidacloprid 2018 intercept P <0.0001, SAP rate slope P <0.0001; phorate 2017 intercept P = <0.0001, SAP rate slope P = 0.0314; phorate 2018 intercept P = <0.0001, SAP rate P <0.0001. In 2018 for both insecticide models, 5.6 kg/ha was identified as a substantial outlier and excluded from the robust models (standardized robust residuals > 11, cutoff = 3). The removal of these outliers reduced both applicable model R² values from ∼0.97 to ∼0.72. Combined-year models per insecticide were examined (data not shown) but did not produce appropriate fits given the variability among years and available data points. From these regression models, a decrease in SWP incidence with increasing SAP rate was observed in both years of the study for both insecticides. When the 2017 - 2018 data were examined within the mixed model, SAP rate was significant overall at P = 0.0641, and SWP incidence began to become different (lower) than treatments without SAP as the rate of SAP increased past 2.24 to 5.6 and 8.96 kg/ha: P = 0.5729, 0.0507, and 0.0213, respectively, with corresponding estimated differences of 1.3, 4.5 and 5.4%. Differences between higher rates of SAP, e.g., 2.24 and 8.96 kg/ha, were significant (P > 0.075).

Results from the susceptibility × insecticide treatment × SAP trial indicated that SAP presence did not have a consistently substantial effect on spotted wilt incidence. Despite this, certain phenomena of cultivar - treatment combinations were observed. Over the combined data, susceptible cultivars treated with phorate and SAP mostly exhibited lower SWP incidence (by 4.8%) than their counterparts without SAP. Previous reports have shown that phorate can lower SWP incidence through induction of plant defense responses (Jain et al., 2015). It may be possible that this combination with SAP promoted a stronger effect compared to phorate alone. Increased spotted wilt was generally observed in TUFRunner 511 and FloRun 157, the susceptible cultivars, and those left as untreated checks or treated with imidacloprid. Previous studies have indicated that imidacloprid can cause an uptick in spotted wilt incidence by promoting activity in thrips movement (Culbreath et al., 2003; Culbreath and Srinivasan 2011).

In the combined analysis, yield was not affected by any of the examined factors or interactions (P > 0.21); accordingly, further yield results discussion will pertain to the location-year data. However, it should be noted that while the following yield results and discussion reference cultivar susceptibility groups out of convenience as well as out of a result of the experimental design, actual yield (and subsequent economic value) differences could very well have been due to other genetic factors (beyond susceptibility to SWP) harbored by the specific genotypes examined in these experiments. Over each of the location-years with the exception of EREC in 2017, yield was affected by cultivar susceptibility (P < 0.02). At PREC, resistant cultivars had statistically higher yields compared to moderately susceptible and susceptible cultivars, while at CPES, the moderately susceptible cultivar had greater yields over susceptible and resistant cultivars. Susceptible cultivars also had increased yield compared to resistant cultivars at CPES. Susceptibility was often a significant factor for yield and appeared to have localized effects, plausibly a result of genotype × environment interactions that were not the focus of this study. Cultivars that performed numerically best at EREC were susceptible (TUFRunner 511), PREC were resistant (Sullivan and TifNV-High O/L), with the moderately susceptible cultivar (Georgia 06G) performing best at CPES (Table 7). Insecticide treatment was also significant at EREC and CPES in 2017 (P < 0.009). At EREC, phorate-treated plots had increased yields over those treated with imidacloprid (5960 compared to 5370 kg/ha), whereas at CPES imidacloprid-treated plots had increased yields (8380 compared to 8110 kg/ha). The effect of susceptibility × insecticide was also a significant factor at EREC (P = 0.0006) in 2017, where phorate-treated plots had greater yields with moderately susceptible and susceptible cultivars, whereas phorate only numerically increased yields in resistant cultivars. Yield was not affected (P > 0.05) by SAP-related effects at EREC (except where specified later for susceptibility × insecticide × SAP in 2018) or PREC. While treatments with SAP overall had lower (P = 0.0435) yields at CPES in 2017 (8110 compared to 8270 kg/ha), this was not observed in other location-years. While the interaction of susceptibility × SAP was consistently significant at CPES in 2017 and 2018 (P = 0.0821 and 0.0833, respectively), the subsequent related results were inconsistent between the two years. In 2017, addition of SAP was associated with reduced yield for the susceptible cultivar (8100 compared to 8460 kg/ha), whereas in 2018 it was associated with greater yield for the susceptible cultivar (7460 compared to 6410 kg/ha); in both years, yield of moderate and resistant cultivars were not affected. In 2018, the effect of susceptibility × insecticide × SAP affected yields (P = 0.0023) at EREC (Table 7), resulting in a difference for treatments in the moderately susceptible group, where imidacloprid with SAP had less yield compared to remaining treatments (Table 7). Differences within the resistant and susceptible groups were not significant. At CPES in 2017, the overall susceptibility × insecticide × SAP interaction was significant at P = 0.0573, and this interaction was more significant when examined among the partitioned (sliced) moderate and resistant susceptibility groups: P = 0.0432 and 0.0128, respectively. For the moderate cultivar, yield from imidacloprid + SAP was not different from imidacloprid without SAP but was greater than all other treatments. From the resistant cultivar data, addition of SAP numerically increased yield for imidacloprid treatment but significantly reduced yield for phorate and untreated treatments (Table 7). Among insecticide treatments, phorate did not consistently result in statistically greater yields in combined analysis of years and locations (Table 7). Previous reports from Herbert et al., (2007) and Tubbs (2013) have indicated yield to be increased following phorate application. However, it has also been reported that phorate has an inconsistent effect on peanut yields (Culbreath et al., 2008; Marois and Wright 2003). Hayat and Ali (2004) saw an increase of yield in tomato plants when including SAP (12.5 g/kg) mixed into the soil, however other reports did not find any significant or consistent differences in other crops (Busscher et al., 2009; Hou et al., 2017).

Treatment economic value was not affected by SAP or its interactions with other examined factors (P > 0.23). Insecticide treatment alone did not affect economic value (P = 0.1029), but insecticide did interact with susceptibility (P = 0.0244). While no significant differences were observed for moderate and resistant cultivars, susceptible cultivars treated with phorate had greater economic value ($2,637/ha) compared to susceptible cultivars treated with imidacloprid ($2,417/ha) (Table 8). Treatments also differed (P = 0.0008) with cultivar susceptibility, with moderately susceptible ($2,634/ha) and susceptible ($2,523/ha) cultivars having better economic value compared to resistant cultivars ($2,417/ha), P = 0.0002 and 0.0503, respectively; the difference between moderate and susceptible cultivars was also was fairly significantly (P = 0.0534). Regarding aflatoxin and grade segregations, two of the 244 samples analyzed from yields taken at EREC and CPES in 2017 and 2018 were classified segregation II, and two more samples had segregation III classifications; of the corresponding treatments, none included SAP. Of the two segregation II samples, one was from an untreated susceptible treatment ($784/ha) and the other was from an imidacloprid treated susceptible cultivar ($712/ha). Both of the segregation III samples ($578 and $749/ha) were from no-insecticide moderately susceptible treatments.

Spotted wilt disease of peanut was not conclusively mitigated using superabsorbent polymer in these studies. Still, decreases in SWP incidence were frequently observed for susceptible cultivars paired with phorate and polymer compared to phorate alone across location-years, with similar albeit more mitigated effects observed from the moderately susceptible cultivar. Further research should be performed to clarify the mechanisms behind this phenomenon. Superabsorbent polymer could be useful in situations where as much additional protection against SWP as possible is warranted. However, in taking into consideration the variability of the results presented from these studies combined with its current estimated material costs, use of SAP in commercial peanut production at this time does not appear to be cost effective and as such is not recommended at this time.