ABSTRACT Peanut rust, caused by Puccinia arachidis Speg., is a foliar disease that plagues peanut production along with early and late leaf spots, Passalora arachidicola (Hori) U. Braun and Nothopassalora personata (Berk. & M.A. Curtis) U. Braun, C. Nakash, Videira & Crous, respectively. Rust can cause up to 80% yield losses without control and is widespread in tropical countries but is also a sporadic problem in the United States. An integrative plant management strategy with rust resistant peanut cultivars is needed to decrease dependence on costly fungicides and increase yields for farmers who cannot afford or do not have access to fungicides. Only moderate levels of rust resistance have been identified in cultivated peanut germplasm, but fortunately, high resistance to rust has been identified in wild Arachis species that can be introgressed into peanut cultivars. In this study, 16 diploid, wild Arachis species, five diploid, interspecific hybrids, 11 unique, allotetraploid interspecific hybrids, and two cultivated peanut controls were tested for resistance to rust. Resistance was evaluated in vitro by incubation time, susceptibility index (calculated based on the number of lesions of different diameters)/ leaf area, total number of lesions/ leaf area, and total number of sporulating lesions/ leaf area. All wild Arachis species tested were very highly resistant to rust, except for A. ipaënsis , the B-genome progenitor of cultivated peanut. Additionally, all interspecific hybrids and synthetic allotetraploids not produced with A. ipaënsis as a parent did not show symptoms for rust. Any of these nine synthetic allotetraploids, BatCor , BatDur 1, BatDur 2, BatSten 1, GregSten , MagCard , MagDio , MagDur , and ValSten 1 are recommended for progression to QTL mapping of rust resistance. These resistance QTLs can be pyramided into peanut cultivars to protect yields in the United States and to increase yields in tropical, developing countries for farmers that cannot afford, or do not have access to, costly fungicides.

ABSTRACT During 2012, 2015, and 2018 a set of 18 peanut ( Arachis hypogaea L.) genotypes (some common and some different) were used to evaluate the effect of planting dates (April, May, and June) on leaf spot disease and pod yield. Within each year, the same genotypes were grown during the three planting dates at the Gibbs Farm near the University of Georgia, Coastal Plain Experiment Station, Tifton, GA using a randomized complete block design with five replications without fungicides or insecticides but with irrigation. Each year, significant differences (P≤0.05) were found among the genotypes within each of these three planting dates for leaf spot disease ratings (0-9 scale) and pod yields. ‘Georganic' in 2012 and 2015; and GA 132705, ‘Georgia-19HP', and ‘Georgia-14N' in 2018 had among the lowest leaf spot ratings. ‘Georgia-12Y' had the highest average pod yield for each year of the three years. Each year during this study, the April planting date had the lowest, and the June planting date had the highest leaf spot disease ratings. Percent coefficient of variation (CV) was consistently lower at the June planting date which suggest the least variability among the peanut genotypes. In the overall average of genotypes, the April planting date resulted in the highest pod yield and the June planting date had the lowest average pod yield. In summary, April planting dates resulted in the highest pod yields, and the lowest leaf spot ratings across each of the three years.

ABSTRACT Four field experiments conducted in 2015 were used to examine the relationships among normalized difference vegetation index (NDVI) measurements from two canopy crop sensors and visual estimates of defoliation by late leaf spot ( Nothopassalora personata ) of peanut ( Arachis hypogaea ) the predominant foliar disease in this study. For each evaluation, reflectance was measured with each the two meters, and leaf spot severity was measured visually within as short a time as possible. Linear and quadratic regression models were used to characterize the relationships between percent defoliation from late leaf spot and NDVI measured with the GreenSeeker (NDVI GS ) and Crop Circle (NDVI CC ) instruments and the relationships between NDVI GS and NDVI CC. NDVI GS decreased with increasing percent defoliation according to linear or quadratic functions in three of the four trials, NDVI CC decreased with increasing percent defoliation according to linear functions in three of four trials. In two of the four trials, NDVI CC increased with increasing NDVI GS according to quadratic functions, but there was no significant regression for those variables in two trials. In three of the four trials , NDVI CC linear regression had a better fit for predicting percent defoliation according to the coefficient of determination (R 2 ). There was no indication for either instrument that the same NDVI reading corresponded with the same level of defoliation across trials. Results indicated that NDVI measurements from the two instruments are not interchangeable.

ABSTRACT During 2011 at Tifton and Plains, GA, there was an early-season drought stress period during May and June. A few drought-tolerant plants were identified and tagged which appeared green and turgid amongst otherwise dry and severely-wilted plants within the pure-line, runner-type peanut ( Arachis hypogaea L.) cultivar, ‘Georgia-10T'. Pod and seeds were harvested from these drought-tolerant individual plant selections (IPS) for increase and testing. During autumn and winter of 2014 to 2015, a greenhouse drought study was utilized to test these IPS's compared to the parental check cultivar. Green and turgid plants were identified within the same check cultivar after exposing the plants to an early-season two-wk drought stress period at 60 and 90 d after planting. Seed from IPS of naturally occurring and artificially drought-induced plants produced similar normal and small-plants. Replicated preliminary yield tests were conducted during 2017 to compare progeny rows from these IPS's to the check cultivar, Georgia-10T. Field trial data indicated that the smaller-plants produced from early-season drought stress had significantly reduced yield, grade, pod size, and seed size as compared to larger plant selections and Georgia-10T parental cultivar. In a greenhouse study conducted during autumn and winter of 2017 to 2018, these small plants had significantly shorter internode length and mainstem height compared to the same small plants treated with gibberellic acid (GA3) which were taller and had longer internode lengths after one and two months. Small plants resulted from artificially and naturally occurring early-season drought-induced stress within the pure-line runner-type peanut cultivar, Georgia-10T, were caused by lack of GA3. The normal and small-plants each have bred true-to-type following several self-generations. The ramification of these findings suggest the importance of early-season irrigation, especially for seed production of peanut cultivars to avoid subsequent development of low-yielding, small-plants induced by drought-stress.

ABSTRACT Spotted wilt of peanut (SWP) ( Arachis hypogaea L.) caused by Tomato spotted wilt virus (TSWV; family Tospoviridae , genus Orthotospovirus ) is a common disease that causes severe economic losses in peanut producing regions of the world. The causal agent is transmitted by thrips (Thysanoptera: Thripidae). Field experiments were conducted in 2017 and 2018 to determine if management of spotted wilt and subsequent productivity of peanut, including economic value, could benefit from applying a superabsorbent polymer (SAP) with standard in-furrow insecticides at planting. To determine this, two individual experiments were performed. In the first study, phorate and imidacloprid were individually applied with (2.24 kg/ha) or without SAP across cultivars susceptible (FloRun 157 or TUFRunner 511), moderately susceptible (Georgia 06G), and resistant (Sullivan or TifNV-High O/L) to TSWV. Nontreated controls were included in all experiments. The second study sought to determine the efficacy of different rates of SAP (0, 2.24, 5.6, and 8.97 kg/ha). The initial study was conducted in three locations across South Carolina and Georgia, while the second was conducted at Blackville, SC. In the first study, incidence of SWP was reduced (P = 0.0547) in multiple location-year analysis with a reduction of 9.4% observed in susceptible cultivars treated with phorate compared to untreated checks and those with imidacloprid. SAP did not affect final SWP incidence or economic value (P > 0.05) and was not consistently significant for yield. From the SAP rate study, a linear relationship of decreasing SWP incidence was observed with increasing SAP application rates for both imidacloprid and phorate in 2017 and 2018. On a means comparison basis, SWP was lower than the no-SAP check at SAP rates of 5.56 and 8.97 kg/ha, but this did not necessarily translate into increased yield. Additional experiments are needed to elucidate the relationship superabsorbent polymer may have to susceptible cultivars and phorate.

ABSTRACT Field, greenhouse, and growth chamber experiments were conducted to determine the level of resistance to Puccinia arachidis Speg. in newly developed breeding lines of peanut ( Arachis hypogaea L.). These lines were developed in the UF150 project of the Peanut Collaborative Research and Support Program (Peanut CRSP) as part of the United States Agency for International Development (USAID). Field experiments were carried out in Citra, FL and Tifton, GA from 2010 to 2013. Five genotypes Tifrust-10 and Tifrust-13, and CRSP breeding lines PTBOL3-3, 97x36-HO2-1-B2G-3-1-2-2, and BOL3-7 had the lowest standardized area under the disease progress curve and final disease severity score for rust. The CRSP breeding lines 97x36-HO2-1-B2G-3-1-2-2 and BOL3-7 also appeared to be highly resistant to late leaf spot, caused by Cercosporidium personatum (Berk & M. A. Curtis Deighton). In growth chamber studies, genotypes with longer latent periods generally had lower infection frequencies at 7, 11, and 16 d after inoculation, and smaller percent diseased areas. Latent period and percent diseased area were significantly correlated with stAUDPC. CRSP breeding lines 97x36-HO2-1-B2G-3-1-2-2 and BOL3-7, and plant introductions PI562530, PI568164, and PI298115, were among the genotypes with the lower scores for these components. Several genotypes with multiple disease resistance in different environments and under high disease pressure were identified in these studies. These results indicate sources of rust resistance in the CRSP breeding lines, including several genotypes that could be used as parents in peanut germplasm enhancement programs, and indicate that latent period, percent diseased area, and lesion diameter may be used as indicators for rust resistance in growth chamber studies.

ABSTRACT Achieving and maintaining an adequate plant stand is a major priority when making planting and early season management decisions in peanut ( Arachis hypogaea L.). Unpredictable and often extreme weather and high disease pressure in the southeastern United States can contribute to poor emergence and below-optimum plant stands. When plant stand is affected, replanting may be agronomically justified. This study was designed to determine i) the effect of plant stand on pod yield, market grade, and disease incidence in peanut seeded in a twin row pattern, (ii) if replanting is a viable option in a field with a below adequate stand and, iii) the best method for replanting peanut when an adequate stand is not achieved. Field trials were established at two locations in south Georgia in 2012 and 2013 to evaluate peanut production at four plant stands (7.4, 9.8, 12.3, and 14.8 plants/m [total plants/m across both units, or ‘twins' of the twin row pattern) and four replant methods (no replant, destroy the original stand and replant at a full seeding rate, add a reduced rate of seed to supplement the original stand with a single row between the original rows, and supplement with two additional rows with one between and the other next to the original rows). Replanting occurred when the stand had been established, an average of 24 days after initial planting. Pod yield at a stand of 12.3 plants/m was 6.6 and 5.8% greater than at a stand of 7.4 and 9.8 plants/m, respectively, with no benefit from increasing plant stand beyond 12.3 plants/m. Market grade was also maximized at 12.3 plants/m. Disease incidence was unaffected by plant stand. Yield was increased by supplementing an initial stand of 9.8 plants/m in both a single additional row and in two additional rows by 8.3 and 6.6%, respectively. A full replant of the original stand always resulted in lower yield, while grade was slightly increased in the full replant treatment. While an initial stand of 12.3 plants/m was needed in order to maintain yield potential, replanting via supplemental seed addition can recover lost yield at stands below this level.

ABSTRACT The University of Georgia Extension recommendation for optimum plant stand in peanut ( Arachis hypogaea L.) is 13.1 plants/m, although previous work has shown that yield potential can be maintained at lower plant stands. The unpredictable and often extreme weather and the ubiquity of pathogens in the region often contribute to poor emergence and poor plant stands. When plant stand is adversely affected, replanting the field may be a practical option. The objectives of this study were to determine i) the effect of plant stand on yield, grade and disease incidence, ii) at what plant stand peanut gains an advantage from replanting and iii) the best method for replanting peanut when an adequate stand is not achieved. Field trials took place in Plains, GA in 2011, 2012, and 2013; and Tifton, GA in 2012 and 2013 to evaluate peanut production at six plant stands (3.3, 4.9, 6.6, 8.2, 9.8, and 11.5 plants/m, in addition to a 13.1 plants/m control) in combination with three replant practices; i) no replant, ii) destroy the original stand and replant at a full seeding rate, and iii) add a reduced rate of seed to supplement the original stand) in a randomized complete block design. A positive linear trend for yield and a negative linear trend for tomato spotted wilt Tospovirus incidence were discovered as plant stand increased. Yield advantages from replanting occurred via supplemental seed addition to initial stands of 3.3 and 8.2 plants/m. Destroying the initial stand and completely replanting was never beneficial when compared to the other two replant practices. Replanting is warranted via supplemental seed addition at plant stands equal-to or below 8.2 plants/m.

ABSTRACT Stand establishment has been a challenge for organic peanut production in the Southeastern United States. Field experiments were conducted in 2007 and 2009 in research plots certified for organic production to evaluate the potential of genotype selection, shelling procedure, and seed treatment with Bacillus subtilis to improve stand establishment and seedling emergence rates, reduce incidence of Aspergillus crown rot, and increase seedling biomass. Seed of 15 peanut genotypes were mechanically shelled or hand-shelled, and treated with B. subtilis or untreated prior to planting in early June. Percent stand was estimated for each plot 17–21 days after planting (DAP), and percent seedlings affected by Aspergillus crown rot and plant biomass were estimated 21–24 DAP. Seed treatment significantly affected stand establishment for three genotypes each year, Georgia-01R both years, C-99R and Tifguard in 2007, and C34-24-69 and C724-19-25 in 2009. In all of these cases, the hand-shelled plots had significantly greater stands than the mechanically shelled plots for untreated and/or B. subtilis treated seed. B. subtilis treatment improved stands for two of the mechanically shelled treatments that significantly responded to hand shelling. Despite low incidence, there was a significant reduction of crown rot in the hand-shelled plots compared to the mechanically shelled plots across years and genotypes. The cultivar Georganic, which has been planted to organic peanut systems in Georgia in recent years, was among the genotypes with the lowest seedling establishment rates and biomass. Based on these results, it is recommended that sorting thresholds for damaged seed be higher for seed destined for organic production, and that breeding efforts that include assessments of factors involved in seedling establishment and growth be prioritized.

ABSTRACT Identifying effective weed control regimes for organic peanut is paramount for improving the feasibility of organic production. Tine cultivation is a proven effective method for reducing in-row weed populations in several crops. Field trials were therefore conducted in 2008 and 2009 to assess the effects of tine cultivation combined with sweep cultivation and supplemental hand-weeding on weed control and overall productivity of two peanut cultivars under organic management. Tine cultivation regimes consisted of two frequencies (once per week or twice per week) for three durations (3 wk, 4 wk, or 5 wk). All cultivation treatments were also cultivated with flat sweeps at least once and hand-weeded periodically during the growing season. A non-cultivated, non-weeded control was included for comparison. All cultivation treatments significantly reduced annual grass populations in 2008 and Florida pusley populations both years. Cultivated treatments also resulted in denser plant stands for peanut (9.2 plants/m to 13.2 plants/m) than the non-cultivated control (5.9 plants/m to 7.9 plants/m). Pod yields in cultivated treatments ranged from 3502 kg/ha to 3823 kg/ha and were all significantly greater than yields in the non-cultivated control (1630 kg/ha). Also, net revenues generated by cultivated treatments ranged from ($3333/ha to $3637/ha) and were greater than that of the control ($1795/ha). Cultivation frequency had little effect on weed control and peanut productivity. However, the 4- and 5-wk durations displayed potential for improving peanut yield, grade, and net revenue over the 3-wk duration, especially when annual grass weeds were predominant. Cultivating once weekly for 4 or 5 wk with a tine cultivator, along with at least one sweep cultivation and supplemental hand-weeding, is a viable, economical option for providing adequate weed control and maximizing productivity of organically-managed peanut at current market premiums.