Integrated Disease Management of Leaf Spot and Spotted Wilt of Peanut

Strong Resistance to Early and Late Leaf Spot in Peanut-Compatible Wild-Derived Induced Allotetraploids

Early (ELS) and late leaf spots (LLS) are two of the most destructive diseases in peanut (Arachis hypogaea). They can cause severe plant defoliation and tremendous yield loss in the absence of fungicide applications. The high costs of fungicides, their potential for deleterious effects on the environment, the tightening of regulations, and the development of fungicide resistance call for additional management strategies to mitigate both diseases. The use of resistant cultivars is an economical way to manage these diseases, but there are limited sources of leaf spot resistance available in cultivated peanuts. Wild peanut species are excellent sources of resistance, but because of the ploidy level, they do not produce fertile progeny when crossed with peanut. To circumvent this, induced allotetraploids were developed so that resistance traits can be introgressed from wild species into peanut. Here we screened 13 induced allotetraploids (BatDur1, BatDur2, BatSten1, GregSten1, IpaCor1, IpaCor2, IpaDur1, IpaDur2, IpaDur3, IpaVillo1, MagDur1, MagSten1, and ValSten1) against ELS and LLS using a detached leaf bioassay to characterize various components of resistance and identify genotypes that can be used for breeding. Strong associations were detected between the measured components of disease resistance (r = 0.91 to 1.0; P < 0.001) and between ELS and LLS bioassays (r = 0.81 to 0.91; P < 0.001). Induced allotetraploids, particularly those derived from A. stenosperma, exhibited fewer and smaller lesions with limited sporulation and reductions in disease progression in both bioassays. Interestingly, allotetraploids derived from the B-genome peanut progenitor A. ipaënsis were almost as susceptible as cultivated genotypes. The overall results reveal several sources of foliar disease resistance that can be used in breeding programs for ELS and LLS resistance.[Formula: see text] Copyright © 2023 The Author(s). This is an open-access article distributed under the CC BY 4.0 International license.

Discrepancies in Serology-Based and Nucleic Acid-Based Detection and Quantitation of Tomato Spotted Wilt Orthotospovirus in Leaf and Root Tissues from Symptomatic and Asymptomatic Peanut Plants

Thrips-transmitted tomato spotted wilt orthotospovirus (TSWV) causes spotted wilt disease in peanuts. A serological test (DAS-ELISA) is often used to detect TSWV in peanut leaf samples. However, in a few studies, DAS-ELISA detected more TSWV infection in root than leaf samples. It was not clear if the increased detection was due to increased TSWV accumulation in root tissue or merely an overestimation. Additionally, it was unclear if TSWV detection in asymptomatic plants would be affected by the detection technique. TSWV infection in leaf and root tissue from symptomatic and asymptomatic plants was compared via DAS-ELISA, RT-PCR, and RT-qPCR. TSWV incidence did not vary by DAS-ELISA, RT-PCR, and RT-qPCR in leaf and root samples of symptomatic plants or in leaf samples of asymptomatic plants. In contrast, significantly more TSWV infection and virus load were detected in root samples of asymptomatic plants via DAS-ELISA than other techniques suggesting that DAS-ELISA overestimated TSWV incidence and load. TSWV loads from symptomatic plants via RT-qPCR were higher in leaf than root samples, while TSWV loads in leaf and root samples from asymptomatic plants were not different but were lower than those in symptomatic plants. These findings suggested that peanut tissue type and detection technique could affect accurate TSWV detection and/or quantitation.

Efficacy and Profitability of Insecticide Treatments for Tomato Spotted Wilt Management on Peanut in South Carolina

Tomato spotted wilt (TSW) is a common and serious disease of peanut (Arachis hypogaea L.) caused by Tomato spotted wilt virus (TSWV; family Tospoviridae, genus Orthotospovirus). Management frequently uses an integrated approach, with cultivar resistance and application of in-furrow insecticide as two critical components. In-furrow insecticides help suppress thrips, which can injure and stunt young growing plants and transmit TSWV, with postemergent application of acephate capable of providing additional thrips control. To examine effects of systemic insecticides (imidacloprid, imidacloprid plus fluopyram, phorate, and acephate) on TSW management, yield, and economic return across cultivar susceptibilities (susceptible, moderately susceptible, and resistant) in South Carolina, a meta-analysis was used to synthesize results from 32 studies conducted between 2009 and 2018. Although efficacy and magnitude of individual treatments varied with susceptibility, imidacloprid increased, whereas phorate generally decreased TSW incidence relative to nontreated controls. In-furrow treatments followed by acephate further reduced TSW incidence and increased profitability. All examined treatments improved yield compared with untreated peanuts except for susceptible cultivars treated with imidacloprid. Imidacloprid plus fluopyram increased yield more than imidacloprid alone for the susceptible group, although there was little difference between these treatments in association with moderately susceptible cultivars. When comparing individual applications, phorate was overall the most profitable option across susceptibilities, although imidacloprid plus fluopyram exhibited analogous profitability for susceptible cultivars. Results from this study can be used to assist producer selection of management options for TSW in peanut.

Evaluation of Alternatives to an Organophosphate Insecticide with Selected Cultural Practices: Effects on Thrips, Frankliniella fusca, and Incidence of Spotted Wilt in Peanut Farmscapes

Peanut growers use a combination of tactics to manage spotted wilt disease caused by thrips-transmitted Tomato spotted wilt virus (TSWV). They include planting TSWV-resistant cultivars, application of insecticides, and various cultural practices. Two commonly used insecticides against thrips are aldicarb and phorate. Both insecticides exhibit broad-spectrum toxicity. Recent research has led to the identification of potential alternatives to aldicarb and phorate. In this study, along with reduced-risk, alternative insecticides, we evaluated the effect of conventional versus strip tillage; single versus twin row seeding pattern; and 13 seed/m versus 20 seed/m on thips density, feeding injury, and spotted wilt incidence. Three field trials were conducted in Georgia in 2012 and 2013. Thrips counts, thrips feeding injuriy, and incidence of spotted wilt were less under strip tillage than under conventional tillage. Reduced feeding injury from thrips was observed on twin-row plots compared with single-row plots. Thrips counts, thrips feeding injury, and incidence of spotted wilt did not vary by seeding rate. Yield from twin-row plots was greater than yield from single-row plots only in 2012. Yield was not affected by other cultural practices. Alternative insecticides, including imidacloprid and spinetoram, were as effective as phorate in suppressing thrips and reducing incidence of spotted wilt in conjunction with cultural practices. Results suggest that cultural practices and reduced-risk insecticides (alternatives to aldicarb and phorate) can effectively suppress thrips and incidence of spotted wilt in peanut.

Evaluation of New High Oleic Virginia-Type Peanut Cultivars for Disease Tolerance, Yield, and Quality

As new cultivars are developed and released by peanut breeding programs, their levels of tolerance to common diseases and the overall profitability of production needs to be determined. Virginia-type peanut cultivars were evaluated for disease resistance/tolerance, yield, and quality when grown under different fungicide programs and in locations varying in disease pressure. Cultivars included a disease susceptible (CHAMPS) and tolerant (Bailey) cultivar and two new high-oleic cultivars, Sullivan and Wynne. Fungicide programs consisted of a leaf spot program, a leaf spot plus Cylindrocladium black rot (CBR) program, a leaf spot plus Sclerotinia blight program, or an untreated check. Cultivars and fungicide programs were arranged in a randomized split-plot design with fungicide treatments in 16-row main plots and cultivars in 4-row subplots. Disease incidence and severity varied among growing seasons and the five fields where experiments were conducted. Overall, Sullivan had good leaf spot tolerance and both Sullivan and Bailey had Sclerotinia and CBR tolerance. All cultivars yielded well in the absence of disease pressure, but Sullivan was consistently the highest yielding cultivar. Grade characteristics varied among cultivars, but while fungicide treatments impacted yield, they had little effect on grade. Net value and profitability of different fungicide programs varied by experiment, but overall Sullivan had the highest net value regardless of fungicide program. Due to slightly higher disease tolerance compared to Bailey, good agronomic characteristics, high yield and quality under a variety of growing environments, and the presence of the high-oleic trait, Sullivan is an excellent cultivar for Virginia-type peanut production in the Virginia-Carolina region.

Three decades of managing Tomato spotted wilt virus in peanut in southeastern United States

Southeastern states namely Georgia, Florida, and Alabama produce two-thirds of the peanuts in the United States. Thrips-transmitted Tomato spotted wilt virus (TSWV), which causes spotted wilt disease, has been a major impediment to peanut production for the past three decades. The cultivars grown in the 1980s were extremely susceptible to TSWV. Early yield losses extended to tens of millions of dollars each year (up to 100% loss in many fields). This situation led to the creation of an interdisciplinary team known as "SWAT: Spotted Wilt Action Team". Initial efforts focused on risk mitigation using a combination of chemical and cultural management practices along with a strong investment in breeding programs. Beginning in the mid 1990s, cultivars with field resistance were developed and integrated with cultural and chemical management options. A Risk Mitigation Index (Peanut Rx) was made available to growers to assess risks, and provide options for mitigating risks such as planting field resistant cultivars with in-furrow insecticides, planting after peak thrips incidence, planting in twin rows, and increasing seeding rates. These efforts helped curtail losses due to spotted wilt. The Peanut Rx continues to be refined every year based on new research findings. Breeding efforts, predominantly in Georgia and Florida, continue to develop cultivars with incremental field resistance. The present-day cultivars (third-generation TSWV-resistant cultivars released after 2010) possess substantially greater field resistance than second-generation (cultivars released from 2000 to 2010) and first-generation (cultivars released from 1994 to 2000) TSWV resistant cultivars. Despite increased field resistance, these cultivars are not immune to TSWV and succumb under high thrips and TSWV pressure. Therefore, field resistant cultivars cannot serve as a 'stand-alone' option and have to be integrated with other management options. The mechanism of resistance is also unknown in field resistant cultivars. Recent research in our laboratory evaluated field resistant cultivars against thrips and TSWV. Results revealed that some resistant cultivars suppressed thrips feeding and development, and they accumulated fewer viral copies than susceptible cultivars. Transcriptomes developed with the aid of Next Generation Sequencing revealed differential gene expression patterns following TSWV infection in susceptible than field resistant cultivars. Results revealed that the upregulation of transcripts pertaining to constitutive and induced plant defense proteins in TSWV resistant cultivars was more robust over susceptible cultivars. On the flipside, the long-term effects of using such resistant cultivars on TSWV were assessed by virus population genetics studies. Initial results suggest lack of positive selection pressure on TSWV, and that the sustainable use of resistant cultivars is not threatened. Follow up research is being conducted. Improvements in TSWV management have enhanced sustainability and contributed to increased yields from <2800kg/ha before 1995 to ∼5000kg/ha in 2015.

Evaluation of Alternatives to Carbamate and Organophosphate Insecticides Against Thrips and Tomato Spotted Wilt Virus in Peanut Production

Thrips are important pests of peanut. They cause severe feeding injuries on peanut foliage in the early season. They also transmit Tomato spotted wilt virus (TSWV), which causes spotted wilt disease. At-plant insecticides and cultivars that exhibit field resistance to TSWV are often used to manage thrips and spotted wilt disease. Historically, peanut growers used the broad-spectrum insecticides aldicarb (IRAC class 1A; Temik) and phorate (IRAC class 1B; Thimet) for managing thrips and thereby reducing TSWV transmission. Aldicarb has not been produced since 2011 and its usage in peanut will be legally phased out in 2018; therefore, identification of alternative chemistries is critical for thrips and spotted wilt management. Here, eight alternative insecticides, with known thrips activity, were evaluated in field trials conducted from 2011 through 2013. In addition, different application methods of alternatives were also evaluated. Imidacloprid (Admire Pro), thiamethoxam (Actara), spinetoram (Radiant), and cyantraniliprole (Exirel) were as effective as aldicarb and phorate in suppressing thrips, but none of the insecticides significantly suppressed spotted wilt incidence. Nevertheless, greenhouse assays demonstrated that the same alternative insecticides were effective in suppressing thrips feeding and reducing TSWV transmission. Spotted wilt incidence in the greenhouse was more severe (∼80%) than in the field (5–25%). In general, field resistance to TSWV in cultivars only marginally influenced spotted wilt incidence. Results suggest that effective management of thrips using alternative insecticides and subsequent feeding reduction could improve yields under low to moderate virus pressure.

Tropical food legumes: virus diseases of economic importance and their control

Diverse array of food legume crops (Fabaceae: Papilionoideae) have been adopted worldwide for their protein-rich seed. Choice of legumes and their importance vary in different parts of the world. The economically important legumes are severely affected by a range of virus diseases causing significant economic losses due to reduction in grain production, poor quality seed, and costs incurred in phytosanitation and disease control. The majority of the viruses infecting legumes are vectored by insects, and several of them are also seed transmitted, thus assuming importance in the quarantine and in the epidemiology. This review is focused on the economically important viruses of soybean, groundnut, common bean, cowpea, pigeonpea, mungbean, urdbean, chickpea, pea, faba bean, and lentil and begomovirus diseases of three minor tropical food legumes (hyacinth bean, horse gram, and lima bean). Aspects included are geographic distribution, impact on crop growth and yields, virus characteristics, diagnosis of causal viruses, disease epidemiology, and options for control. Effectiveness of selection and planting with virus-free seed, phytosanitation, manipulation of crop cultural and agronomic practices, control of virus vectors and host plant resistance, and potential of transgenic resistance for legume virus disease control are discussed.

Predicting favorable conditions for early leaf spot of peanut using output from the Weather Research and Forecasting (WRF) model

Early leaf spot of peanut (Arachis hypogaea L.), a disease caused by Cercospora arachidicola S. Hori, is responsible for an annual crop loss of several million dollars in the southeastern United States alone. The development of early leaf spot on peanut and subsequent spread of the spores of C. arachidicola relies on favorable weather conditions. Accurate spatio-temporal weather information is crucial for monitoring the progression of favorable conditions and determining the potential threat of the disease. Therefore, the development of a prediction model for mitigating the risk of early leaf spot in peanut production is important. The specific objective of this study was to demonstrate the application of the high-resolution Weather Research and Forecasting (WRF) model for management of early leaf spot in peanut. We coupled high-resolution weather output of the WRF, i.e. relative humidity and temperature, with the Oklahoma peanut leaf spot advisory model in predicting favorable conditions for early leaf spot infection over Georgia in 2007. Results showed a more favorable infection condition in the southeastern coastline of Georgia where the infection threshold were met sooner compared to the southwestern and central part of Georgia where the disease risk was lower. A newly introduced infection threat index indicates that the leaf spot threat threshold was met sooner at Alma, GA, compared to Tifton and Cordele, GA. The short-term prediction of weather parameters and their use in the management of peanut diseases is a viable and promising technique, which could help growers make accurate management decisions, and lower disease impact through optimum timing of fungicide applications.

Field Evaluations of Leaf Spot Resistance and Yield in Peanut Genotypes in the United States and Bolivia

Field experiments were conducted in 2002 to 2006 to characterize yield potential and disease resistance in the Bolivian landrace peanut (Arachis hypogaea) cv. Bayo Grande, and breeding lines developed from crosses of Bayo Grande and U.S. cv. Florida MDR-98. Diseases of interest included early leaf spot, caused by the fungus Cercospora arachidicola, and late leaf spot, caused by the fungus Cercosporidium personatum. Bayo Grande, MDR-98, and three breeding lines, along with U.S. cvs. C-99R and Georgia Green, were included in split-plot field experiments in six locations across the United States and Bolivia. Whole-plot treatments consisted of two tebuconazole applications and a nontreated control. Genotypes were the subplot treatments. Area under the disease progress curve (AUDPC) for percent defoliation due to leaf spot was lower for Bayo Grande and all breeding lines than for Georgia Green at all U.S. locations across years. AUDPC for disease incidence from one U.S. location indicated similar results. Severity of leaf spot epidemics and relative effects of the genotypes were less consistent in the Bolivian experiments. In Bolivia, there were no indications of greater levels of disease resistance in any of the breeding lines than in Bayo Grande. In the United States, yields of Bayo Grande and the breeding lines were greater than those of the other genotypes in 1 of 2 years. In Bolivia, low disease intensity resulted in the highest yields in Georgia Green, while high disease intensity resulted in comparable yields among the breeding lines, MDR-98, and C-99R. Leaf spot suppression by tebuconazole was greater in Bolivia than in the United States. This result indicates a possible higher level of fungicide resistance in the U.S. population of leaf spot pathogens. Overall, data from this study suggest that Bayo Grande and the breeding lines may be desirable germplasm for U.S. and Bolivian breeding programs or production.

Interactive Effects of Planting Date and Cultivar on Tomato Spotted Wilt of Peanut

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.

Impact of Early Spring Weather Factors on the Risk of Tomato Spotted Wilt in Peanut

Peanut growers in the southeastern United States have suffered significant economic losses due to spotted wilt caused by Tomato spotted wilt virus (TSWV). The virus is transmitted by western flower thrips, Frankliniella occidentalis, and tobacco thrips, F. fusca, and was first reported in the southeast in 1986. The severity of this disease is extremely variable in individual peanut fields, perhaps due to the sensitivity of the vector population to changing weather patterns. The objective of this study was to investigate the impact of early spring weather on spotted wilt risk in peanut. On-farm surveys of spotted wilt severity were conducted in Georgia peanut fields in 1998, 1999, 2002, 2004, and 2005. The percent spotted wilt intensity (%) for cv. Georgia Green was recorded and categorized into three intensity levels: low, moderate, and high. Meteorological data were obtained from the Georgia Automated Environmental Monitoring Network for the period between March 1 and April 30. Statistical analysis was conducted to identify weather variables that had significant impact on spotted wilt intensity. The results indicated a high probability of spotted wilt if the number of rain days during March was greater than or equal to 10 days and planting was before 11 May or after 5 June. The total evapotranspiration in April (>127 mm) and the average daily minimum temperature in March (>6.8°C) similarly increased the risk of spotted wilt. Knowing in advance the level of spotted wilt risk expected in a peanut field could assist growers with evaluating management options and significantly improve the impact of their decisions against spotted wilt risk in peanut.

Comparison of ELISA and RT-PCR assays for the detection of Tomato spotted wilt virus in peanut

Diagnosis of Tomato spotted wilt virus (TSWV) in peanut can be accomplished by enzyme-linked immunosorbent assay (ELISA) or reverse transcription polymerase chain reaction (RT-PCR) but there has been no report of a direct comparison of the success of the two assays in evaluating infection rates of field-grown peanut. We collected peanut root samples from field-grown plants, 76 in 2006 and 48 in 2007, and tested these samples by both ELISA and RT-PCR assays for the presence of TSWV. Out of 124 samples, 50 (40.3%) and 57 (46.0%) were positive for TSWV by ELISA and RT-PCR respectively. In 13.7% of these samples, ELISA and RT-PCR differed in their results. However, Chi square analysis showed no significant difference between the results for these two assays. This result supports the conclusion that ELISA and RT-PCR are comparable for detecting TSWV infection rates in field-grown peanuts.

Response of New Field-Resistant Peanut Cultivars to Twin-Row Pattern or In-Furrow Applications of Phorate for Management of Spotted Wilt

Field experiments were conducted at Marianna, FL in 2006 and Tifton, GA in 2006 and 2007 to compare new peanut (Arachis hypogaea) cultivars to the moderately resistant cv. Georgia Green and the highly resistant cv. AP-3 for field resistance to Tomato spotted wilt virus (TSWV), genus Tospovirus, and to determine the effects of in-furrow application of phorate insecticide and use of twin-row versus single-row patterns on incidence of spotted wilt in these cultivars. Cvs. Georgia Green, AP-3, Georgia-03L, Georgia-01R, Florida-07, McCloud, and York were evaluated in all five experiments, and Tifguard was added in experiments at Tifton. All cultivars except McCloud had lower incidence of spotted wilt than Georgia Green in all experiments. McCloud was intermediate in resistance to TSWV and had lower incidence of spotted wilt than Georgia Green in four of five experiments. Use of the twin-row pattern also reduced incidence of spotted wilt in McCloud in both years. On Georgia Green, phorate reduced incidence of spotted wilt in 2007 and twin-row pattern reduced incidence in both years. Phorate had no effect on spotted wilt in AP-3, Georgia-03L, McCloud, Georgia-01R, or Tifguard in either year. Twin-row pattern reduced either final incidence or area under the disease progress curve in all cultivars in at least 1 year of the study. All of these new cultivars should reduce the risk of losses to spotted wilt compared with Georgia Green. In highly resistant cultivars, especially AP-3, York, and Tifguard, use of phorate insecticide or twin-row pattern may not be necessary, and may not provide noticeable benefit in reduction of spotted wilt or increased yield.

Evaluation of Resistance to Cylindrocladium Parasiticum of Runner-Type Peanut in The Greenhouse and Field

Identification and utilization of peanut cultivars with resistance to Cylindrocladium black rot (CBR) is a desirable approach to manage this disease. The objectives of this study were to improve greenhouse and field screening techniques for resistance to CBR, and to evaluate the reaction of selected runner-type peanut genotypes. Georgia-02C (moderately resistant to CBR) and C-99R (CBR-susceptible) were used in comparing the effectiveness of different inoculation methods in the greenhouse. Disease development was affected by both size and density of microsclerotia in soil. Use of microsclerotia at a size of ≥150 to &lt;250 µm and a density of 1 to 5 microsclerotia/g soil provided the best separation the CBR-resistant cultivar Georgia-02C and the susceptible C-99R based on root rot severity. Genotypes with varying resistance to CBR were evaluated by growth in a naturally infested field, and by inoculating plants in the field and greenhouse. Disease incidence and severity at harvest were the most effective parameters for evaluating CBR resistance in the field and greenhouse, respectively. The cultivars Georgia-02C and Georganic had the lowest disease incidence, whereas C-99R and DP-1 had the highest disease incidence in a naturally infested field in 2005 and 2006. Incidence of CBR was moderate for Georgia-01R in both years, but was inconsistent for C34-24-85. Georgia-02C and Georganic also showed partial resistance to CBR in greenhouse tests. Inoculated plants in the field had similar reaction with Georgia-02C and Georganic showing higher CBR resistance than C-99R and DP-1 in both 2006 and 2007. The root rot severities for genotypes Georgia-02C and Georganic were lower than those for C-99R and DP-1. Incidence of CBR in the naturally infested field was significantly correlated with CBR incidence in the inoculated plants in the field (r  =  0.84, P ≤ 0.01), but neither was correlated with disease ratings for greenhouse experiments. Peanut genotypes are most reliably screened by inoculating plants in the field or using uniformly infested fields. Further study is needed to improve greenhouse screening procedures.

Use of Resistant Cultivars and Reduced Fungicide Programs to Manage Peanut Diseases in Irrigated and Nonirrigated Fields

Field experiments were conducted in 2004 and 2005 to evaluate the response of several peanut cultivars to standard and reduced-input fungicide programs under production systems which differed in the duration of crop rotation, disease history within a field, or in the presence or absence of irrigation. Effects on early leaf spot (caused by Cercospora arachidicola), late leaf spot (caused by Cercosporidium personatum), and southern stem rot (caused by Sclerotium rolfsii), pod yields, and economic returns were assessed. Standard fungicide programs were similar for both sets of experiments and included applications of pyraclostrobin, tebuconazole, azoxystrobin, or chlorothalonil. Reduced-fungicide programs, comprising combinations of the aforementioned fungicides, resulted in two and four applications for the cultivar and irrigation experiment, respectively. Two additional programs (a seven-spray chlorothalonil and a nontreated control) were included in the cultivar experiment. Fungicide programs provided adequate levels of leaf spot suppression, and stem rot incidence was similar among fungicide programs within the two management systems. In the cultivar experiment, returns were significantly lower for the reduced program compared with the full program and seven-spray chlorothalonil program; however, they were significantly higher than the nontreated control. Significant differences in leaf spot, stem rot, and yield were observed among cultivars in both experiments. Overall, leaf spot intensity was lowest for the cvs. Georgia-03L and Georgia-01R and greatest for Georgia Green and Georgia-02C. Georgia-03L, Georgia-02C, and AP-3 consistently had lower incidence of stem rot than the other cultivars. Pod yields for all cultivars were equivalent to or greater than Georgia Green in both experiments; however, the performance of reduced-fungicide programs was inconsistent.

Effects of Cover Crop Residue and Preplant Herbicide on Early Leaf Spot of Peanut

Epidemics of early leaf spot, caused by Cercospora arachidicola, of peanut (Arachis hypogaea) are delayed in strip-tilled compared to conventionally tilled fields. This effect may be due to applications of glyphosate used to kill the winter cover crop in strip-tilled fields and/or the presence of cover crop residue at the soil surface of strip-tilled fields. Preplant herbicide (no herbicide, glyphosate, and paraquat), reciprocal residue (plus residue in conventionally tilled plots and minus residue in strip-tilled plots), and added straw mulch were evaluated to determine their effects on early leaf spot epidemics (AUDPC based on incidence and severity, and final percent defoliation) in conventionally tilled and strip-tilled plots. Additional experiments were conducted to characterize the effects of mulch (straw, fumigated straw, and plastic straw [Textraw]) treatments on disease, and to study tillage effects on disease in nonrotated peanut fields. Glyphosate and paraquat had no effect on AUDPC values or defoliation. The addition of straw to conventionally tilled plots significantly reduced disease levels. Cover crop and straw treatments had no significant effect on disease in the strip-tilled plots. AUDPC values were highest in the bare soil plots, lowest in the straw and fumigated straw plots, and intermediate in the plots with Textraw. Fewer initial infections were detected in the Textraw plots compared to the bare soil plots based on results of a trap leaf experiment. Strip-tillage did not consistently suppress early leaf spot epidemics in nonrotated fields. These results show that the presence of cover crop residue is partly responsible for the early leaf spot suppression observed in strip-tilled fields. Cover crop residue may interfere with the dispersal of primary inoculum from overwintering stroma in the soil to the plant tissues.

Characterization of early leaf spot suppression by strip tillage in peanut

ABSTRACT Epidemics of early leaf spot of peanut (Arachis hypogaea), caused by Cercospora arachidicola, are less severe in strip-tilled than conventionally tilled fields. Experiments were carried out to characterize the effect of strip tillage on early leaf spot epidemics and identify the primary target of suppression using a comparative epidemiology approach. Leaf spot intensity was assessed weekly as percent incidence or with the Florida 1-to-10 severity scale in peanut plots that were conventionally or strip tilled. The logistic model, fit to disease progress data, was used to estimate initial disease (y(0)) and epidemic rate (r) parameters. Environmental variables, inoculum abundance, and field host resistance were assessed independently. For experiments combined, estimated y(0) was less in strip-tilled than conventionally tilled plots, and r was comparable. The epidemic was delayed in strip-tilled plots by an average of 5.7 and 11.7 days based on incidence and severity, respectively. Tillage did not consistently affect mean canopy temperature, relative humidity, or frequency of environmental records favorable for infection or spore dispersal. Host response to infection was not affected by tillage, but infections were detected earlier and at higher frequencies with noninoculated detached leaves from conventionally tilled plots. These data suggest that strip tillage delays early leaf spot epidemics due to fewer initial infections; most likely a consequence of less inoculum being dispersed to peanut leaves from overwintering stroma in the soil.