© 2006 Plant Management Network.
Management of Early Leaf Spot of Peanut as Affected by Fungicide and Date of Spray Program Initiation
A. K. Culbreath, R. C. Kemerait, Jr., and T. B. Brenneman, Department of Plant Pathology, Coastal Plain Experiment Station, The University of Georgia, Tifton 31793-0748
Culbreath, A. K., Kemerait, R. C., Jr., and Brenneman, T. B. 2006. Management of early leaf spot of peanut as affected by fungicide and date of spray program initiation. Online. Plant Health Progress doi:10.1094/PHP-2006-0214-01-RS.
A fungicide program for control of early leaf spot (Cercospora arachidicola) of peanut (Arachis hypogaea) in Georgia typically consists of a full-season program initiated ca. 30 days after planting (DAP) that includes seven applications made on 14-day intervals. Field tests were conducted in Tifton and Plains, GA in 2001 and 2002 to compare full-season and reduced programs of the fungicides pyraclostrobin (168 g a.i./ha), chlorothalonil (1.26 kg a.i./ha), and tebuconazole (227 g a.i./ha). Reduced fungicide programs were achieved by delaying initial applications at 14-day intervals from ca. 30 DAP to 45 through 105 DAP, totaling from 6 to as low as 2 applications per season, respectively. In 2003 and 2004, full-season programs of the fungicides pyraclostrobin and chlorothalonil were compared to reduced programs of pyraclostrobin. Reduced programs that received 6 and 5 applications were achieved by delaying the initial applications from ca. 30 DAP to 45 and 60 DAP, respectively. In 2001 and 2002, control of early leaf spot with pyraclostrobin was superior to that with chlorothalonil or tebuconazole within the respective initiation dates. Delaying the initial application of pyraclostrobin up to 60 DAP gave better control of leaf spot than for full-season programs of chlorothalonil or tebuconazole. Fungicide programs had no effect on yield at Plains in 2001 or Tifton in 2002. At Tifton in 2001 and Plains in 2002, yield decreased according to quadratic functions of time of spray initiation, with no indication of yield reduction until fungicide programs were initiated after 60 to 70 DAP. In 2003 and 2004, reduced programs of pyraclostrobin resulted in final levels of leaf spot that were similar to or lower than those in plots receiving a full-season program with chlorothalonil. Across both years, yields of reduced programs with pyraclostrobin were similar to those of full programs of either chlorothalonil or pyraclostrobin.
Management of early leaf spot, caused by Cercospora arachidicola S. Hori, (Fig. 1) and late leaf spot, caused by Cercosporidium personatum (Berk. & M. A. Curtis) Deighton, diseases of peanut (Arachis hypogaea L.) depends on multiple fungicide applications. On the most widely-planted peanut cultivars in the southeastern U.S., leaf spot diseases can cause extensive defoliation (Fig. 2) and yield loss. Chlorothalonil became the standard fungicide for leaf spot control in the mid-1970s (14) and is still an important tool for managing leaf spot diseases. Chlorothalonil is a protectant fungicide, and application prior to infection by the leaf spot pathogens is essential for achieving satisfactory control. Hence, the date of initiation of spray programs can be critical for optimal leaf spot management. The standard use pattern for chlorothalonil in the southeastern U.S. consists of a full-season spray program totaling seven sprays per season beginning ca. 30 days after planting (DAP) with subsequent applications made every 14-days until two weeks prior to when the crop will be inverted (10). Shokes et al. (13) reported incremental increases in final defoliation caused by early and late leaf spots as initiation dates for chlorothalonil programs were delayed by 14 days from 34 to 188 DAP, and decreases in yield with initiation dates later than 72 DAP.
Tebuconazole is a systemic ergosterol biosynthesis-inhibiting fungicide registered for use on peanut in 1994. Its use has improved the control of one or both leaf spot diseases compared to chlorothalonil alone (1,2,11). Typically, tebuconazole applications are made for control of stem rot (Sclerotium rolfsii Sacc.) and Rhizoctonia limb rot (Rhizoctonia solani Kühn) as well as leaf spot diseases. In a standard tebuconazole "4-spray block" program, applications begin ca. 30 DAP, with the first two applications of a seven-spray program consisting of 1.26 kg/ha of chlorothalonil per application. Four consecutive applications of tebuconazole at 227 g/ha are then applied beginning ca. 60 DAP. The final spray is also chlorothalonil.
Brenneman and Culbreath (1) showed that tebuconazole was superior to chlorothalonil for leaf spot control when applied according to an advisory program where applications were made according to rain events and precipitation forecasts rather than the calendar. Labrinos et al. reported that tebuconazole had systemic activity against C. personatum, and that tebuconazole increased the incubation period of C. personatum and decreased the size of late leaf spot lesions and the amount of sporulation the fungus produced compared to chlorothalonil (11). Results from these studies suggest that tebuconazole might be better than chlorothalonil for leaf spot control when initial applications are delayed. However, direct comparison of chlorothalonil and tebuconazole in fungicide programs reduced by delaying initial applications has not been reported.
Pyraclostrobin is a strobilurin-type fungicide that has shown activity against C. arachidicola (6,7). Pyraclostrobin was registered for use on peanut in the U.S. in 2002. Pyraclostrobin has been reported to provide control of early leaf spot that is superior to that achieved with chlorothalonil applied on similar schedules (6,7). Previous reports also indicated that pyraclostrobin applied on a 21-day schedule was as effective for controlling leaf spot as applications of chlorothalonil on a 14-day schedule, and that the superior efficacy of pyraclostrobin raised the possibility of reducing the number of fungicide applications without compromising leaf spot control or yield (6,7). Neither of these studies compared the fungicides using reduced programs with delayed initial applications. The purpose of this work was to determine the relative effects of reduced fungicide programs with pyraclostrobin, chlorothalonil, and tebuconazole, achieved by delaying initial applications, on control of early leaf spot of peanut.
Full Season and Reduced Fungicide Programs
Peanut seed (18 seeds per m of row) were planted in fields of Tifton loamy sand at the Coastal Plain Experiment Station, Lang Farm, Tifton, GA on 24 May 2001, and 27 May 2002, 5 June 2003, and 28 May 2004, and in fields of Faceville sandy loam at the Southwest Georgia Branch Station, Plains, GA on 7 May 2001 and 7 May 2002. The cv. ‘Georgia Green’ was used in all tests except in 2002 at Tifton when the cv. ‘AT-201’ was used and in 2004 at Tifton when the cv. ’Carver‘ was used. All fields had been planted to cotton (Gossypium hirsutum L.) the previous year and to peanut two years prior. Plots received aldicarb (Temik 15 G, Bayer CropScience, Research Triangle Park, NC) (0.75 to 1.0 kg a.i./ha) in-furrow at planting. The smallest experimental unit in all tests was 7.6 m long and 1.8 m wide and contained two single rows.
In 2001 and 2002, treatments were arranged in a split-plot design with four replications. Whole plot treatments consisted of spray program initiation dates. The initial application of each spray program was scheduled for ca. 30, 44, 58, 72, 86, and 100 days after planting (DAP), resulting in a total of 7, 6, 5, 4, 3, and 2 applications, respectively (Table 1). For the test at Plains in 2001, the first application for the full-season program was made ca. 14 days later than the planned date of 30 DAP. For the full-season program that typically consisted of 7 applications, spray dates were 32, 46, 60, 71, 83, 99, and 119 DAP in 2001 and 28, 43, 59, 71, 84, 95, and 109 DAP at Tifton, and 45, 57, 72, 87, 101, 113, and DAP in 2001 and 36, 55, 70, 80, 92, 105, and 121 DAP in 2002 at Plains. For the reduced fungicide programs, initial applications were delayed by 2,4,6,8, and 10 weeks by omitting the first spray, first two sprays, first three sprays, first four sprays, and first five sprays, respectively (Table 1). Following the initial applications, reduced spray programs followed the same 14-day schedule as the full-season program.
Table 1. Full season and reduced fungicide programs evaluated at Tifton and Plains, GA in 2001 and 2002.
x Numbers 1 through 7 represent sequential fungicide applications made on ca. 14-day intervals. The first application in the full-season program was made ca. 30 days after planting. Dashes (-) represent sprays omitted in the reduced programs. Actual application dates varied slightly among tests.
y The full-season program and the, 6 and 5 application programs were also tested for pyraclostrobin in 2003 and 2004.
Fungicide treatments included: (i) pyraclostrobin (Headline 2.09 EC, BASF Corp, Research Triangle Park, NC) applied at 168 g a.i./ha; (ii) chlorothalonil (Bravo WeatherStik 720 F, Syngenta Crop Protection, Greensboro, NC) applied at 1.26 kg a.i./ha; and (iii) tebuconazole (Folicur 3.6 F, Bayer CropScience, Research Triangle Park, NC) applied at 227 g a.i./ha. All tests also included a nontreated control.
In 2003 and 2004, the experimental design was a randomized complete block with four replications. Full season programs of pyraclostrobin (168 g a.i./ha) and chlorothalonil (1.26 kg a.i./ha) were compared to reduced programs of pyraclostrobin in which initial applications were delayed by 2 and 4 weeks by omitting the first and second sprays respectively (Table 1). Full season programs consisted of 6 applications in 2003 and 7 applications in 2004 made on 14-day intervals. Spray dates for the full-season programs were 33, 43, 60, 74, 89 and 104 DAP in 2003 and 32, 45, 55, 69, 83, 98 and 116 DAP in 2004.
Fungicide applications were made using a tractor-mounted CO2-propellant sprayer with three D3-23 hollow-cone spray nozzles per row. Fungicides were applied in 114 liters of water per ha at a pressure of 345 kPa. In all tests, the tractor traveled all plots each time an application was made to ensure that differences in soil compaction or limb damage (3) due to tractor traffic would not confound treatment effects on yield.
Early and late leaf spot was assessed for each plot by use of the Florida 1 to 10 scale, a disease index where 1 = no leaf spot, and 10 = plants completely defoliated and dead because of leaf spot (5). Values of 1 through 4 on the scale reflect increasing incidence of leaflets with spots, and occurrence of spots in lower versus upper canopy of the plots. Values 4 through 10 reflect increasing levels of defoliation (5). Plots were rated several times during the season in each test. Final ratings were made just prior to digging and inverting the peanut plants. Plants were dug and inverted on 26 September 2001, 10 October 2002, 6 October 2003, and 4 October 2004 at Tifton, and on 18 September 2001 and 24 September 2002 at Plains. Incidence of stem rot was also evaluated immediately after plots were inverted, as was incidence of Cylindrocladium black rot (Cylindrocladium parasiticum Croos, Wingfield, & Alfeas) at Plains in 2001. Incidence of stem rot was low in most cases with few significant treatment effects. Likewise there were no significant treatment effects on Cylindrocladium black rot incidence. Results from those evaluations are not presented.
Plants were allowed to dry in the field, and pods were harvested mechanically. Data for final leaf spot disease index values, other disease ratings, and yield were subjected to analysis of variance. Since initiation dates varied among the tests in 2001 and 2002, data were analyzed within each year and location. Tests from 2003 and 2004 were analyzed across years. Fisher's protected least significant differences were calculated for mean separations (15). All subsequent reference to significant effects of factors, interactions or differences among means indicates significance at P < 0.05 unless otherwise stated. In the 2001 and 2002 tests, linear and quadratic regression analyses were used to examine the effects of initial application date on the final leaf spot index and yield.
Leaf Spot Control
The severity of leaf spot epidemics ranged from moderate to extremely heavy across the various tests, with nontreated controls almost completely defoliated in all Tifton tests. Early leaf spot was the predominant foliar disease in all tests in 2001-2003. Late leaf spot was also heavy in 2003 and was the predominant foliar disease late in the season in 2004.
There were significant fungicide × spray program effects for final early leaf spot levels in all four tests in 2001 and 2002. In both locations and years, final early leaf spot levels increased linearly with time of initial fungicide application for chlorothalonil and tebuconazole, whereas final early leaf spot levels increased according to a quadratic function of time of initial application of pyraclostrobin (Figs. 3 and 4). Final early leaf spot levels in plots treated with pyraclostrobin were lower than those treated with chlorothalonil or tebuconazole within respective spray initiation timings of 72 DAP or earlier in both locations in 2001 (Fig. 3) and at Plains in 2002 (Fig. 4). In 2002 at Tifton, final early leaf spot levels were lower for pyraclostrobin than for tebuconazole within every spray initiation treatment and lower than for chlorothalonil for the spray program initiation timings of 59, 71, and 84 DAP initiation treatments (Fig. 4). The relative efficacy of chlorothalonil and tebuconazole differed among the tests, with the two fungicides providing similar levels of control for most initiation treatments in 2001 at both locations (Fig. 3) and in 2002 at Plains (Fig. 4). However, in 2002 at Tifton, chlorothalonil provided better leaf spot control than tebuconazole at spray program initiation timings of 43, 59, and 84 DAP (Fig. 4).
Significant year × fungicide program effects on final late leaf spot ratings for the tests conducted in 2003 and 2004 were noted. In 2003, full-season and reduced spray programs with pyraclostrobin resulted in final leaf spot levels that were lower than the full-season program with chlorothalonil (Fig 5). In 2004, late leaf spot levels were similar for the full-season program with chlorothalonil and the full and reduced programs with pyraclostrobin at each of the final two evaluation dates (Fig. 5).
Fungicide Program Effects on Yield
There were no fungicide effects or fungicide × spray program interaction effects on yield in 2001 or 2002. Therefore, yield data were pooled over fungicides. Neither fungicide nor spray program affected yield in 2001 at Plains or in 2002 at Tifton (Fig. 6). Yields were severely affected by Cylindrocladium black rot at Plains in 2001 and by tomato spotted wilt, caused by Tomato spotted wilt virus at Tifton in 2002. Neither fungicide treatment nor spray program had a significant effect (P > 0.05) on either of those diseases. At Plains in 2002 and Tifton in 2001, several fungicide spray programs had yields that were substantially higher than those of the control plots. In those two tests, yield decreased with time of spray initiation according to quadratic functions, with little indication of a yield decrease until the spray programs were initiated later than 70 DAP at Plains or 60 DAP at Tifton (Fig 6).
No significant year × spray program interaction effects on yield for the tests in 2003 and 2004 were recorded. Therefore, data presented are pooled yields for the spray programs over the two years (Fig. 7). All spray programs had yields higher than the nontreated control, but there were no differences among the full-season program with chlorothalonil or pyraclostrobin, or the reduced programs with pyraclostrobin (Fig. 7).
Potential for Reduced Fungicide Programs by Delaying Initial Applications
Increases in leaf spot with increasing delays of initial application of chlorothalonil observed in this study were similar to those reported by Shokes et al. (13). Such findings indicate that full-season programs starting as early as 30 DAP can be important for control of leaf spot with this fungicide when used in a calendar-based spray program. Early applications were also important for early leaf spot management with tebuconazole. Delaying the initial applications of either of these fungicides could compromise early and late leaf spot control if conditions are favorable for their development. Better control of leaf spot achieved with the fungicide applications made 60 DAP or earlier may not correlate with greater yield. Shokes et al. reported no significant reduction in yield until initial sprays of chlorothalonil were delayed to 72 DAP (13). Similarly, in this study there was little indication of yield decrease until initial sprays were delayed to 60 to 70 days.
The results from this study corroborate previous reports that pyraclostrobin is superior to chlorothalonil or tebuconazole for leaf spot control when applied on similar schedules (6,7). Results also indicate that initial applications of pyraclostrobin at rates of 168 g/ha applied as late as 60 DAP can provide control of early leaf spot that is similar to or better than that achieved with chlorothalonil or tebuconazole regimes initiated ca. 30 DAP. Results from 2001 and 2002 indicate that even longer delays could provide acceptable levels of leaf spot control. Such delays would not be advisable because of risk of hastening development of fungicide resistance (4), but these results provide an indication that pyraclostrobin would be superior to chlorothalonil or tebuconazole when timely fungicide applications are not made. The delayed applications of pyraclostrobin that were evaluated in 2003 and 2004 were more aligned with what might be a more typical delay in applying fungicides because of weather, equipment problems, or scheduling challenges than many of the longer application delays assessed in 2001 and 2002.
Although tebuconazole and chlorothalonil were comparable within most spray programs at both locations in 2001 and at Plains in 2002, chlorothalonil was superior to tebuconazole for early leaf spot control in three spray programs at Tifton in 2002. Variability in the relative performance of chlorothalonil and tebuconazole among these tests has not been explained. These results corroborate a report by Hagan et al. (8) in which standard application regimes of block applications of tebuconazole similar to those described previously provided early leaf spot control that was similar to or better than that of full-season applications of chlorothalonil in 2000 and 2001, but inferior to chlorothalonil in 2002.
Largely due to the risk of developing problems with resistance to the strobilurin fungicides in the leaf spot pathogen populations, the full-season use of pyraclostrobin evaluated in this study is not advisable for commercial production (4), and this use pattern is not permitted on the pyraclostrobin label. Current use patterns for pyraclostrobin on peanut include delaying the first application to approximately 37 DAP and waiting three weeks until the first application of tebuconazole. However, similar application of pyraclostrobin should be of use with any subsequent fungicide regime that does not include other strobilurin fungicides. Based on the results of this study and the previous studies (6,7), such a delay in initial application coupled with lengthening the interval for the subsequent fungicide application should not compromise leaf spot control. Such a program results in a reduction of one fungicide application compared to a full-season program beginning 30 DAP. The estimated cost (cost of fungicide plus cost of application) for one application of pyraclostrobin at the rate used in this test would be approximately $39/ha, compared to approximately $46/ha for two applications of chlorothalonil. Further delay in the initial application could result in additional savings, but other factors must be addressed. In many fields, management of stem rot and Rhizoctonia limb rot would need to be considered. These tests provided no information on the impact of delayed fungicide applications on stem rot or limb rot severity. In addition, recommendations for preventing problems with fungal resistance to the strobilurin fungicides include applying them only in a preventative manner (4). Longer delays in initial application might also increase the likelihood of the fungicide being applied after infections are established. However, longer delays in initial applications and use of extended application intervals may still also be possible if integrated with the use of conservation tillage, resistant cultivars, crop rotation, or a combination of these factors that suppress leaf spot epidemics (12). Investigations are underway in which the effects of reduced fungicide programs using pyraclostrobin are being evaluated when combined with those factors. Although all treatments in these experiments were applied on a regular "calendar-based" schedule, pyraclostrobin might be of even greater utility if applied according to a weather-based advisory such as AU-Pnuts (9).
This study did not directly address the differences in post-infection activity of the fungicides evaluated. However, differences in leaf spot severity among the three fungicides with delayed initial applications indicate that pyraclostrobin may have substantial activity against infections already established.
These results indicate that full-season programs beginning 30 to 45 DAP can be important for leaf spot control with chlorothalonil or tebuconazole in Georgia. However, reduced fungicide programs with pyraclostrobin achieved by making initial applications 2 to 4 weeks later can provide leaf spot control similar to or better than that achieved with chlorothalonil or tebuconazole when the initial application is made ca. 30 DAP. Therefore, pyraclostrobin may also increase the flexibility of timing initial applications without compromising control of leaf spot, thus reducing number of fungicide applications required and the costs of fungicide programs for leaf spot.
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