© 2001 Plant Health Progress.
Applications of Mixtures of Copper Fungicides and Chlorothalonil for Management of Peanut Leaf Spot Diseases
A. K. Culbreath, T. B. Brenneman, and R. C. Kemerait, Jr., Department of Plant Pathology, The University of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793-0748
Early leaf spot and late leaf spot (Fig. 1) of peanut caused by Cercospora arachidicola Hori and Cercosporidium personatum (Berk. & M. A. Curtis) Deighton, respectively, are constant threats to peanut (Arachis hypogaea L.) production worldwide. Left unchecked on susceptible cultivars, either of these diseases can cause complete defoliation (Fig. 2) and substantial pod loss. Because of frequently favorable environmental factors and the use of cultivars without substantial resistance to prominent fungal pathogens, management of leaf spot diseases and the important soilborne diseases southern stem rot (Sclerotium rolfsii Sacc.) and Rhizoctonia limb rot (Rhizoctonia solani Kuhns) in Georgia, Alabama, and Florida is heavily dependent upon the use of fungicides. Six or seven fungicide applications are made per season in many fields.
Copper fungicides once were commonly used for suppression of leaf spot diseases of peanut (10). Use of copper fungicides ceased after chlorothalonil and benomyl were registered and provided better leaf spot control. Use of benomyl on peanut in the southeastern U.S. was discontinued after populations of C. arachidicola and C. personatum developed that were insensitive to that fungicide (10), but chlorothalonil is still effective and is used extensively for control of leaf spot diseases in peanut. Neither chlorothalonil nor copper fungicides provide control of southern stem rot or Rhizoctonia limb rot. Fungicide regimes in the southeastern U.S. typically include applications of flutolanil, azoxystrobin, or tebuconazole for management of these two diseases. Flutolanil has minimal effect on leaf spot diseases (6), and only two applications of azoxystrobin are recommended (7).
In 1994, propiconazole and tebuconazole were registered for management of foliar diseases and tebuconazole was registered for management of southern stem rot and Rhizoctonia limb rot of peanut. Both tebuconazole and tank-mix combinations of chlorothalonil + propiconazole have been reported to provide better control of leaf spot diseases of peanut than chlorothalonil alone (1,5). Because of concerns for developing populations of pathogens with reduced sensitivity to ergosterol biosynthesis inhibiting (EBI) fungicides (8), it is recommended that propiconazole and tebuconazole not be applied in the same field and growing season (7). A fungicide alternative that could provide disease control better than chlorothalonil alone without increasing the risk of developing resistance to the EBI or strobilurin fungicides would be an asset to peanut growers. In addition, standard rates of chlorothalonil (0.84 kg/ha) + propiconazole (0.063 kg/ha) typically are more expensive than standard rates of chlorothalonil alone (1.26 kg/ha). Less expensive fungicides or combinations that provide adequate leaf spot control also could be very beneficial to growers, regardless of the type of fungicide used for management of soilborne diseases. Copper fungicides often are less expensive than chlorothalonil, but when applied alone are inferior to chlorothalonil for leaf spot control (4). Pre-mix formulations of chlorothalonil and copper oxychloride (3Cu(OH)2•CuCl2) and tank-mix combinations of reduced rates of chlorothalonil and copper hydroxide (Cu(OH)2) may cost less and still provide better control of leaf spot diseases than standard rates of chlorothalonil alone. The purpose of this study was to determine the effect of a range of rates of a pre-mix formulation of 24% chlorothalonil and 24% copper oxychloride, and tank mix combinations of reduced rates of chlorothalonil with copper hydroxide on peanut leaf spot severity and peanut yield. Comparison of these mixtures to standard rates of chlorothalonil alone and regimes that include EBI fungicides currently labeled in the U.S. for use on peanut was particularly of interest.
Chlorothalonil + Copper Oxychloride Rate Response Tests
Experiments were conducted at the University of Georgia Southwest Branch Experiment Station Plains, and the University of Georgia Coastal Plain Experiment Station Black Shank Farm, Tifton, GA in 1997 and 1998 to determine the effects of pre-mix combinations of copper oxychloride and chlorothalonil on peanut leaf spot. Soil type at Plains was Greenville sandy clay loam soil. The soil type at Tifton was loamy sand (pH = 6.0). All fields had been planted to cotton (Gossypium hirsutum L.) the previous year. The cultivar used in all tests was Georgia Green, with seeding rate of 112 kg of seed per ha. Planting dates were 2 May 1997 and 13 May 1998 at Plains, and 27 May 1997 and 26 May 1998 at Tifton. Aldicarb (Temik 15G, 1.2 to 1.3 kg ai/ha) was applied in furrow at planting for control of thrips in all tests.
In all tests, a randomized complete block experimental design was used with four replications. Plots consisted of 2 rows (1 bed), 7.6 m long. In Tifton, row spacing was a uniform 0.91 m (1.83 m bed); in Plains, rows were 0.71 m apart within the bed and 0.91 m between rows in adjacent beds (1.63 m bed). In all tests, plots were separated by two nonsprayed border rows, and blocks were separated by 2.4 m fallow alleys. Calcium sulfate was applied as gypsum (560 to 1120 kg/ha) to all plots 40 to 50 days after planting.
Treatments consisted of: i) a non-sprayed control; ii) chlorothalonil (Bravo WeatherStik, Syngenta Crop Protection, Greensboro, NC) at 1.26 kg ai/ha; iii) chlorothalonil (Terranil 6L, Terra-Riverside, Sioux City, IA) at 1.26 kg ai/ha; iv) pre-mix combination of chlorothalonil (0.42 kg ai/ha) + copper oxychloride (0.42 kg ai/ha); v) pre-mix combination of chlorothalonil (0.56 kg ai/ha) + copper oxychloride (0.56 kg ai/ha); vi) pre-mix combination of chlorothalonil (0.70 kg ai/ha) + copper oxychloride (0.70 kg ai/ha); vii) pre-mix combination of chlorothalonil (0.84 kg ai/ha) + copper oxychloride (0.84 kg ai/ha); viii) tank mix combination of chlorothalonil (Bravo WeatherStik) at 0.84 kg ai/ha + propiconazole (Tilt 3.6 EC, Syngenta Crop Protection, Greensboro, NC) at 0.063 kg ai/ha ; and ix) block applications of chlorothalonil (Bravo WeatherStik) at 1.26 kg/ha applied in sprays 1, 2, and 7, and tebuconazole (Folicur 3.6 F, Bayer Corp., Kansas City, MO) (0.23 kg ai/ha) in sprays 3 to 6. Treatments 4 to 8 were a range of rates of a liquid flowable formulation of a pre-mix combination of 24% chlorothalonil and 24% copper oxychloride (Terranil CU, Terra-Riverside, Sioux City, IA). For all treatments except the tebuconazole block treatment, the same fungicide or combination was used for all applications. Fungicide applications were initiated 30 to 35 days after planting. Subsequent applications were made at 14-day intervals, for a total of seven applications. Fungicides were applied in the equivalent of 114 liters of water per ha at a pressure of 345 kPa using a tractor-mounted CO2-powered sprayer with three D2-13 nozzles per row.
Leaf spot intensity (severity and defoliation) was assessed by use of the Florida 1 to 10 scale where 1 = no leaf spot, and 10 = plants completely defoliated and killed by leaf spot (2). Multiple leaf spot evaluations were made in each test, but final leaf spot ratings were made immediately prior to digging. Plots were dug and inverted on 15 Sep. 1997 and 25 Sep. 1998 at Plains and 1 Oct. 1997 and 12 Oct. 1998 at Tifton. Immediately after plants were inverted, loci of southern stem rot were counted for each plot, where a locus represented 31 cm or less of linear row with one or more plants infected (9). Inverted plants were allowed to dry in the field for 7 to 11 days until pods were harvested mechanically. Pods were adjusted to 12% w/w moisture, and yields were determined for each plot.
For both sets of experiments, data from each year were analyzed independently by analysis of variance (11). Data were combined for analysis across locations, and means comparisons from the pooled analysis were used when test x treatment effects were not significant (P > 0.05). Subsequent reference to significant differences among means indicates significance at P < 0.05 unless otherwise stated.
Relative treatment effects on leaf spot intensity ratings and yield were consistent across locations in 1997, but location x treatment effects were significant for both final leaf spot intensity ratings and yield in 1998. Results are presented with data pooled across locations in 1997 (Fig. 3A and D) and for each location in 1998 (Fig. 3B, C, E, and F). There was no significant difference (P > 0.05) between the two formulations of chlorothalonil for leaf spot intensity or yield, and analyses of leaf spot ratings and yield were conducted using data pooled from these two treatments. Across both tests in 1997, final leaf spot ratings in plots receiving chlorothalonil + copper oxychloride at rates 0.56 + 0.56 kg or greater were less than in plots receiving chlorothalonil alone at 1.26 kg/ha, and were similar (P > 0.05) to those of plots treated with chlorothalonil + propiconazole (Fig. 3A). Leaf spot ratings were lowest (P < 0.05) in plots treated with chlorothalonil-tebuconazole block of sprays across locations in 1997.
In 1998, location effects and location x treatment effects were significant for both leaf spot ratings and yield. Leaf spot epidemics were much more severe at Tifton than in Plains (Fig. 3B, C). At both locations, all treatments that included chlorothalonil + copper oxychloride at rates of 0.42 + 0.42 kg/ha or higher had leaf spot ratings similar (P > 0.05) to or lower than (P < 0.05) chlorothalonil alone (Fig. 3B, C). Combinations of chlorothalonil + copper oxychloride at 0.70 + 0.70 kg/ha or higher had leaf spot ratings that did not differ (P > 0.05) from those of chlorothalonil + propiconazole in all four tests (Fig. 3A, B, C). Block applications of chlorothalonil and tebuconazole were similar (P > 0.05) to chlorothalonil alone for leaf spot control at either location in 1998.
Across locations, all treatments in 1997 had yields higher (P < 0.05) than that of the plots receiving no fungicide (Fig. 3D). Only chlorothalonil + propiconazole treatments had yields greater than that of chlorothalonil alone. Yields of treatments receiving chlorothalonil + copper oxychloride at 0.56 + 0.56 kg/ha or higher had yields similar (P > 0.05) to those of chlorothalonil + propiconazole.
In 1998 at Plains, only three of the chlorothalonil + copper oxychloride pre-mix treatments and the chlorothalonil - tebuconazole block treatment had yields that were greater (P < 0.05) than those of the non-treated control (Fig. 3E). The large increase in yield for the tebuconazole treatment was due primarily to control of southern stem rot. At Tifton, all treatments resulted in greater (P < 0.05) yields than those of the nontreated control, but there were no other differences in yield among the various fungicide treatments (Fig. 3F).
Treatments, per ha, of 0.84 kg of chlorothalonil + 0.84 kg of copper oxychloride provided control of leaf spot similar (P > 0.05) to that achieved with chlorothalonil + propiconazole, and better than that of standard rates of chlorothalonil alone. In three of four tests, there was little additional improvement in leaf spot control with rates per ha above 0.56 kg of chlorothalonil + 0.56 kg of copper oxychloride. In that fourth test, rates as low as 0.56 kg of chlorothalonil + 0.56 kg of copper oxychloride provided leaf spot control similar (P > 0.05) to that of full rates of chlorothalonil alone.
Results from this study corroborated previous reports that combinations of chlorothalonil + propiconazole provided better leaf spot control than chlorothalonil alone at 1.26 kg/ha (5). In all four tests in this study, rates of 0.70 kg of chlorothalonil + 0.70 kg copper oxychloride or higher provided leaf spot ratings that did not differ (P > 0.05) from those of standard rates of chlorothalonil + propiconazole.
Based largely on these results, minimum rates per ha of chlorothalonil + copper oxychloride recommended for control of leaf spot in peanut were reduced from 0.84 kg of chlorothalonil + 0.84 kg of copper oxychloride to 0.63 kg of chlorothalonil + 0.63 kg of copper oxychloride (7).
Chlorothalonil + Copper Hydroxide Tests
Effects of tank-mix combinations of chlorothalonil and copper hydroxide on peanut leaf spot were examined in two tests in Tifton, GA in 1998, and in one test in each of Tifton and Plains, GA in 1999 and 2000. Experimental design, crop histories, soil types, and crop management were similar to those for the respective locations in the previously described tests. Planting dates were 24 to 26 May at Tifton in all three years, and 4 May 1999 and 8 May 2000 at Plains.
In 1998, treatments in both tests consisted of: i) nontreated control; ii) chlorothalonil (Bravo WeatherStik) at 1.26 kg/ha; iii) tank mix combination of chlorothalonil at 0.63 kg ai/ha + copper hydroxide (Kocide 4.5 LF, Griffin Corp., Valdosta, GA) at 0.63 kg ai/ha; and iv) chlorothalonil-tebuconazole block application regime previously described.
In 1999, treatments at Plains included those used in 1998 + the pre-mix combination of chlorothalonil (0.56 kg/ha) + copper oxychloride (0.56 kg/ha) and tank mix combinations of chlorothalonil (Bravo WeatherStik 720) + propiconazole treatments as previously described. The nontreated control and tebuconazole treatments were not included at Tifton.
In 2000, the chlorothalonil + copper oxychloride treatment was omitted, and a third formulation of chlorothalonil (Equus 720, Griffin Corp., Valdosta, GA), applied alone at 1.26 kg/ha and at 0.84 kg/ha in a tank mix with copper hydroxide at 0.63 kg/ha, was used. Fungicides in all tests were applied at 14-day intervals after initial applications 30 to 35 days after planting.
Early leaf spot was the predominant foliar disease in all six tests. Immediately prior to digging, leaf spot intensity was assessed in each plot as previously described. Plants were dug and inverted on 23 September 1999 and 21 September 2000 at Plains and 4 to 12 October in all tests at Tifton. Southern stem rot was evaluated after plants were inverted, and pods were harvested as previously described. Only treatments that included tebuconazole reduced incidence of southern stem rot compared to chlorothalonil alone in any test. Therefore, data from these ratings are not shown.
Treatment effects on leaf spot ratings and yield were consistent across both tests in 1998 (Fig. 4). Final leaf spot ratings of plots treated with chlorothalonil + copper hydroxide tank mix were lower than (P < 0.05) ratings of plots treated with chlorothalonil alone (Fig. 4), and were similar (P > 0.05) to ratings of plots that were treated with block applications of chlorothalonil and tebuconazole. There were no differences in yield among the three fungicide treatments; all three treatments had yields higher than (P < 0.05) the control.
In 1999, leaf spot ratings in plots treated with chlorothalonil + propiconazole were better than in either chlorothalonil + copper treatment in 1999. Both chlorothalonil + copper treatments had leaf spot ratings that were similar (P > 0.05) to those of chlorothalonil at Tifton, but higher than (P < 0.05) those of chlorothalonil at Plains. Leaf spot ratings in plots treated with the chlorothalonil-tebuconazole block were similar (P > 0.05) to those of plots treated with chlorothalonil alone at Plains in 1999 (Fig. 5). Yields did not differ (P > 0.05) among the various fungicide regimes in 1999.
Leaf spot ratings and yields in both chlorothalonil treatments did not differ (P > 0.05) at either location in 2000 and are reported as one mean (Fig. 6). Leaf spot epidemics were more severe at Tifton than at Plains. Location x treatment interactions were significant for leaf spot ratings and yield. Pod yields at Plains were affected greatly by Cylindrocladium black rot (Cylindrocladium parasiticum). Leaf spot severity ratings were lower (P < 0.05) in all treatments than in the nontreated control in both locations, and leaf spot severity ratings were similar (P > 0.05) among all treatments that received fungicides. At Tifton, yields did not differ (P > 0.05) among treatments that received fungicides. Only the chlorothalonil-tebuconazole block treatment had yields greater than (P < 0.05) that of the control at Plains. Based largely on these results, tank mix combinations of chlorothalonil at 0.84 kg/ha + copper hydroxide at 0.63 kg/ha were added to Georgia recommendations for leaf spot control for the 2001 season.
Summary and Conclusions
From field tests that included a range of intensities of leaf spot epidemics, mixtures of chlorothalonil with copper oxychloride or copper hydroxide have the potential to provide control of leaf spot diseases of peanut comparable to, and in some cases superior to, that achieved with standard rates of chlorothalonil alone. In five of six tests, 0.56 kg of chlorothalonil + 0.56 of kg copper oxychloride per ha was as good as or better than 1.26 kg of chlorothalonil alone per ha for leaf spot control, and yields were similar (P > 0.05) for those two treatments in all six trials. Higher rates of that combination controlled leaf spot as well as (P > 0.05) chlorothalonil + propiconazole. In three of four tests, leaf spot ratings in plots treated with chlorothalonil at 0.63 kg/ha + copper hydroxide at 0.63 kg/ha were similar to (P > 0.05) or better than (P < 0.05) those treated with chlorothalonil alone at 1.26 kg/ha. In both tests using chlorothalonil at 0.84 kg/ha + copper hydroxide at 0.63 kg/ha, leaf spot ratings in that treatment were similar to (P > 0.05) ratings for the chlorothalonil alone at 1.26 kg/ha.
The higher yields obtained from tebuconazole treatments in one test were primarily due to control of southern stem rot and Rhizoctonia limb rot not provided by the other fungicides. The difference was not observed when those diseases occurred at lower incidence. There was no consistent yield advantage to any of the chlorothalonil, chlorothalonil + propiconazole, or chlorothalonil + copper treatment regimes. Relative costs of these fungicides vary from year to year and across the peanut-producing region of the southeastern U.S. within each year, but chlorothalonil + copper combinations may cost less than full rates of chlorothalonil alone. From average estimates for the 2001 growing season, the combination of chlorothalonil (Bravo WeatherStik) at 0.63 kg/ha and copper hydroxide (Kocide 4.5 LF) at 0.63 kg/ha cost approximately $17.64/ha per application. Cost of combinations of the same rate of copper hydroxide with chlorothalonil (Bravo WeatherStik) at 0.84 kg/ha (approximately $21.19/ha per application) was similar to that of chlorothalonil (Bravo WeatherStik) alone at 1.26 kg/ha ($21.35/ha per application), and only slightly less than the cost of chlorothalonil (Bravo WeatherStik) at 0.84 kg/ha + propiconazole (Tilt) at 0.063 kg/ha ($21.74/ha per application). The formulation of chlorothalonil and copper oxychloride used in these experiments is no longer commercially available. However, at rates per ha of 0.56 kg of each of the chlorothalonil and copper oxychloride components, tank mix combinations that would emulate the mixture used in these tests might be even less expensive than the chlorothalonil + copper hydroxide treatments. Based on these results, peanut growers should be able to take advantage of price differentials between mixtures of chlorothalonil + copper oxychloride or copper hydroxide and full rates of chlorothalonil alone or chlorothalonil + propiconazole when they occur without increasing the risks of losses in yield to leaf spot diseases.
Use of mixtures of copper fungicides with chlorothalonil should be similar to use of chlorothalonil alone with regard to fungicide resistance management when used in various alternation or block patterns with tebuconazole or azoxystrobin. There have been no reports of resistance in C. arachidicola or C. personatum to either chlorothalonil or copper fungicides, even after many years of use. The efficacy of the fungicide use patterns on peanut for preventing problems with resistance to EBI and strobilurin fungicides remains to be determined. However, the chlorothalonil and copper combinations should not increase the risk of problems with resistance to either of these classes of fungicides.
This research was supported by state and Hatch funds allocated to the Georgia Agricultural Experiment Station. Additional funding was provided by Georgia growers through grants from the Georgia Agricultural Commodity Commission for Peanuts. The efforts of Mike Heath are gratefully acknowledged.
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