© 2002 Plant Management Network.
Peanut Disease Management Utilizing an In-furrow Treatment of Azoxystrobin
Steve L. Rideout, Former Graduate Research Assistant, Timothy B. Brenneman, Professor, and Albert K. Culbreath, Professor, Department of Plant Pathology, The University of Georgia, Coastal Plain Experiment Station, Tifton 31793-0748
Corresponding author: Steve L. Rideout. firstname.lastname@example.org
Rideout, S. L., Brenneman, T. B., and Culbreath, A. K. 2002. Peanut disease management utilizing an in-furrow treatment of azoxystrobin. Online. Plant Health Progress doi:10.1094/PHP-2002-0916-01-RS.
Applications of azoxystrobin in-furrow have shown inconsistent results in terms of yield and disease control in experimental peanut plots. Two trials were conducted in both 2000 and 2001 to better define the effects of this treatment on peanut disease control and pod yield. Treatments included: azoxystrobin in-furrow alone (102 g ai/ha), azoxystrobin mid-season alone (two applications at 335 g ai/ha), azoxystrobin both in-furrow and mid-season, and a nontreated control. Destructive sampling was conducted throughout the growing season to determine treatment effects. The in-furrow treatment of azoxystrobin had minimal impact on plant stand counts, tomato spotted wilt incidence, yield, and crop value. However, in-furrow applications of azoxystrobin did suppress levels of Aspergillus crown rot (Aspergillus niger) and early season (prior to 60 days after planting) southern stem rot (Sclerotium rolfsii). However, season-long control of stem rot was only accomplished through the use of mid-season applications of azoxystrobin. Similarly, yields and crop value were only significantly increased when in-season applications of azoxystrobin were made. These trials were conducted under optimal planting and growing conditions using high quality seed. Testing under adverse conditions or heavier disease pressure, which may reveal more benefits from a treatment of azoxystrobin in-furrow, needs to be conducted.
Environmental and edaphic factors prevalent in the Coastal Plain region of the southeastern United States favor the production of peanut (Arachis hypogaea L.). However, these same conditions favor intense disease development on peanut. One disease of particular importance in this region is southern stem rot caused by the soilborne fungus Sclerotium rolfsii Sacc. Over the past decade, Georgia peanut farmers have incurred average annual losses of 30 to 40 million dollars due to southern stem rot.
In the past, chemical management of southern stem rot was accomplished through an application of pentachloronitrobenzene (PCNB) at pegging and to a lesser extent, with an application of the insecticide chlorpyrifos which has some suppressive activity against southern stem rot (1,2,11). However, in the last decade, the fungicides tebuconazole, flutalonil, and azoxystrobin have provided increased suppression of southern stem rot in peanut (1,13). Initial applications of these fungicides are generally made around 60 days after planting followed by one to three subsequent applications (depending on the fungicide selected) on a calendar schedule. While the calendar-based stem rot management scheme is usually adequate, favorable environmental conditions may trigger disease onset earlier in some growing seasons, causing substantial damage prior to the initial fungicide application. In years where early epidemics of southern stem rot occur, improved disease control should result from either scheduling initial fungicide applications when weather patterns favor disease development or at planting or early in the growing season to delay the onset of southern stem rot.
Few early season management tools aimed at suppressing the initial stages of southern stem rot have been investigated. Currently, a limited number of producers in Georgia make an in-furrow application of PCNB at planting, primarily to improve seedling health and stand establishment. This treatment may suppress early season southern stem rot, however, such results have not been documented. In 2000, azoxystrobin (Abound 2.08F, Syngenta Crop Protection, Greensboro, NC) received a label for peanut as an in-furrow treatment. Small, often non-significant yield increases have been seen with azoxystrobin applied in-furrow (unpublished data), but the reasons for these yield gains have not been well documented. Theoretically, the treatment could suppress levels of saprophytic growth of S. rolfsii in the soil around the peanut roots, or it may inhibit initial infection of peanut plants early in the growing season. In addition to the potential benefits for enhanced early season stem rot control, azoxystrobin applied in-furrow may also lower the incidence of seedling diseases, such as those caused by Rhizoctonia solani Kühn and Aspergillus niger Teigh. By reducing the incidence of seedling diseases, plant stands will be increased thereby reducing the incidence of Tomato spotted wilt virus (TSWV), of the genus Tospovirus, family Bunyaviridae (4,5,10,12). Though these theories are sound, they are largely unproven at this time. Further research needs to be conducted to validate or reject these hypotheses.
Five trials conducted at the University of Georgia’s Coastal Plain Experiment Station in Tifton, GA, from 1998 to 2001 showed an increased yield of 815 kg/ha in plots where azoxystrobin was applied in-furrow (Abound 2.08F at 102 g ai/ha) when compared with the nontreated control in the absence of further mid-season fungicide applications targeting soilborne diseases (unpublished data). However, yield was significantly higher for the in-furrow treated plots in only three of the five trials summarized here. Similarly, trends towards reduced levels of tomato spotted wilt and southern stem rot were noted when plots receiving the in-furrow treatment (tomato spotted wilt incidence = 4.6%, stem rot incidence = 31.9%) were compared with the nontreated control (tomato spotted wilt incidence = 7.3%, stem rot incidence = 36.7%). Again, this trend was not consistently significant as disease incidence for both tomato spotted wilt and southern stem rot were significantly reduced in the in-furrow treated plots for one of the five trials for each disease.
In five additional trials, also conducted in Tifton, the trend towards increased yield when using azoxystrobin in-furrow was less noticeable when those treatments were followed by mid-season applications of azoxystrobin at 335 g/ai/ha 60 and 90 DAP (3,unpublished data). Mean yield for plots receiving only mid-season applications of azoxystrobin was 4580 kg/ha, while plots receiving both the in-furrow and mid-season treatments had a mean yield of 4582 kg/ha. Significant yield increases were not obtained in the five trials with in-furrow treatment. In these five trials, southern stem rot was significantly reduced in only one trial, and tomato spotted wilt incidence was never significantly affected by the in-furrow treatment of azoxystrobin. Mean disease incidence values for plots only receiving mid-season applications were 13.7% and 19.4% for tomato spotted wilt and stem rot, respectively, compared with 12.3 and 17.9% for tomato spotted wilt and stem rot in those receiving both the in-furrow and mid-season azoxystrobin treatments.
Several other trials evaluating the effects of azoxystrobin in-furrow on yield and disease levels have been published in Fungicide and Nematicide Reports (7,8,9,16). Results from these trials are in agreement with those obtained from the preliminary University of Georgia data set in that few, if any, yield or disease control benefits were derived from the use of the in-furrow azoxystrobin treatment.
The increased use of in-furrow fungicides and their sporadic effectiveness in prior studies illustrate the need for more detailed research on this relatively new concept. Thus, the objectives of this study were to examine the effects of in-furrow applications of azoxystrobin on peanut yields and disease control. In particular, the effects of the in-furrow treatment on the initial growth and season long development of southern stem rot were examined.
Two trials were conducted in 2000 and 2001 to examine the effects of an azoxystrobin in-furrow treatment on peanut diseases and yield. One trial per year was conducted at both the Blackshank and Gibbs Farms at the University of Georgia’s Coastal Plain Experiment Station in Tifton, GA. The plot area at the Gibbs Farm was a fine-loamy, kaolinitic, thermic Plinthic Kandiudults (Tifton loamy sand, 2-5% slope, pH=5.2) and had been planted in corn (Zea mays L.) the previous year. Soil type for the trials at the Blackshank Farm was a loamy, kaolinitic, thermic Arenic Plinthic Kandiudults (Fuquay sand, 2-5% slope, pH=6.3 for 2001, 6.1 for 2000) that had been previously planted in peanut (2001 location) and watermelon [Citrullus lanatus (Thunb.) Matsum. & Nikai] (2000 location). The runner type peanut cultivar Georgia Green was used in all trials (23 seeds per m per row, 0.91 m row spacing). Peanut seeds were treated with 249 g per 100 kg of seed of a captan (45%), PCNB (15%), and carboxin (10%) seed treatment mixture (Vitavax PC, Gustafson LLC, Plano, TX) prior to planting. Planting dates for the four trials ranged from 9 May to 17 May. Irrigation requirements, in addition to insect, nematode, and weed control were accomplished using standard recommendations of the University of Georgia. Foliar diseases, primarily early and late leaf spot [caused by Cercospora arachidicola Hori and Cercosporidium personatum (Berk. & M. A. Curtis) Deighton, respectively] were controlled in the entire plot area by broadcast applications of chlorothalonil (Bravo Ultrex, Syngenta Crop Protection, Greensboro, NC) at 1.3 kg ai/ha on a two-week schedule in 2000. In 2001, a mixture of chlorothalonil (1.3 kg ai/ha) and propiconazole (Tilt 3.6 EC, Syngenta Crop Protection, Greensboro, NC) at 126 g ai/ha was applied for the first two applications, followed by chlorothalonil (1.3 kg ai/ha) on a two-week schedule.
A split-plot design with four to six replicates per treatment was used. Each individual plot consisted of a two-row bed (7.6 m x 1.8 m). Main plots were azoxystrobin (102 g ai/ha) at planting or nontreated and sub-plots were treated with azoxystrobin (335 g ai/ha per application) at 60 and 90 days after planting (DAP) or nontreated for a total of four possible treatment combinations.
In-furrow treatments of azoxystrobin were applied at planting using a single ConeJet TX-6 hollow cone nozzle (TeeJet Technologies, Springfield, IL) calibrated to deliver a total output of 47 liter/ha. Nozzles were placed in the furrow opener on the planter so that fungicide application and seed drop into the furrow were simultaneous. The spray pattern uniformly covered the width of the open seed furrow as well as the seed itself. Mid-season applications of azoxystrobin were made at 60 and 90 DAP using a CO2 pressurized sprayer calibrated to deliver 188 liter/ha total output with three ConeJet TX-6 hollow cone nozzles per row.
Data Collection. Destructive sampling, as described by Bowen (6), was conducted seven or eight times per trial during the growing season. Prior to 60 DAP, 7.6 m of row per replication, for both nontreated and azoxystrobin in-furrow treated plots, were dug by hand and each individual peanut plant was assessed for the presence of S. rolfsii and/or A. niger. After 60 DAP, 3.8 m of row per replication were assessed in the same manner for each of the four treatment combinations. For each row section sampled at each assessment date, total number of plants present was recorded and incidence for both stem rot and Aspergillus crown rot were calculated. Plants were counted in these ratings if they showed signs of S. rolfsii and/or A. niger or diagnostic symptoms of either disease.
Tomato spotted wilt was rated within several days prior to inversion for all trials based on the incidence of row feet exhibiting aboveground symptoms of infection. Southern stem rot, as well as Cylindrocladium black rot (Cylindrocladium parasiticum Crous, Wingfield, & Alfenas) in one trial, were assessed immediately following inversion by determining the incidence of disease loci (< 30 cm per locus) per plot (15). After several days of field drying, plots were harvested using a combine and dried to approximately 10% before determining yields and grading.
Statistical Analyses. Plant stand counts, disease incidence values for stem rot, Aspergillus crown rot, tomato spotted wilt and Cylindrocladium black rot, as well as yield and crop value were analyzed using analysis of variance (SAS Institute, Cary, NC). Prior to 60 DAP, only two different treatments existed, plots receiving only the application of azoxystrobin in-furrow and non-treated controls. Statistical comparisons between these two treatments were made using student t-tests (P < 0.05). With the onset of mid-season azoxystrobin applications at 60 DAP, two more treatments were present, plots receiving the in-furrow treatment and mid-season applications (at 60 and 90 DAP) and plots receiving only mid-season sprays. Statistical analyses conducted on data after 60 DAP compared four treatments and differences in treatment means were determined using analysis of variance and Waller-Duncan mean separation tests (k-ratio=100). Where appropriate and where no significant treatment-trial interactions were present, combined analyses were conducted across the two trials within years.
Effects of In-Furrow Treatments with Azoxystrobin
Plant Stand Effects. Treatment with azoxystrobin in-furrow did not produce phytotoxicity in any of the trials in this study. Plant stand counts from the two earliest sampling dates for each year showed little difference in seedling emergence between nontreated plots and those treated with azoxystrobin in-furrow (Fig. 1). Only at 21 DAP in 2001 did plots treated with azoxystrobin in-furrow have a significantly higher plant stand count than the non-treated plots. Plant stand counts from 35 DAP and later in the growing season were similar.
Aspergillus crown rot and tomato spotted wilt. Aspergillus crown rot disease incidence was low (< 5%) in these four trials. However, significant reductions in crown rot incidence were consistently observed in plots treated with azoxystrobin in-furrow (Fig. 2). Suppression of crown rot was seen in all four trials across both years and was first observed around 35 to 50 DAP, which typically is the period of highest activity for A. niger. Tomato spotted wilt was observed in both seasons, but incidence was moderate in 2000 and low in 2001. Although plots receiving the in-furrow treatment consistently had the lowest incidence of spotted wilt, differences were not statistically significant (Fig. 3).
Southern stem rot. Virtually no Sclerotium rolfsii was actively growing in the peanut rhizosphere at 21 DAP and incidence at 35 DAP was low (< 5% disease incidence). No significant differences in stem rot between the non-treated plots and the azoxystrobin in-furrow treated plots at 21 and 35 DAP were noted (Table 1). However, a significantly lower (P < 0.05) incidence of stem rot was found in treated versus nontreated plots in all four trials at the 49 DAP sampling date. However, suppression of stem rot was observed in only two of the four tests (both conducted in 2000) at 59 DAP. No further suppression of stem rot due to the in-furrow treatment of azoxystrobin was found at sampling dates later than 58 DAP in any of the trials.
Table 1. Peanut plants showing signs and/or symptoms of Sclerotium rolfsii in plots treated with azoxystrobin in-furrow or nontreated.
w Destructive assessments were made on 7.6 m row per replication. There were four to six replications per trial. Each individual plant was dug and assessed for signs/symptoms of S. rolfsii.
x DAP = days after planting.
y Application of azoxystrobin (102 g ai/ha) in-furrow at planting.
z Student's t-tests were performed to determine differences in treatments. Entries in bold font were significant at the P < 0.05 level.
In all four trials, mid-season applications of azoxystrobin at 60 & 90 DAP significantly (P < 0.05) suppressed stem rot levels at most sampling dates after the 60 DAP rating (Table 2). However, plots that received both an in-furrow and mid-season application of azoxystrobin had the same stem rot damage levels as those receiving only the mid-season treatments.
Table 2. Peanut plants showing signs and/or symptoms of Sclerotium rolfsii in plots treated with applications of azoxystrobin in-furrow and mid-season.
w Destructive assessments were made on 3.8 m row per replication. There were four to six replications per trial. Each individual plant was dug and assessed for signs/symptoms of S. rolfsii.
x DAP = days after planting.
y Treatments were as follows: Azoxystrobin in-furrow alone (102 g ai/ha) at planting (Azoxystrobin IF), nontreated, azoxystrobin applied mid-season alone (335 g ai/ha per application) at 60 and 90 DAP (Azoxystrobin MS), and azoxystrobin applied both in-furrow and mid-season using the same rates (AIF + AMS).
z Means for individual trials and assessment dates that are followed by common letters are not significantly different according to Waller-Duncan tests (k-ratio = 100).
Stem rot incidence ratings taken prior to harvest (immediately after inversion) mirrored the trends seen in the late season destructive assessments. In both 2000 and 2001, plots receiving mid-season applications of azoxystrobin, regardless of whether an in-furrow application was made, showed significantly (P < 0.05) less stem rot when compared to those plots that did not receive the mid-season application of the same fungicide (Fig. 4). When compared to the non-treated control, the in-furrow application of azoxystrobin did not significantly suppress stem rot levels.
Cylindrocladium black rot. Cylindrocladium black rot was only found in one of the four trials in this study. Mean disease incidence ratings were 15.2, 12.0, 14.0, and 8.0% for non-treated plots, plots receiving only the in-furrow treatment, plots receiving only the in-season treatment, and plots receiving both in-furrow and in-season treatments, respectively. In this one trial, no significant differences in the incidence of Cylindrocladium black rot were observed among treatments.
Pod yield and crop value. Pod yield and resulting crop values were
significantly higher in 2001 than in 2000. However, similar trends were noted in
both years for both variables. Yield and value were both increased significantly
(P < 0.05) when mid-season applications of azoxystrobin were made (Fig. 5),
but application of azoxystrobin in-furrow did not increase crop yield or value,
regardless of whether mid
The use of azoxystrobin in-furrow was safe when applied using the rate and delivery methods examined in this study. Similar studies have shown plant injury from in-furrow applications of tebuconazole (unpublished data), but no symptoms of phytotoxicity were observed with azoxystrobin in any of these trials. Higher rates used in preliminary trials also showed no crop injury (unpublished data). In addition, the application method used in this study results in a high level of fungicide exposure for the seed, presumably maximizing the risk of injury. It appears unlikely that azoxystrobin applied in this manner is phytotoxic to peanut.
Beneficial effects of azoxystrobin on plant stand in these tests were minimal, however, these trials were planted under optimal levels of soil temperature and moisture, using a high seeding rate of quality seeds. Planting earlier in the year in cooler soils may lead to less than optimal seed germination and increased seedling damping-off caused by R. solani. In such a situation, the advantages to plant stand realized from an application of azoxystrobin in-furrow may be greater than observed in this study, although further research is necessary to verify this theory.
Planting dates for these trials were also optimal (early May) to minimize the effects of tomato spotted wilt (4,5,10,14). Early to mid-April planting dates have been shown to intensify levels of this disease (4,5,10,14). Although the reasons for this phenomenon are not well understood, lower plant stands have also been shown to enhance tomato spotted wilt incidence (4,5,10,12). It is a reasonable assumption that by ensuring a healthy and vigorous plant stand, tomato spotted wilt levels could be reduced. In this study, conducted under optimal conditions, the azoxystrobin in-furrow treatment had no significant effect on tomato spotted wilt levels. Under adverse planting conditions, increases in plant stand or uniformity of emergence derived from damping-off control may also reduce tomato spotted wilt levels and increase the benefit of the azoxystrobin in-furrow applications. Plant stands observed in these trials were all within the "low risk of spotted wilt" category as defined by the University of Georgia’s 2002 Spotted Wilt Risk Index (4).
Treatment with azoxystrobin in-furrow suppressed levels of early season Aspergillus crown rot in all four trials. However, crown rot incidence was generally low, and probably had no impact on pod yields and crop value. The consistently high level of control observed in these trials indicates that additional benefits beyond those seen in this study may be derived from in-furrow applications of azoxystrobin in fields with a history of high crown rot levels and/or when environmental conditions favor disease development.
Suppression of early season stem rot (prior to 60 DAP) was also seen in all four trials in this study. However, that suppression did not last the entire growing season. Therefore, in fields where stem rot is a problem, in-furrow treatment with azoxystrobin does not give sufficient season-long control of stem rot, and mid-season applications of a fungicide are still warranted. In 2000 and especially in 2001, early season incidence of stem rot was low; therefore, activity of the pathogen prior to 60 DAP had little impact on disease levels later in the year when most damage is incurred by stem rot. Early season stem rot suppression seen in the azoxystrobin in-furrow treated plots may be of more benefit in years when epidemics of stem rot are more severe prior to the 60 DAP initiation of calendar-based fungicide applications. Further testing is necessary to determine the extent of these benefits.
In these studies, the treatment with azoxystrobin in-furrow had no significant impact on pod yield or crop value. Other small plot research trials have found the same trend regarding the in-furrow treatment, although some have shown significant yield increases. Trials showing a benefit have generally been conducted in fields with significant levels of stem rot where no additional post emergence fungicides were applied for stem rot control. Under optimal planting and growing conditions and with adequate disease management practices (i.e., mid-season fungicide applications and treated seeds), an application of azoxystrobin was seen to provide little benefit in the two years of this study. However, with early planting, reduced seeding rates or seed quality, high crown rot pressure, or early epidemics of stem rot, the benefits of in-furrow applications of azoxystrobin are more likely to be observed. Unfortunately, adverse events such as these are difficult to predict, making the grower’s decision on whether to apply azoxystrobin in-furrow more complicated. Therefore, the usage of azoxystrobin in-furrow may best be viewed as insurance against adverse conditions that could be detrimental to the peanut crop.
The authors wish to thank Jimmy Mixon, Lewis Mullis, Pat Hilton, Amy Davis, Jamie Barnhill, Unnessee Hargett, and Don Hickey for their technical assistance. Special thanks also goes to Syngenta Crop Protection for the funding of this study.
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