Introduction

Pyraclostrobin is a broad-spectrum strobilurin fungicide that has demonstrated translaminar movement through the leaf epidermis and mesophyll layers but is not redistributed like a true systemic fungicide 1,2,23. In previous trials, the level of control of early leaf spot, caused by the fungus Cercospora arachidicola Hori, and late leaf spot, caused by the fungus Cercosporidium personatum (Berk. & Curtis) Deighton, on peanut (Arachis hypogaea L.) obtained with 3 or more pyraclostrobin applications as part of a recommended seven-application calendar program was often superior to that provided by other fungicides 9,13. Hagan et al 14 noted that a 2-wk calendar program that included three or four applications of pyraclostrobin controlled stem rot, caused by the fungus Sclerotium roflsii Sacc., as effectively as recommended tebuconazole, azoxystrobin, and flutolanil + chlorothalonil programs. Due to exceptional residual activity, pyraclostrobin applied at rates of 0.16 to 0.27 kg ai/ha is labeled for early and late leaf spot control at treatment intervals up to 3-wk compared to the 2-wk interval recommended for other fungicides 18. At extended application intervals, multiple pyraclostrobin application programs have proven as effective in controlling early leaf spot as recommended 2-wk chlorothalonil, tebuconazole, and azoxystrobin programs 9,13. However, to comply with Fungicide Resistance Action Committee (FRAC) guidelines for strobilurin fungicides, no more than two applications of pyraclostrobin as part of a 7-application program may be made to a given peanut field per production season ().

Recommended seven-application calendar-based leaf spot and stem rot control programs may account for more than 25% of the variable cost in the 2006 Alabama peanut production budget (). Due to decreasing profitability of peanut production, emphasis on lowering production costs by reducing fungicide inputs has increased. The recent release of peanut cultivars with partial resistance to early and/or late leaf spot 7,10,11,12, as well as the registration of the highly efficacious fungicide pyraclostrobin 20,23, may allow treatment intervals to be extended from approximately 2- to 3- or possibly 4-wk intervals without jeopardizing disease control or yield. However, a significant increase in late leaf spot severity was previously seen on the partially disease resistant cultivar Southern Runner when the application interval with chlorothalonil at 1.26 kg ai/ha was extended from 2- to 3-wk 6. In addition, a sizable yield reduction was associated with increased disease severity on partially leaf spot resistant peanut cultivars in one out of three years with the extended 3-wk treatment interval 3,6. Monfort et al 19 also noted greater early leaf spot damage on partially leaf spot resistant cultivars treated at extended intervals, but yield for the recommended and extended calendar programs for tebuconazole and azoxystrobin, but not chlorothalonil, were similar. In addition, Cantonwine et al 8 reported early leaf spot intensification when application intervals with chlorothalonil were lengthened from 2- to 4-wk, yet yield response did not significantly differ among the 2-, 3-, and 4-wk treatment schedules in 2002 or among the 3- and 4-wk treatment schedules in 2003.

The AU-Pnuts leaf spot advisory was designed to optimize fungicide application timing when weather patterns are most conducive to the development of leaf spot diseases on peanut 17. According to this advisory, a fungicide application is triggered by the number of accumulated rain or irrigation events > 2.5 mm [0.10 inch] within a 24 hr period and the 5-d average rainfall forecast. When peanut seedlings first emerge, the counting of rain events for the first fungicide application begins. Regardless of the 5-d average rainfall forecast, the first fungicide application is made no later than the sixth rain event. Beginning 10 d after the first and each subsequent fungicide application, a) three rain events, b) the 5-d average rainfall forecast is ≥ 50%, or c) a combination of one or two rain events and the 5-d average rainfall forecast triggers an application 17. Reductions of 1.25 and 2.5 fungicide applications per season on the leaf spot susceptible peanut cultivar Florunner 17 and partially leaf spot resistant cultivar Southern Runner 16 were obtained with the AU-Pnuts leaf spot advisory, respectively. Compared with the recommended 2-wk calendar leaf spot treatment schedule, Brenneman and Culbreath 6 saved two fungicide applications in two of three years with the AU-Pnuts advisory. Similar reductions in numbers of fungicide applications, noted with an AU-Pnuts azoxystrobin/chlorothalonil program by Bowen et al 3, were accompanied with an increased risk of damaging early leaf spot outbreaks and subsequent yield loss. In this study we evaluated the impact of 2-, 3, and 4-wk calendar treatment schedules and the AU-Pnuts advisory on the effectiveness of selected rates of pyraclostrobin for control of early leaf spot and stem rot on susceptible and partially resistant peanut cultivars and on their yields.

Materials and Methods

Results

Discussion

Portillo et al 20, Culbreath et al 9, and Hagan et al 13 have previously demonstrated that pyraclostrobin often controls early and late leaf spot on peanut better than most registered fungicides. While the application rates evaluated in these earlier studies were similar, the treatment programs included an additional one to five more applications than the two specified on the current pyraclostrobin federal label and in FRAC guidelines for strobilurin fungicides. In the current study, seven-spray fungicide programs that included two pyraclostrobin applications at 60 and 90 DAP consistently provided better control of early leaf spot than the standard 2-wk calendar chlorothalonil-alone program. As indicated by disease ratings that did not exceed 3.9 over the three year study period, symptoms on peanuts protected with the above pyraclostrobin/chlorothalonil programs were restricted to light leaf spotting in the lower and mid-canopy, as well as a very low level of premature defoliation. However, while leaf spot control obtained with the 2-wk programs that included pyraclostrobin was superior to that with chlorothalonil-alone, there was no consistent gain in yield with the pyraclostrobin/chlorothalonil programs.

Culbreath et al 9 noted that the difference in leaf spot control with the 0.17 and 0.22 kg ai/ha rates of pyraclostrobin was minimal. In our comparison of the 0.16 and 0.27 kg ai/ha rates, early leaf spot ratings were numerically lower in all three years with the higher concentration of pyraclostrobin. Yields also tended to be greater with the 0.27 kg ai/ha programs than with the 0.16 kg ai/ha pyraclostrobin, but this effect was significant only in one of three years.

As has been previously noted 13, efficacy of pyraclostrobin for the control of early leaf spot declined when application intervals were extended beyond the recommended 2-wk interval. Over both rates of pyraclostrobin, significantly better early leaf spot control was seen in two of three study years with the 2- rather than the 3-wk application intervals. However, the decline in early leaf spot control that was observed between the 2- and 3-wk pyraclostrobin/chlorothalonil programs only resulted in a minimal increase in the level of leaf spotting and defoliation. While disease increase was minimal, lower yields were seen in two of three study years, and regression analysis indicated that the difference in disease could result in 3 to 6% yield reduction.

The AU-Pnuts leaf spot advisory was designed to eliminate one or more fungicide applications without an appreciable decline in the control of leaf spot diseases or peanut yield 6,16,17. Bowen et al 3 recently noted similar control of early leaf spot and yield response in two of three years with the recommended 2-wk calendar and AU-Pnuts azoxystrobin/chlorothalonil programs along with a reduction of two to three fungicide applications with the standard AU-Pnuts advisory rules on the partially leaf spot resistant peanut cultivar C-99R. In the current study, AU-Pnuts scheduling allowed a savings of one fungicide application in each of the two years in which this was evaluated. When averaged across pyraclostrobin rates, the yield difference between the pyraclostrobin 2-wk and the AU-Pnuts schedules ranged from less than 100 kg/ha in 2004 to 400 kg/ha in 2005.

Two applications of pyraclostrobin at 0.16 or 0.27 kg ai/ha, as evaluated in this study, failed to reduce the incidence of stem rot when compared with chlorothalonil, a fungicide that is considered to have little if any activity against the causal fungus S. rolfsii 6. Differences in the level of stem rot control provided by pyraclostrobin in a previous study 13 may be related to application number. In the study by Hagan et al 13, plots treated with three applications of pyraclostrobin at 0.11 or 0.16 kg ai/ha had lower stem rot incidence compared to a standard 2-wk calendar chlorothalonil-alone program. In addition, the same three-application pyraclostrobin/chlorothalonil programs gave the same level of stem rot control as recommended azoxystrobin, flutolanil, and tebuconazole programs 13. However, in a concurrent Alabama study 14, a program that included three applications of pyraclostrobin at 0.11 kg ai/ha failed to reduce the incidence of this disease below that noted on peanut treated season-long with chlorothalonil alone; yield for these two treatments also were similar. Finally, stem rot control and yield response was far superior for chlorothalonil + flutolanil and azoxystrobin/chlorothalonil programs than for the above pyraclostrobin/chlorothalonil program 14. Overall, pyraclostrobin, even at the elevated label rate of 0.27 kg ai/ha, does not appear to be the optimum choice for preventing destructive stem rot outbreaks on peanut.

When applied according to label directions and FRAC guidelines, 2-wk calendar programs that included two applications of pyraclostrobin gave better control of early leaf spot than the standard 2-wk calendar chlorothalonil-alone program. In addition, the 3-wk pyraclostrobin/chlorothalonil programs for the control of early leaf spot and yield response were usually comparable to that obtained with the above chlorothalonil-only program. However, yield response with the 3-wk pyraclostrobin/chlorothalonil program fell below that of the 2-wk programs in 2004 and 2005 by 350 and 450 kg/ha, respectively. At current market prices, the yield difference between the 2- and 3-wk pyraclostrobin/chlorothalonil programs, after accounting for the cost of the two chlorothalonil applications saved with the 3-wk program, translates into a loss of $44 to $56/ha. Similar patterns of yield declines at extended application intervals with calendar and advisory schedules have previously been reported for chlorothalonil and tebuconazole on partially leaf spot resistant peanut cultivars 6. Higher leaf spot ratings and lower yields observed in this study suggests that the application interval for a fungicide program that includes two applications of pyraclostrobin should be more frequently than once every 4-wk where peanuts are cropped every second year or other situations where destructive disease outbreaks are expected. Performance of the AU-Pnuts pyraclostrobin/chlorothalonil treatments was intermediate between that of the 2- and 3-wk calendar treatments in 2004 and was equal to the 3-wk treatment in the following year.

Carver and Andru II peanuts are considered among the most susceptible runner peanut cultivars to leaf spot diseases 12,18. However, final early leaf spot ratings for these cultivars were similar to those recorded for DP-1 and C-99R in 2003 and 2004, respectively, and were lower than for C-99R in 2005. In previous studies conducted in Alabama 14 and Georgia 19, C-99R was significantly less susceptible to early leaf spot than Georgia Green 11. In addition, Cantonwine et al 7 noted that DP-1 was among the most early leaf spot resistant runner peanut cultivar. Differences between our results and those previously reported may be due to the period of time between the final fungicide application (between 9 and 14 September) and final rating dates, especially for the later maturing cultivars. DP-1 and C-99R were rated for early leaf spot 4-wks after the latest fungicide application, and disease would have increased during that time. Thus, this report is not seeking to refute previous findings about disease reactions of the cultivars. The early maturing (maturity group 3) cultivar Andru II typically had less stem rot damage than the later maturing Carver, DP-1, or C-99R. As previously noted by Hagan et al 12, Andru II may avoid stem rot by maturing before additional colonization of the pods and stems can occur. In contrast, late maturing (maturity group 5) cultivars, like DP-1 and C-99R, are exposed to attack by S. rolfsii for an additional 20 to 30 days beyond the optimum maturity date for Andru II. Incidence of stem rot on Carver was similar to levels noted on C-99R in 2004 and 2005 but lower than those observed on DP-1. Lower stem rot damage levels for Andru II did not translate into higher yield. Yield for C-99R was significantly higher than for Carver in 2004 and 2005, and for Andru II in 2004. In the first year of this study, Carver significantly out-yielded both Andru II and DP-1.

In summary, extending fungicide application intervals may not be the best option for reducing production inputs, while maximizing peanut yield, particularly where cropping patterns or other production practices favor destructive outbreaks of leaf spot diseases and stem rot. The recently developed Georgia Fungal Risk Index, which quantifies the threat posed by leaf spot disease, stem rot, and TSWV on the basis of rotation pattern, cultivar selection, and management practices, is a tool that may allow peanut producers to customize fungicide programs field by field basis 4. Regardless, peanut producers need to carefully consider the increased risk of sizable yield losses against the cost savings from eliminating one or two fungicide applications before adopting an extended application program with any recommended fungicide on their peanut crop.

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