J. P. Beasley

Development of a Method of Risk Assessment to Facilitate Integrated Management of Spotted Wilt of Peanut

Tomato spotted wilt virus (TSWV) is one of a growing list of tospoviruses adversely affecting agriculture worldwide (36,38). Spotted wilt, the disease caused by TSWV, was first observed in peanut (Arachis hypogaea L.) growing regions of the southeastern United States in 1986 (41), and its importance steadily increased in Georgia through 1997 to the extent that annual losses exceeded

Effect of new peanut cultivar Georgia Browne on epidemics of spotted wilt

40 million. Since 1997, annual losses have sharply declined to approximately

Row Pattern and Seeding Rate Effects on Agronomic, Disease, and Economic Factors in Large-Seeded Runner Peanut

10 million in 2000 (60) and

Peanut Seed Vigor Evaluation Using a Thermal Gradient

3.3 million in 2003 (15). The only known means of virus transmission is via vectors belonging to a few species of thrips (36,50). Only first instar larvae of Frankliniella occidentalis (Pergande), one vector species, can acquire the virus from an infected plant (57). After acquisition, the virus replicates in the vector and the viruliferous thrips is capable of transmission for the duration of its life (50,56,59). In Georgia, the primary vectors are tobacco thrips, F. fusca (Hinds), and western flower thrips, F. occidentalis (53). Most spotted wilt in peanut is thought to be the result of primary transmission, but some secondary transmission probably occurs as well (22), mostly by F. fusca, which readily reproduces on peanut (53,54). Peanut plants may initiate symptom expression at any stage of plant development (6), but severe stunting (Fig. 1) typically occurs on younger plants, whereas the initiation of symptoms on older plants typically results in generalized yellowing and/or localized leaf mottling (Fig. 2). Linear regression indicates that both number of seed and seed yield per infected plant increase with time from planting until symptom expression (27). Incidence and severity of tomato spotted wilt of peanut have been extremely variable in Georgia peanut fields (11). Research programs conducted since 1990 have identified several contributing factors affecting disease severity and have provided estimates of their relative importance. Some factors are more important than others, but no single factor can be manipulated to control the disease (26). Using available research data, a spotted wilt risk index was developed as a means of helping peanut growers to assess risk levels associated with specific combinations of production practices and thereby to avoid high-risk situations. Pertinent production practices were assigned point values that were weighted relative to their influence on final spotted wilt severity. As new research data became available, the index was modified and validated with extensive small plot trials and on-farm surveys. This article describes how the index was developed and modified and reports the results of on-farm surveys used to validate the index for peanut grown under a wide array of production practices. Factors Affecting Severity of Tomato Spotted Wilt in Peanut Peanut cultivar. Soon after spotted wilt became established in Georgia, the cultivar ‘Southern Runner’ was noted to have about half the incidence of that seen on the more widely grown ‘Florunner’ (5,28). This serendipitous discovery was followed by the discovery of a series of other varieties, some of which were produced from crosses with ‘Southern Runner’, having similar levels of resistance. ‘Georgia Browne’ (25), ‘Georgia Green’ (29), ‘Florida MDR 98’ (32), ‘ViruGard’ (31), ‘C99R’ (30,40,58), and several advanced breeding lines (29,30) have been shown to exhibit lower incidence of spotted wilt than ‘Florunner’ and other equally susceptible cultivars. The mechanism of resistance exhibited by these cultivars is unknown, but since thrips populations on resistant cultivars do not appear to be significantly lower than those on susceptible cultivars, differences in cultivar susceptibility are not thought to be due to differential preference by vectors (25,28–31). Planting date. Prior to severe outbreaks of spotted wilt in Georgia, planting date was found to influence the incidence of the disease on peanuts grown in southern Texas (47), where peanuts planted early and late in the normal planting season tended to have more spotted wilt than peanuts planted in the middle of the planting season, and those planted within a recommended “window” expressed less severe symptoms. Although actual planting dates are slightly different, a similar trend was Corresponding author: Steve L. Brown E-mail: bugbrown@arches.uga.edu

Interactive effects of planting date and cultivar on tomato spotted wilt of peanut.

Epidemics of spotted wilt, caused by tomato spotted wilt tospovirus, were monitored in plantings of groundnut cultivars Georgia Browne, Southern Runner and Florunner in 5 tests during 1990-93 at Attapulgus, Georgia, USA, and in plantings of the 3 cultivars plus Marc I and AT-127 in 3 tests in Colquitt County, Georgia, in 1993. Final incidence of spotted wilt and area under the disease progress curve values for Georgia Browne were similar to those for Southern Runner but lower than those for Florunner, Marc I and At-127. Pod yields for Georgia Browne were higher than those for Florunner in all tests at both locations and higher than those for the 4 other cultivars in Colquitt County in 1993. Pod yields were similar for Georgia Browne and Southern Runner in 5 tests in Attapulgus. Numbers of tobacco thrips (Frankliniella fusca), western flower thrips (F. occidentalis) or larvae of undifferentiated Frankliniella spp. that colonized the cultivars were similar in most cases. Differences among the cultivars in incidence of spotted wilt could not be attributed to differences in thrips population.

Fall-Bedding for Reduced Digging Losses and Improved Yield in Strip-Till Peanut

Recent peanut cultivar releases are trending to a larger seed size, but have great resistance to tomato spotted wilt virus (TSWV). Larger-seeded cultivars cost more to plant than smaller at an equivalent population. Reduced seeding rates could save growers on seed costs and impede the spread of southern stem rot, but can reduce plant stands which can lower yields and increase TSWV incidence. Therefore, the objectives of this experiment were to compare seven peanut cultivars (Georgia Green, Georgia-06G, AT 3085RO, Florida-07, Tifguard, AP-3, and Georgia-03L) in single and twin row patterns at three seeding rates (17, 20, and 23 seed/m) on a sandy loam soil at Plains, GA for disease incidence, agronomic, and economic performance. Measured variables included yield and grade, plant height and stand, TSWV and southern stem rot incidence, and adjusted net revenue in 2008 and 2009. Twin rows outperformed single rows whenever differences occurred. The only factors consistently affected by reducing seeding rate were plant height and stand, both decreased at the lowest seeding rate. There was a trend toward lower yields (approximately 6% reduction) at the 17 seed/m rate in twin row pattern, although net returns were not diminished compared to the higher seeding rates since lower seed costs offset yield reductions. The cultivars Georgia-06G and Florida-07 had the highest yield and adjusted net revenue among the seven cultivars in both years. Tifguard and Georgia Green had lowest overall yields and would not be preferred cultivars in sandy loam soils. This study demonstrates that twin rows have higher yield, plant stands, and net revenue, plus reduced TSWV incidence than single row pattern, and a reduction in seeding rate to 17 seed/m can be made without serious risk of lost revenue. However, benefits of reducing seeding rate in twin rows were not as pronounced as they were for single rows, and exhibited a greater potential for lower yield. A grower planting in single rows would likely have the most to gain from planting fewer seed, especially under heavy southern stem rot pressure, but planting in twin rows would still be a preferred option over single rows.

Genotypic Differences in Current Peanut (Arachis hypogaea L.) Cultivars in Phenology and Stability of These Traits under Different Irrigation Scheduling Methods

Experiments conducted from 2007 to 2009 evaluated germination of 11 peanut runner-type cultivars. Germination was evaluated in Petridishes incubated over a thermal gradient ranging from 14 to 30°C at 1.0 C increments. Beginning 24 hr after seeding, peanut was counted as germinated when radicles were greater than 5 mm long, with removal each day. Germination was counted daily for seven days after seeding. Growing-degree day (GDD) accumulation for each temperature increment was calculated based on daily mean temperature for that Petri dish. Two indices were obtained from a logistic growth curve used to elucidate seed germination by cultivar: (1) maximum indices of germination and (2) GDD value at 80% germination (Germ80), an indication of seed vigor the lower the Germ80 value, the greater the seed lot vigor. Based on the two indices, seed lots “AT 3081R”, “AP-3”, “GA-06G”, and “Carver” had the strongest seed vigor (Germ80 26 to 47 GDD) and a high maximum incidence of germination rate (80 to 94%). Seed lots of “C99-R”, “Georgia-01R”, “Georgia-02C”, and “Georgia-03L” had inconsistent seed performance, failing to achieve 80% germination in at least two of three years.

Effect of Gypsum Application Rate, Soil Type, and Soil Calcium on Yield, Grade and Seed Quality of Runner Type Peanut Cultivars

Field experiments were conducted at Gainesville and Marianna, FL in 2004 and 2005 in which severity of spotted wilt, caused by Tomato spotted wilt virus, and pod yield were compared in six peanut (Arachis hypogaea) cultivars. The six cultivars included the moderately field resistant cultivars ANorden, C-99R, and Georgia Green; the highly field resistant cultivars AP-3 and DP-1; and the susceptible cultivar SunOleic 97R. There were four trials at each location, with four planting dates that ranged from late March to early June. Tomato spotted wilt severity in moderately resistant and susceptible cultivars was lower at Gainesville than at Marianna in both years in moderately resistant and susceptible cultivars. Trends in incidence for the two locations were less evident for AP-3 and DP-1. At Gainesville, there were few differences in tomato spotted wilt severity, and severity ratings were similar for Georgia Green and SunOleic 97R in two of four trials in 2004 and across all trials in 2005. At Marianna, severity ratings were lower for Georgia Green than for SunOleic 97R in six of the eight trials, and severity of tomato spotted wilt was lower for AP-3, C-99R, and DP-1 than for Georgia Green in all eight trials. In 2004, there was a trend toward decreasing severity ratings for Georgia Green and SunOleic 97R with later planting dates, but not for AP-3 or DP-1 at Marianna. Split-plot field experiments were also conducted at Tifton, GA in 2005 through 2007 in which incidence of tomato spotted wilt and pod yield were compared for peanut cultivars AP-3 and Georgia Green across planting dates ranging from late April through late May. Incidence of tomato spotted wilt was lower for AP-3 than for Georgia Green within each planting date of all years, and planting date effects were smaller in AP-3, if observed at all, than in Georgia Green. In most planting dates of all three trials, yields were higher for AP-3 than for Georgia Green. The relationships between yield and planting date were not consistent. These results indicate that the level of field resistance in AP-3 and DP-1 cultivars is sufficient to allow planting in late April without greatly increasing the risk of losses to tomato spotted wilt.

Plant Population and Replant Method Effects on Peanut Seeded in Single Rows

Abstract Most peanut (Arahcis hypogaea L.) production occurs under highly intensive conventional tillage systems. With recent volatility in input prices, reducing tillage trips is a viable way of reducing production costs. However, growers can experience yield loss when switching from conventional tillage to strip-tillage in peanut on certain soil types due to the lack of an elevated bed at harvest time. Studies were conducted to compare standard strip-till with strip-till on two-row raised beds as well as rip and beds prepared in the fall. Comparisons were made on a coarse textured soil at Tifton, GA and a fine textured soil at Plains, GA. The three bed types, with and without wheat cover, were evaluated over two years at both locations. No effects of cover or interactions with bed type were present. At Plains, the rip and bed and raised bed reduced digging losses by 62 and 47%, respectively. Soil compaction within the harvest depth was reduced by 3.3 and 4.7 times by the raised bed and rip and bed, resp...

Hysteresis response of daytime net ecosystem exchange during drought

Understanding differences among peanut (Arachis hypogaea L.) cultivars in growth and phenology and the interactions with environment (G X E interactions) for these traits allows predictions for yield potential or performance in variable environments. Despite the importance of this information, very little quantitative data exists on the differences in aboveground growth, canopy architecture, and reproductive phenology for currently grown peanut cultivars. This study quantified differences in these traits among eight peanut cultivars and explored whether irrigation scheduling method (a factor of environment) affected the development in these traits through the season in 2004 and 2005. As expected, year to year variability in environmental conditions (most likely timing of rainfall events during the growing season) significantly affected growth habit across cultivars. However, the irrigation scheduling method, despite differences in total water applied among methods during the season, had no effect on any of the measured traits. This result is likely due to the fact that all methods were adequately supplying crop water demand. Genetic variability in all of the measured growth and phonological traits was strong despite the expectation that cultivars were genetically similar. Further, the lack of significant interactions between year and cultivar for most of the plant growth and reproductive characteristics also indicated a strong genetic component to these traits. One overall trend noted was that late-maturing cultivars had, on average, higher maximum values of LAI, stem mass, and leaf mass measured in the late growth period. Differences in isotopic composition were also strong among cultivars; the cultivars Georgia-02C and Tifrunner had significantly higher isotopic levels (and thus water-use efficiency) than Georgia-01R, Georgia Green, and AP3 across years. Aside from the obvious relationships between pod number and weight, the strongest predictors of reproductive output were late-season traits including leaf weight and LAI. This study successfully documented variability among peanut cultivars in many important traits linked to overall production.