A field trial was conducted at the Wiregrass Research and Extension Center in Headland, AL during the 2022 and 2023 growing seasons. The experiment was planted in a split-split plot design within a randomized complete block with four replications. Main plots consisted of 24 rows and contained three planting dates: late April (high risk to thrips infestation), mid-May (low risk), and mid-June (moderate risk). These risk levels were determined by the Georgia Peanut Rx guide (Kemerait et al. 2004). The sub-plot consisted of eight rows and contained three varieties selected based on the Peanut Rx guidelines determined by the spotted wilt index rating: GA-12Y (field resistant to TSWV; ‘5’ rating on PeanutRx), GA-06G (moderately field resistant, but historically high yielding; ‘10’ rating on PeanutRx), and AU-NPL 17 (moderately field resistant; ‘10’ rating on PeanutRx) (Kemerait et al. 2024). Lastly, the sub-sub plot was four rows, either treated or not treated with an in-furrow application of Thimet according to the product label (20 g of phorate, 5 lbs/acre). Plots were 9.1 m long and spaced 0.9 m apart and planted at a seeding rate of 20 seed per row meter. Peanuts were grown under dryland conditions in the 2022 season and were irrigated in the 2023 season. All plots were managed for high yield with regards to insect, weed and disease management according to recommendations from the Alabama Cooperative Extension System.

In both years, the percentage stand was evaluated by randomly sampling the number of plants per 9 meters of row in the center two rows of each plot at 14 days after planting (DAP). Whole plot vigor and thrips injury ratings were made at 14, 21, and 28 DAP. Vigor was rated on a 0 to 10 scale, with 0 being no vigor and 10 being maximum vigor, and is based on the overall health, growth, and fullness of the plants in the plot. Thrips injury was rated on a 0 to 5 scale, with 0 being no injury and 5 being complete absence of leaf tissue, measured based on the severity of thrips damage symptoms (silvery speckling, leaf crinkling, etc., scale shown in Figure 1) seen in the plot. Due to F. fusca and F. occidentalis being the primary thrips pests of peanuts in Alabama, both species being vectors for TSWV, and both species exhibiting similar foliar feeding patterns, thrips injury ratings were deemed an appropriate measure for determining infestation levels in this trial.

Fractional green canopy cover (FGCC) and virus incidence was recorded at 42, 49, and 56 DAP. FGCC was recorded with the Canopeo application on iPhone in the center two rows of each plot to assess for growth stunting and general lack of vitality. The Canopeo app was developed at Oklahoma State University App Centre and works by analyzing R/G and B/G ratios and the excess green index to produce a binary image, wherein white pixels represent green matter and black pixels represent all other colors (Patrignani and Ochsner 2015). Canopeo then quantifies the FGCC as a percentage, which is then used to estimate canopy coverage. This method has been found to be a fast, reliable, and nondestructive way to accurately assess plant health in small-plot cotton and peanut trials (Graham et al. 2019; Yash et al. 2022). Percent virus incidence was recorded by dividing the number of row meters that contained plants which exhibited symptoms of TSWV (stunted growth, leaf chlorosis) by the total number of row meters (18.3 m) in the center two rows of each plot. Finally, the center two rows of each plot were harvested and yield was taken at approximately 150 DAP.

Combined percent stand counts, seedling vigor, thrips injury ratings, Canopeo ratings, TSWV incidence and yield data were analyzed with a mixed model of variance (PROC GLIMMIX, SAS 9.4, SAS Institute Inc. Cary, NC). Planting date, variety, insecticide and their interactions were considered fixed effects in the model. Replication, replication by planting date and plant date nested in replication by year were considered random effects. Degrees of freedom were estimated using the Kenward Rogers Method. Means and standard errors were calculated with PROC MEANS. Means were separated using LSMEANS and were considered significant at α=0.05. Unless indicated, two- and three-way interactions were not significant and are not discussed.

Variety (F = 7.37; df = 2, 18; P = 0.005) and planting date (F = 34.56; df = 2, 6; P < 0.001) significantly impacted percent stand. Varieties GA-12Y and GA-06G had significantly higher stands than AU-NPL17 (Table 1). Peanuts planted at early (April) and mid (May) planting dates had significantly higher stand than peanuts planted at the late (June) plant date (Table 1). No significant difference was observed for insecticide (F = 0.49; df = 1, 99; P = 0.485) (Table 1).

Similarly, variety (F = 18.15; df = 2, 18; P <0.001) and planting date (F = 7.65; df = 2, 6; P <0.001) significantly impacted seedling vigor. GA-12Y and GA-06G again significantly outperformed AU-NPL17 (Table 1). However, the early (April) and late (June) planted peanuts both averaged ≈0.3 vigor points higher than peanuts planted at the Mid (May) date (Table 1). No significance was found for insecticide treatment (F = 0.24; df = 1, 99; P = 0.624) for seedling vigor.

A significant two-way interaction of variety by planting date was observed for thrips injury (F = 6.27; df = 2, 99; P = 0.003). In general, Early (April) planted peanuts had higher thrips ratings than Mid (May) or Late (June) planted peanuts. However, at both the Mid (May) and Late (June) planting dates AU-NPL17 had more thrips injury than either GA-12Y or GA-06G (Table 2). There was also a significant interaction of insecticide by planting date (F = 6.27; df = 2, 99; P = 0.003) for thrips injury. While Thimet significantly reduced injury in the early (April) and mid (May) planting dates, there was no reduction in injury for Thimet in the late (June) planting date (Table 3).

There was no impact of variety (F =1.41; df = 2, 18; P = 0.270) or insecticide (F = 0.40; df = 1, 96; P = 0.527) on Canopeo ratings. However, there was an effect of planting date (F = 59.71; df = 2, 6; P = <0.001), where late planted peanuts had the highest Canopeo ratings, followed by mid and early planted peanuts (Table 4).

Significant differences for variety (F = 10.85; df = 2, 18; P = 0.001) and planting date (F = 11.95; df = 2, 6; P = 0.008) were found for TSWV incidence. The highest TSWV incidence was found in GA-06G, with GA-12Y and AU-NPL17 showing similar and lower TSWV incidence (Table 4). Planting date (F = 1.83; df = 1, 96; P = 0.180) also had a significant effect on TSWV incidence, where peanuts planted at the early (April) plant date had significantly higher TSWV incidence than those planted at the mid (May) or late (June) planting dates (Table 4). No significant effect of insecticide treatments (F = 1.83; df = 1, 96; P = 0.180) was observed for TSWV incidence.

A significant difference was found for yield in insecticide (F = 7.9; df = 1, 114.2; P = 0.006) and variety (F = 3.03; df = 2, 18; P = 0.073) treatments. Plots that received Thimet had significantly higher yield than plots without Thimet treatment (Table 4). Mid-planted peanuts had significantly higher yield than early-planted peanuts, with the Late-planted date falling between the other two dates.

In our study, AU-NPL17 had significantly lower stands than GA-12Y and GA-06G. These results are similar to those found by Faske et al. (2020) who observed AU-NPL17 to have lower stands than GA-12Y and GA-06G. We also found that percent stand counts were reduced at the late (June) planting date. Zurweller et al. (2023) also reported lower stand in later planted peanuts (May 30). In that study, the authors suggested lower stands could have been due to warmer soils and less moisture. In our study, we did not monitor soil temperatures, but we did have adequate moisture for germination.

AU-NPL 17 had significantly lower average vigor than GA-12Y and GA-06G, which could be attributed to lower stands observed with this variety, which could suggest less seedling vigor. Peanuts at the mid (May) planting date had significantly lower vigor than planting dates, however the differences were minor (0.3 points on 0-10 scale) and statistical differences may be a result of the trial design.

Two interactions were observed for thrips injury. In the first, variety by planting date, the mid and late-planted GA12-Y and GA-06G had significantly less thrips injury than AU-NPL 17 at these planting dates. This may again be attributed to the low stand in AU-NPL 17. Lower plant populations are known to correlate with higher incidences of TSWV (Tillman et al. 2015; Strayer-Scherer and Graham 2021). AU-NPL 17 likely had more thrips injury at the mid and late planting dates due to the reduced stands at these dates, which has been seen previously (Zurweller et al. 2023). Peanut stands with fewer plants down the row are thought to result in higher thrips populations on individual plants, resulting in higher injury (Brown et al. 2005). In the second interaction, insecticide by planting date, Thimet usage correlated with significant reductions in thrips injury at the early and mid-planting dates, but not at the late planting date. Despite Peanut Rx guidelines suggesting the late plant date is at moderate risk compared to the mid plant date (low risk), late planted peanuts were observed to have the least amount of thrips injury in both years of this study (University of Georgia 2021). Based on thrips injury trends for planting date over the two years assessed, Thimet correlated with a significant reduction in feeding injury when thrips pressure was high but did not result in reduced damage when pressure was low.

The late plant date showed the highest canopy closure across treatments. This differs from previous research, which shows later planting dates correlate with decreased canopy closure (Bateman et al. 2020). While the decreased stand at the late plant date would lead to the assumption that it would also have decreased canopy closure, a study done by Plumblee et al. (2018) observed no correlation between seed density and canopy closure. Plumblee et al. (2018) also concluded that factors such as thrips population, flight patterns, feeding injury must be driving differences in canopy closure, which aligns with our observations of significantly lower thrips injury followed by significantly higher canopy closure, and vice versa. Another consideration for reduced canopy closure at the later planting date could be related to the increased risk of leaf spot diseases at later planting dates (Gonzales et al. 2023, Kermerait et al. 2024). However, this is unlikely because we did not observe leaf spot symptoms in this trial and all plots, regardless of planting date, were sprayed with fungicides on 14-day intervals to minimize impacts of fungal diseases, especially leaf spot.

The higher field resistant variety, GA-12Y, had significantly lower virus incidence than GA-06G or AU-NPL 17, reflective of Peanut Rx guidelines on TSWV resistant varieties. AU-NPL 17 and GA-06G did not significantly separate for virus incidence, but the highest incidence was found to be in GA-06G (12.9%) followed by AU-NPL 17 (10.7%). Though the Peanut Rx guide states GA-06G and AU-NPL 17 are of equal susceptibility to TSWV, other sources state that AU-NPL 17 may be more field resistant, especially given its high yield potential (Hollis 2017). While peanuts planted at the mid and late planting dates did not separate as they did for thrips injury and canopy closure, the early planting date was observed to have significantly higher TSWV incidence. The lack of separation between mid and late plant dates for TSWV incidence could be attributed to the less populous, but more efficient TSWV vectors, western flower thrips (F. occidentalis), have been found to migrate through Alabama at higher densities in late May to early June (Brown et al. 2005; Frank et al. 2020; Cook et al. 2003).

Insecticide treatment was found to have a significant impact on yield in this study. Plots that received an in-furrow application of Thimet yielded ≈350 kg/ha higher than plots without Thimet treatment (Table 4). The overall trend of peanuts treated with Thimet having higher yields than non-treated peanuts is similar to other studies (Zurweller et al. 2023; Brandenburg et al. 2021; Mahoney et al. 2018). This suggests that thrips injury can reduce yields, regardless of TSWV incidence in Alabama and thrips should be managed to maintain yields. Previous research has shown that using Thimet consistently results in higher yields than non-treated peanuts or peanuts with other treatments (Culbreath et al. 2008, Anco et al. 2020). This due to both controlling thrips and reducing TSWV incidence. The systemic activity of phorate may influence plant physiology and gene expression, potentially enhancing resistance to TSWV and improving overall yield. Systemic insecticides, like Thimet, are absorbed by plants and translocated throughout plant tissues influencing various physiological processes such as growth, development and defense mechanisms (Nauen and Bretschneider 2002, Thaler et al. 2002). These insecticides can induce changes in gene expression, particularly genes involved in plant defense responses, enhancing the ability of the plant to resist pathogens and pests (Nauen and Bretschneider 2002, Thaler et al. 2002).

Yield results also showed a significant interaction for planting date. Yield was significantly lower for early-planted peanuts than for any other treatment. This aligns with the literature which states TSWV causes significant yield loss (Strayer-Scherer and Graham 2021; Culbreath et al. 2003; Sundaraj et al. 2014). The reduced yield during the early plant date correlates with the significantly higher virus incidence observed at that date. The Mid-planted peanuts had the highest yield out of the three, which aligns with the Peanut Rx recommendations to plant in mid-May.

This study shows the importance of a systematic approach to TSWV management in peanuts to maximize yield. We found that planting date was the most important factor in thrips injury and TSWV incidence. Overall, the use of Thimet was important, but more so at the early planting date (April) when thrips injury and TSWV infection risk is the highest. These results demonstrate that farmers should plant high yielding cultivars in Mid-May with Thimet in areas with high TSWV incidence to reduce virus pressure and maintain yield potential.