Peanut (Arachis hypogaea L.), an important agronomic crop in Alabama, was planted on approximately 74,000 ha (183,000 acres) in 33 of 67 counties in 2020. With an estimated yield of 3811 kg/ha (3400 lb/A), Alabama’s peanuts had a farm-gate value in 2022 of $144 million (NASS, 2022). In the southeastern United States, yield and quality of peanut is threatened by several diseases including early and late leaf spots (caused by Passalora arachidicola and Nothopassalora personata, respectively), southern stem rot (caused by Agroathelia rolfsii, anamorph of Sclerotium rolfsii), and tomato spotted wilt virus (a Tospovirus), as well as several plant parasitic nematodes.

In Alabama, Florida, Georgia, and Texas, a widespread and potentially damaging parasite of peanut is the peanut root-knot nematode (PRKN, Meloidogyne arenaria race 1 (Neal) Chitwood) (Timper et al., 2018). The host range of M. arenaria race 1 also includes pepper (Capsicum spp.), tobacco (Nicotiana spp.), and tomato (Solanum spp.) (Sasser and Carter, 1982). However, corn (Zea mays) and cotton (Gossypii spp.), which are commonly rotated with peanut, are likely poor hosts (Rodríguez-Kábana and Touchton, 1984). In production fields with heavy infestations of M. arenaria race 1, peanut yield may be less than 50% of expectations (Rodríguez-Kábana et al., 1991; Timper et al., 2018). Javanese root-knot nematode races 3 and 4 (Meloidogyne javanica Chitwood), which also parasitizes peanut, has not been reported in Alabama but was identified on peanut in Florida (Cetintas et al., 2003) and Georgia (Minton et al., 1969). In addition, compared to sound, undamaged pods, nematode damaged pods may be more readily colonized by fungi such as Aspergillus flavus, with subsequent aflatoxin production occurring under favorable conditions (Timper et al., 2004; Timper et al., 2013; Bowen, 2009).

The primary options available to peanut producers for managing M. arenaria are crop rotation, resistant cultivars, and nematicides (Rodríguez-Kábana and Canullo, 1992; Majumdar et al., 2023). Crop rotation is the most sustainable means for suppressing M. arenaria populations below damage thresholds and for maximizing yield; however, economic considerations, such as low commodity prices for rotation crops and modest yield projections for rainfed corn and cotton, often limit cropping options for southeastern peanut producers (McSorley et al., 1992; Rodríguez-Kábana and Canullo, 1992). The cultivars COAN and NemaTAM, which are introgressive backcrosses between a PRKN-resistant, interspecific amphiploid hybrid and the cultivar Florunner, were the first PRKN-resistant runner market-type cultivars released by the Texas Agricultural Experiment Station (Simpson and Starr, 2001; Simpson et al., 2003). However, COAN and NemaTAM, like their Florunner parent, proved highly susceptible to the Tomato spotted wilt orthotospovirus (TSWV), the causal virus of tomato spotted wilt, an endemic and potentially damaging disease across the southeastern United States. In addition, these PRKN-resistant cultivars failed to match the yield of contemporary TSWV-resistant commercial cultivars (Holbrook et al., 2008). Tifguard (now an obsolete cultivar) not only demonstrated PRKN and TSWV resistance but also superior yield when exposed to both pathogen systems compared with the obsolete commercial standard Georgia Green (Holbrook et al., 2008).

Branch et al. (2014) noted that the PRKN-resistant Georgia-14N (tested as GA 082522; Branch and Brenneman, 2015) often produced greater yield than the contemporary PRKN-susceptible cultivars, Georgia-07W and Georgia Greener, and statistically matched those for Tifguard. TifNV-High O/L (Holbrook et al., 2017), another PRKN-resistant cultivar, has more recently become commercially available. TifNV-High O/L and Georgia-14N had similarly greater yield than Georgia-06G in M. arenaria infested fields (Brenneman et al. (2017). Grabau et al. (2024) also reported greater yields for TifNV-High O/L than Georgia-06G under severe PRKN pressure. Holbrook et al. (2017), Campbell et al. (2019), and Strayer-Scherer et al. (2021) reported comparable yields for the recently released PRKN-resistant cultivars and current PRKN-susceptible commercial standards in the absence of damaging M. arenaria populations. However, adoption of PRKN resistant cultivars is limited by the perception of reduced yield, particularly in the absence of damaging M. arenaria populations, and concerns about grades (i.e., proportion of sound mature kernels) compared with current commercial standards (Starr et al., 2002; Grabau et al., 2020). In the absence of this damaging nematode, Starr et al. (2002) reported that the yield potential of earlier PRKN-resistant cultivars, i.e., COAN and NemaTAM, did not match that of susceptible commercial cultivars.

Nematicides are a widely used option for nematode management in peanut. Currently, the nematicides 1,3-dichloropropene (1,3-D, Telone II, Teleos Ag Solutions, Pinehurst, NC), along with the nematicides/insecticides AgLogic Aldicarb 15GG (AgLogic Chemical, LLC, Chapel Hill, NC) and oxamyl (Vydate C-LV, Corteva Agriscience United States, Indianapolis, IN), as well as the fungicide/nematicide fluopyram (Bayer CropScience) are recommended for nematode control in Alabama (Majumdar et al., 2023). Of these nematicides, only the latter two (aldicarb and fluopyram) are widely used on Alabama peanuts.

Aldicarb is a carbamate insecticide with efficacy for controlling nematodes and thrips on peanut. When compared with non-treated controls, aldicarb treatments had significantly reduced M. arenaria populations along with significantly improved plant vigor and yield on a PRKN-susceptible peanut cultivar have been reported by Rodríguez-Kábana et al. (1981, 1985b). The current aldicarb label specifies in-furrow placement at 1.1 kg a.i. ha^-1 with the costly option of side dressing and incorporating an additional 1.6 kg a.i. ha^-1 at-pegging in fields with high nematode pressure. Rodríguez-Kábana et al. (1981) noted superior yield response with equivalent rates of banded compared with in-furrow applications of aldicarb.

Fluopyram is a broad spectrum, succinate dehydrogenase inhibitor (SDHI) fungicide with nematistatic activity against numerous plant parasitic nematodes (Hungenberg et al., 2011) and fungal plant pathogens in the Ascomycetes and Deuteromycetes, particularly in the family Sclerotinaceae (Labourdette et al., 2011). Averaged over seven years, Hagan et al. (2024b) reported significant yield gains with aldicarb, fluopyram + imidacloprid, or two applications of fluopyram products on Georgia-06G compared to non-treated controls. Similarly, Grabau et al. (2020) had previously noted significant pod yield gains with fluopyram, aldicarb, or 1,3-D treatments compared with the non-treated control, in one of two study years when substantial PRKN pressure was encountered. Wade et al. (2016) reported that aldicarb reduced pod and root damage as did 1,3-D and fluopyram (+ imidacloprid).

The objective of this study was to compare the interaction of selected commercial PRKN-resistant peanut cultivars and the nematicides fluopyram (formulated with imidacloprid) and aldicarb on pod yield, M. arenaria populations, and root and pod galling. In addition, the differential response of selected cultivars to early and late leaf spots, and stem rot, as well as non-target impacts of nematicides on these diseases in an irrigated production system on a site with a resident population of M. arenaria in Southeast Alabama was assessed.

Overall, weather patterns during the production season did not greatly differ among study years (Table 3). A higher average temperature was seen in 2016 compared to other years, but differences were generally < 2°C. In each year, rainfall was 18 to 40% lower than the 30-year average and irrigation was inadequate to reach that average. Rainfall was particularly lacking in 2018, with only 60% of the historical average and fewer raindays than other years.

Vigor was significantly (P < 0.01) affected by nematicide treatment in 2016 and cultivar in 2017; no single factor affected plant vigor in 2018 and the cultivar × nematicide interaction was not significant in any year (Table 4). Averaged over nematicide treatments, in 2017 TifNV-High O/L had significantly greater vigor than the other two cultivars and also had numerically greater vigor in 2018; Tifguard in 2016 also had numerically greater vigor than the other cultivars. In 2016, the fluopyram treatment, averaged over cultivars, had > 12% greater vigor than other nematicides, but this was not a trend seen in other years.

In 2016, leaf spot damage, across nematicide treatments, differed significantly (P = 0.03) among cultivars with Tifguard having more damage than the other two cultivars; nematicide treatment and the cultivar × nematicide interaction did not affect LSDEF (Table 5). In 2017, averaged over cultivars, the non-treated control had greater (P = 0.099) LSDEF than the fluopyram and aldicarb treatments; cultivar and the cultivar × nematicide interaction did not affect LSDEF. All factors significantly (P < 0.04) affected LSDEF in 2018. Averaged over nematicide treatments, Georgia-06G had the greatest and Georgia-14N had the lowest LSDEF while TifNV-High O/L had intermediate LSDEF (Table 5). Also in 2018, across cultivars, the fluopyram treatment had lower LSDEF than the non-treated control, while the aldicarb treatment was similar to both. The two-way interaction was significant (P = 0.016) in 2018, likely because Georgia-06G with aldicarb or left untreated had significantly greater LSDEF than all other treatments, but with fluopyram, Georgia-06G had LSDEF similar to other treatments (Fig. 1).

Cultivar significantly (P < 0.0005) affected stem rot in each study year, but nematicide and the cultivar × nematicide interaction did not (P > 0.20) (Table 5). Georgia-06G consistently had greater stem rot than Tifguard, TifNV-High O/L or Georgia-14N; the latter nematode-resistant cultivars had > 65% lower stem rot than the former susceptible cultivar (Table 5). Similarly, root and pod galling was significantly (P < 0.0005) impacted by cultivar, but not by nematicide or the cultivar × nematicide interaction (P > 0.06) (Table 4). As with stem rot, Georgia-06G consistently had greater RKDam than the PRKN-resistant cultivars (Table 4). The nearly significant (P= 0.061) cultivar × nematicide interaction in 2016, was likely due to reduced damage on Georgia-14N with fluopyram compared to aldicarb, while on Tifguard and Georgia-06G, nematode damage was similar for all treatments (data not shown).

In each study year, averaged over nematicide treatments, final PRKN juvenile populations were significantly (P < 0.0003) affected by cultivar but not by nematicide or the cultivar × nematicide interaction (P > 0.05) (Table 6). Averaged across nematicides, Georgia-06G consistently had greater RKpop than other cultivars—350%, 964%, and 706% greater than populations on Tifguard or TifNV-High O/L in 2016, and 2017 and 2018, respectively, with even lower populations on Georgia-14N. While not statistically compared across years, average RKpop in Georgia-06G was greatest in 2017, somewhat lower in 2016, and considerably lower in 2018 (Table 6). The nearly significant (P = 0.058) cultivar × nematicide interaction in 2017 was likely due to numerically greater PRKN juvenile populations with fluopyram on Georgia-14N and TifNV-High O/L while on Georgia-06G this treatment had substantially lower populations than non-treated or aldicarb treatments (data not shown).

Yield was significantly (P < 0.01) impacted by cultivar but not by nematicide (P > 0.10) in each study year. In 2016, Georgia-14N had significantly greater yield than other cultivars, while in 2017 and 2018, TifNV-High O/L outperformed Georgia-06G and Georgia-14N. The nearly significant (P = 0.0823) effect of nematicide treatments in 2016 on yield indicated that non-treated controls, averaged over cultivars, had significantly lower yield than fluopyram. The cultivar × nematicide interaction was not a significant factor (P > 0.50) in 2016 or 2017 but was (P = 0.016) in 2018 (Table 6) when non-treated Georgia-06G had greater yield than with aldicarb and aldicarb-treated TifNV-High O/L had greater yield than with fluopyram (Fig. 2).

This study sought to evaluate the performance of PRKN-resistant peanut cultivars, with and without nematicides, in comparison to the current commercial susceptible standard cultivar, Georgia-06G. In addition, the possible non-target effects of the nematicides, aldicarb and fluopyram, on leaf spot and stem rot diseases were assessed. In fields infested with M. arenaria Race 1, in each of three study years, the cultivars, Tifguard, Georgia-14N, and TifNV-High O/L had little or no galling on roots and pods such that nematode damage was consistently and significantly lower than on Georgia-06G. Similarly, significant and consistent reductions in M. arenaria juvenile populations were observed on the PRKN-resistant cultivars compared with Georgia-06G. Georgia-06G also had greater leaf spot and defoliation severity in one study year and consistently greater stem rot, but these damages did not appear to consistently detract from its yield.

The decreased galling and limited M. arenaria juvenile counts observed in the current study on PRKN-resistant cultivars aligns with previous work. Branch et al. (2014) reported a significant reduction in galling on the roots and pods of Georgia-14N compared with the PRKN-susceptible cultivars Georgia-07W and Georgia Greener. A significant reduction in the egg mass index, which is a measure of M. arenaria reproduction, along with a lower root gall index for TifNV-High O/L as compared with the obsolete susceptible cultivars Florida-07, Georgia Green, and FloRun 107 was noted by Holbrook et al. (2017). In addition, Brenneman et al. (2017) did not observe root galling on either Georgia-14N or TifNV-High O/L, even in fields with high resident M. arenaria populations. Holbrook et al. (2008) had also noted significantly lower root-gall and egg mass indices along with egg counts with the obsolete resistant cultivars COAN, NemaTAM, and Tifguard compared with the now obsolete susceptible cultivar Georgia Green. Grabau et al. (2024) similarly noted significant reductions in root-galling on TifNV-High O/L compared to Georgia-06G. In contrast to the consistent reductions in juvenile populations with the PRKN-resistant cultivars herein, particularly in 2016 and 2017, enhanced plant vigor was observed in only in 2017 for Georgia-14N and TifNV-High O/L compared with the susceptible industry standard Georgia-06G.

In 2016 and 2017, juvenile populations of M. arenaria on Georgia-06G were in excess of 200 per 100 cm³ soil while with the PRKN-resistant cultivars juvenile populations were consistently < 80. Given that the damage threshold for M. arenaria Race 1 on peanut is 10 juveniles per 100 cm³ of soil (Jagdale et al., 2013), substantial reductions in yield were expected with Georgia-06G. However, yield differences between nematode resistant cultivars, compared to the susceptible commercial cultivar, were inconsistent. Here, significant yield gains, when compared with the susceptible standard Georgia-06G and averaged over nematicide treatments, were obtained with Georgia-14N in 2016 and with TifNV-High O/L in 2017 and 2018. In 2016, average yield of Tifguard was statistically similar to that of Georgia-06G, despite significantly lower root and pod galling and substantially reduced PRKN juvenile populations on Tifguard. Holbrook et al. (2008) noted greater yields for Tifguard compared to the standard check cultivar, Georgia Green, when grown in fields with little to no nematode pressure. Georgia Green, now obsolete, had lower yield potential than Georgia-06G (Branch, 2007), suggesting that Tifguard and Georgia-06G might have similar yield potential as seen herein in 2016. In the two later years of the current study, Georgia-14N had yields that were similar to Georgia-06G while TifNV-High O/L had greater yields than Georgia-06G. On sites with high nematode populations, Brenneman et al. (2017) consistently observed yield gains with Georgia-14N and TifNV-High O/L compared with Georgia-06G. Together, these studies suggest that the yield potentials of Georgia-14N and TifNV-HighO/L are comparable to that of Georgia-06G. Yield differences from year-to-year between cultivars observed among these three studies could have arisen from a variety of factors, including soil type, weather, and management practices.

Widespread adoption of PRKN-resistant runner-type cultivars has been limited due to grower perception that yields will be lower than with standard (susceptible) commercial cultivars (Starr et al., 2002; Grabau et al., 2020). In irrigated OVT cultivar trials conducted annually from 2017 to 2022 in Southeast Alabama with minimal stem rot and PRKN pressure, Georgia-14N had greater yield than Georgia-06G in one of six years, while in four of six years TifNV-High O/L had comparable or greater yields than Georgia-06G (Hagan et al., 2024b). Herein, yield of the PRKN-resistant cultivars, under light to moderate PRKN pressure, consistently matched and often exceeded the yield of the PRKN-susceptible Georgia-06G. In a series of concurrent Southwest Alabama trials with minimal root-knot pressure, yields of Georgia-14N and TifNV-High O/L matched or exceeded that of Georgia-06G and many other commercial cultivars (Hagan et al., 2020a; Hagan et al. 2020b; Hagan et al., 2021). As noted previously, yield potential of TifNV-High O/L and, to a lesser extent, Georgia-14N is comparable to Georgia-06G under minimal and high PRKN pressure and are suitable replacements for PRKN-susceptible cultivars across the southeastern peanut production region.

In this study in 2016, plant vigor was significantly greater with fluopyram treatment compared with the non-treated control. In the later two study years, both aldicarb and fluopyram numerically, but not significantly, improved vigor. In contrast, Rodríguez-Kábana et al. (1982) reported a significant improvement in plant vigor (subjective appearance index) with aldicarb, applied at various rates and with varying methods, compared to the non-treated control; this work was done at two sites in the same year.

Despite low leaf spot (LSDEF) and stem rot pressure during the study period, the PRKN-resistant cultivars evaluated here may display possible tolerance or partial resistance to these diseases. Georgia-14N and TifNV-High O/L had reduced leaf spot damage in 2018 but not in 2017 compared with Georgia-06G. Leaf spot damage on Tifguard in 2016 was greater than on Georgia-06G or Georgia-14N, but, overall, leaf spot pressure was low. Reduced stem rot was also noted for Georgia-14N, Tifguard, and TifNV-High O/L in each year, compared to Georgia-06G, with Georgia-14N having consistently lower stem rot than Tifguard or TifNV-High O/L. Branch et al. (2014) had previously noted that Georgia-14N had significantly reduced disease than Tifguard; ‘disease’ was a combination of tomato spotted wilt and stem rot at digging, expressed as total disease incidence. Under low tomato spotted wilt pressure, stem rot incidence in Alabama studies has continued to be lower on Georgia-14N and TifNV-High O/L compared with Georgia-06G (Hagan et al., 2020a, 2021). In a second 2019 Alabama study, Georgia-14N but not TifNV-High O/L suffered significantly less stem rot damage than Georgia-06G (Hagan et al., 2020b). In the same study, reduced leaf spot-incited defoliation for Georgia-14N compared with Georgia-06G was recorded (Hagan et al., 2020b). Significant differences in leaf spot defoliation and stem rot incidence were not always noted between the above PRKN- susceptible and resistant cultivars herein. However, in uniform peanut OVT trials conducted annually in Alabama from 2017 to 2022, no differences in the occurrence of leafspot diseases or stem rot were noted between Georgia-14N, TifNV-High O/L, and Georgia-06G (Hagan et al., 2024b).

Modest but significant reductions in year-end leaf spot severity and defoliation were obtained with in-furrow applications of fluopyram in 2018 on Georgia-06G. Hagan et al. (2024a) also reported reduced leaf spot incited defoliation in peanuts with fluopyram compared to aldicarb on Georgia-06G. In addition, in the previous study, enhanced leaf spot control was obtained when the in-furrow application of fluopyram was followed by an at-peg application of fluopyram + prothioconazole compared with fluopyram alone. Extended suppression of early and late leaf spots with in-furrow applications of fluopyram was recently reported by Culbreath et al. (2021). As noted by Hagan et al. (2024a), yield gains obtained with fluopyram in-furrow followed by fluopyram + prothioconazole at pegging, compared with aldicarb, may be attributed in part to reduced leafspot-incited defoliation.

As was previously noted by Hagan et al. (2024a), fluopyram and aldicarb did not impact stem rot incidence. The absence of activity of fluopyram against this disease is not surprising as the causal fungus, Agroathelia rolfsii, is a Basidiomycete and this fungicide has not shown significant activity against this class of fungi (Labourdette et al., 2011). In multiple trials on peanut in Alabama, aldicarb, regardless of application placement, timing, and rate, did not significantly impact stem rot incidence on Florunner, a cultivar highly susceptible this disease (e.g., Rodríguez-Kábana et al., 1985a; Rodríguez-Kábana et al., 1991).

When averaged over cultivars, reductions in late-season juvenile PRKN populations were not seen with either fluopyram or aldicarb compared with the non-treated control. In a concurrent study at the same location, similar results were noted on Georgia-06G with these same nematicides along with a fluopyram + imidacloprid followed by an at-peg application of fluopyram + prothioconazole (Hagan et al., 2024a). Similarly, Grabau et al. (2024) noted comparable PRKN populations at harvest on Georgia-06G in one of two study years; however, in the preceding year, with greater nematode populations, both aldicarb and fluopyram in-furrow reduced nematode populations compared to non-treated controls. Wade et al. (2016) had also noted that final M. arenaria juvenile counts were not reduced by 1.1 kg a.i. ha^-1 aldicarb or fluopyram (+ imidacloprid). While Rodríguez-Kábana et al. (1982) failed to record a reduction in final M. arenaria juvenile counts with in-furrow applications of 1.1 and 2.2 kg a.i. ha^-1 aldicarb, significantly lower juvenile counts were noted for banded applications of aldicarb at 1.1 to 4.4 kg a.i. ha^-1 compared to the non-treated control. Significant reductions in nematode populations with banded compared with in-furrow applications of aldicarb were also observed by Rodríguez-Kábana et al. (1981). Overall, nematicides such as fluopyram along with labeled rates of aldicarb are unlikely to give season-end reductions in M. arenaria juvenile populations.

In the current study, yields averaged over cultivars were only improved with fluopyram, compared to no treatment, in one study year. In 2017 and 2018, yields were comparable among no nematicide, aldicarb, or fluopyram treatments. Grabau et al. (2024) had also noted similar yields among nematicide treatments (non-treated, aldicarb, fluopyram, and two applications of fluopyram products) with Georgia-06G. This contrasts to the significant yield gains with aldicarb, applied according to current label specifications, that were recently reported by Hagan et al. (2024a); however, previous research had questioned the efficacy of the now specified in-furrow placement of the 1.1 kg a.i. ha^-1 of aldicarb for controlling PRKN and providing yield protection. Rodríguez-Kábana et al. (1982) noted significantly greater yield gains on peanut with banded compared with in-furrow applications of 1.1 kg a.i. ha^-1 aldicarb with the latter placement having greater yield than the non-treated control. Enhanced performance of banded compared with in-furrow applications of aldicarb has been attributed to a uniform distribution through the pegging and root zone, which then minimizes phytotoxicity risk along with maximizing product efficacy (Rodríguez-Kábana et al., 1981). Herein, the in-furrow placement is likely the cause for the absence of a significant yield gain with aldicarb. Thrips control with aldicarb is not impacted by product placement, so in-furrow placement is effective for controlling this insect pest on peanut (Majumdar et al., 2023).

Overall, weather patterns during the production season did not differ greatly among study years. While 2016 had an overall higher average temperature than 2018, average minimum daily temperatures were lower in 2016 than 2018. Georgia-14N performed very well in 2016; it may be that this cultivar is tolerant of or is more productive at higher daily temperatures than Georgia-06G or Tifguard. All three years had less than normal rainfall, with 2018 having 40% lower rainfall and fewer raindays compared with the 30-year average. Despite supplemental irrigation, drier weather patterns likely resulted in reduced nematode activity and delayed disease development, which subsequently was reflected in greater yield as was noted in a concurrent nematicide study (Hagan et al. 2024a).

Along with crop rotation, resistant cultivars are the preferred method for managing PRKN, while maintaining pod yields and peanut profitability, especially when compared to nematicide use (Rodríguez-Kábana et al.,1987). Superior yield gains compared with the PRKN-susceptible cultivar Georgia-06G were consistently observed over the study period with one of the PRKN-resistant cultivars. In addition to sometimes erratic yield gains, the nematicides aldicarb and fluopyram add $33 to $49 ha^-1 to variable production costs. For fluopyram, added product costs can be partially offset by using the Peanut R_X program (Kemerait et al., 2020) to delete one or possibly more early season fungicide applications. In addition to plant parasitic nematodes, aldicarb has activity against thrips, thereby allowing some savings that are achieved by deleting the insecticide targeting this pest. Overall, nematicides are best adapted for use on susceptible cultivars in fields with light to moderate resident M. arenaria populations (Brenneman et al., 2017) as compared with PRKN-resistant cultivars which are best suited to those fields with moderate to high resident PRKN populations.