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Use of a resistant peanut cultivar with copper fungicides and reduced fungicide applications for control of late leaf spot
361
Use of a resistant peanut cultivar with copper fungicides and reduced fungicide applications for control of late leaf spot A. K. C u l b r e a t h * , T. B. B r e n n e m a n a n d C. K. Kvien
University of Georgia, Coastal Plain Experiment Station, Departments of Plant Pathology and Agronomy, Tifton, Georgia, USA
Abstract
Keywords
In field tests conducted in 1988 and 1989, chlorothalonil, propiconazole, or diniconazole applied on a 2 l-day schedule provided control of late leaf spot (Cercosporidiumpersonatum) of peanut (Arachis hypogaea) that was sufficient to prevent yield losses in Southern Runner, a cultivar with moderate resistance to C. personatum. Copper hydroxide in 1988 and 1989, copper oxysulphate in 1988 and copper resinate fungicides in 1989 applied in a 14-day schedule produced yields comparable to chlorothalonii applied in a 14-day schedule, although leaf spot control provided by all of these materials was inferior to that provided by chlorothalonil, propiconazole or diniconazole. In 1990, significant yield loss due to leaf spot did not occur for even the untreated plots. In 1988 and 1989, highest yields were from plots treated with diniconazole, primarily because of superior control of soil-borne diseases by this fungicide. Resistant cultivars; Cercosporidium personatum; Arachis hypogaea; chlorothalonil; fungicidal control
Introduction
The use of cultivars (cvs) resistant to pathogens is one of the safest and most economical methods for control of plant diseases. Although peanut cvs with complete resistance to all pathogens are not available, partial resistance to one or more pathogens may be useful in reducing chemical inputs and the cost of disease control. The peanut (Arachis hypogaea L.) cv. 'Southern Runner' has a moderate level of resistance to several pathogens (Gorbet et al., 1986; Brenneman, Branch and Csinos, 1990; Culbreath, Demski and Todd, 1990), including Cercosporidium personatum (Curt. & Berk.) Deighton, the cause of late leaf spot. Late leaf spot is one of the most severe diseases of peanut world-wide and is responsible for losses totalling tens of millions of dollars annually in Georgia alone (Thompson, 1990). In the southeastern United States, leaf spot control on susceptible cvs, such as 'Florunner', is achieved by the use of multiple applications of the fungicide chlorothalonil. Excellent control of late leaf spot is achieved with chlorothalonil on susceptible cvs, but at considerable cost to growers. The fungicide applied to peanut contributes to the large pesticide load entering the environment over a large area of Georgia and the southeastern United States. Resistance in Southern Runner is not sufficient to prevent yield losses to leaf spot without application of fungicides (Gorbet et al., 1986). However, use of Southern Runner could reduce the number of *To whom correspondence should be addressed, at Plant Pathology
Department, The University of Georgia Coastal Plain Station, Tifton, GA 31793-0748, USA
fungicide applications required for peanut production. In addition to moderate resistance to infection by C. personatum, Southern Runner has continual leaf production throughout the season. This results in some replacement of leaves lost due to leaf spot disease (Pixley et al., 1990). The inorganic fungicide copper hydroxide [Cu(OH)2 ] and other copper-containing fungicides are registered for use on peanut, and the cost per application of these materials is lower than that of chlorothalonil. Copper fungicides were used extensively for leaf spot control in the southeastern United States until the development of the more effective fungicides, benomyl and chlorothalonil (Smith and Littrell, 1980). Copper fungicides, such as copper hydroxide, copper oxysulphate and copper resinate, do not provide leaf spot control that is adequate to prevent yield losses when applied to the standard susceptible cv., Florunner (Hammond, Backman and Lyle, 1977; Littrell, 1985). Copper fungicides with a moderately resistant cultivar may, however, provide adequate control of leaf spot. In addition to chlorothalonil and the copper fungicides, several experimental fungicides of the triazole (ergosterol biosynthesis inhibitors) class were evaluated for managing leaf spot diseases on peanut. Propiconazole (Hancock and Weete, 1984) and diniconazole (Csinos, Kvien and Littrell, 1987) controlled leaf spot when applied at rates of active ingredient lower than those required for chlorothalonil (Csinos et al., 1987). In addition to activity against C. personatum, these materials have potential for controlling soil-borne diseases (Csinos et al., 1987; Brenneman et al., 1991). The objective of this study was to determine the effects of 14- and 21-day ftmgicide spray
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Control of peanut leaf spot: A. K. Culbreath etaL
schedules of copper fungicides, chlorothalonil and two ergosterol biosynthesis-inhibiting fungicides on leaf spot severity on Southern Runner peanut.
Materials and methods Experiments were conducted in 1988, 1989 and 1990 at the University of Georgia Coastal Plain Experiment Station Gibbs Farm, Tifton, GA. Test sites in 1988 and 1990 were fallow and the 1989 site had been planted to cotton (Gossypium hirsutum L.) the previous growing season. Soil type in all years was a Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Paleudults) with pH of 5.7, 5.8 and 5.8 in 1988, 1989 and 1990, respectively. The experimental design was a randomized complete block with plots that each consisted of a single raised bed (7.6 m x 1.8 m) with two rows per bed. Plots were separated by one bed (1.8m wide) of two unsprayed border rows and 2.4m fallow alleys on the ends. Southern Runner was planted on 17 May, 18 May and 14 May in 1988, 1989 and 1990, respectively. Aldicarb (Temik 15G) (1.12 kg a.i. ha- 1) was applied in the planting furrow at the time of planting for insect control. Gypsum (calcium sulphate) was applied at 560 kg ha- 1 broadcast at early pegging each year. Management of weeds and insects was according to University of Georgia recommendations (Johnson et al., 1987). Treatments in all three years consisted of combinations of different fungicides and application schedules. In 1988, fungicides used included the industry standard chlorothalonil, 1.26kga.i. ha -1 (Bravo 720, ISK Biotech, Mentor, OH), copper hydroxide [Cu(On)2)] , 1.68kg metallic Cu ha- 1 (Kocide 101, 77W, Griffin Corp., Valdosta, GA), copper oxysulphate, 1.51 kg metallic Cu ha- ] (Cuproxat, Gilmore Chemical Co., Memphis, TN) and experimental organic dimethyl sterol-inhibiting fungicides, propiconazole, 0.12 kg a.i. ha- i (Tilt 3.6 EC, Ciba-Geigy Corp., Greensboro, NC) and diniconazole, 0.12kg a.i. ha- 1 (Spotless 25 WP, Valent USA Corp., Memphis, TN). Treatments in 1989 and 1990 consisted of application of chlorothalonil (1.26 kg a.i. ha- 1), Cu(OH) 2 as Kocide 101 (1.68 kg metallic Cu ha-1) and as an experimental dry flowable formulation of copper hydroxide (1.68 kg metallic Cu ha-1) (Copper Hydroxide 77 DF, Terra Corp., Riverside, CA) (henceforth referred to as copper hydroxide DF), and copper resinate, 0.24 kg metallic Cu ha- 1 (TermCop 5E, Tennessee Chemical Co., Copperhill, TN) each on 14- and 21-day spray intervals. In addition, propiconazole (0.12kg a.i. ha-1) and diniconazole (0.12kg a.i. ha-1) were applied at the 21-day spray intervals only. In all three years, fungicides were applied using a CO2-powered sprayer with a 1.9m boom, equipped with three D2-13 hollow-cone nozzles per row. Fungicides were applied in 501 water ha 1 at 345 kPa. Two spray schedule regimens were examined in all years, including a grower standard 14-day schedule and a 21-day schedule. The grower standard schedule sprays were initiated on 23 June 1988, 23 June 1989 and 29 June 1990. The 21-day schedule sprays were begun on 14 July 1988, 23 June 1989 and 29 June 1990. Seven 14-day-schedule
CROP PROTECTION Vol. 11 August 1992
applications were made in each year, and four, five and five 21-day-schedule applications were made in 1988, 1989 and 1990, respectively. Leaf spot severity ratings were made on all plots on 14 September and 14 October 1988, 24 August and 5 October 1989 and 18 September and 17 October 1990, using a scale of 1-10 where 1 represents no leaf spot and 10 represents plants completely defoliated and killed by leaf spot (Chiteka et al., 1988). Data were subjected to analysis of variance. Fisher's protected 1.s.d. values were used for evaluation of significance of treatment differences (Steel and Torrie, 1980). Differences denoted in the text are significant at p < 0.05 unless otherwise stated.
Results Disease and yield in 1988 Final leaf spot ratings for all fungicide treatments were lower than those of unsprayed plots (Table 1). Applications of copper hydroxide, copper hydroxide DF or copper oxysulphate on a 14- or 21-day schedule gave leaf spot control that was inferior to that of chlorothalonil, diniconazole or propiconazole in the corresponding spray schedules (Table 1). Plants treated with copper hydroxide or copper hydroxide DF at 14-day intervals produced yields as high as those of plants treated with diniconazole, propiconazole or chlorothalonil at that same interval. Plants treated with copper hydroxide or copper hydroxide DF at 21-day intervals, or copper oxysulphate at either interval, produced yields that did not differ significantly from yields of untreated plots (Table 1). Leaf spot ratings for plants treated with chlorothalonil at 21-day intervals were higher than for those receiving the same treatment at 14-day intervals, but yields were similar for the two treatments. Diniconazole applied on the 14-day schedule was the only treatment that reduced severity of limb rot, and incidence of stem rot was lower than for the untreated plots.
Disease and yield in 1989 Application of Cu(OH)2 at either 14- or 21-day intervals did not significantly reduce leaf spot ratings compared with unsprayed plots, whereas application of the Cu(OH)2 DF formulation reduced leaf spot severity at both spray intervals (Table 2). Neither of these provided control equal to that achieved with 14- or 21-day applications of chlorothalonil or 21-day schedule applications of propiconazole or diniconazole (Table 2). Yields of plants treated with either copper hydroxide material at either 14- or 21-day intervals did not differ from those treated with chlorothalonil, and were higher than the yield of unsprayed plants. Application of chlorothalonil at 21-day intervals resulted in leaf spot ratings that did not differ significantly from those of plants treated with chlorothalonil on a 14-day schedule, and yields for these two treatments were similar. Only diniconazole applied on the 2 l-day schedule reduced incidence of stem rot in the 1989 test. Pod yield of plants treated with diniconazole at 21-day intervals ranked high-
Control of peanut leaf spot: A. K. Culbreath et aL
363
Table 1. Comparison of fungicides applied at 14- and 21-day intervals for control of late leaf spot on Southern Runner peanut, 1988
Treatment
Rate"
Spray schedule
Leaf spot ratingb
Limb rot e
Stem rot d
Yield (kg ha- 1)
Untreated check
--
Copper hydroxide
1.68 1.68
--
7.9
28.8
5.8
2949
14-day 21-day
5.8 6.6
31.5 25.0
5.0 3.3
3694 3085
Copper oxysulphate
1.51 1.51
14-day 21-day
5.6 6.1
37.0 32.3
4.8 5.3
3185 3258
Chlorothalonil
1.26 1.26
14-day 21-day
2.5 4.3
42.3 42.3
6.3 6.0
3412 3694
Propiconazole
0.12 0.12
14-day 21-day
4.2 4.7
40.8 33.8
6.5 8.3
3630 3303
Diniconazolee
0.12 0.12
14-day 2l-day
4.0 4.0
6.3 24.0
0 5.0
3984 4365
0.6
13.5
4.3
658
1.s.d. (p <0.05) f
"Rate expressed as kg of metallic copper or a.i. ha ~; bleaf spot ratings made on 14 October using the Florida 1-10 scale where 1= no disease and 10= plants completely defoliated and killedby leaf spot (Chiteka et al., 1988);Cpercentageof limbs showingsymptoms or signsof Rhizoctonia limb rot; anumber of diseaseloci (0.3 m of affected row or less) per 13.3m of row; elUPAC chemical name: (E)-(R), E- l-(2,4-dichlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-l-yl)-l-penten-3-ol);fFisher's least significantdifference for comparison of means within columns
Table 2. Comparison of fungicides applied at 14- and 21-day intervals for control of late leaf spot on Southern Runner peanut, 1989 and 1990
Leaf spot ratingh Treatment
Rate"
Untreated
Stem rot c
Yield (kg ha 1)
Spray schedule
1989
1990
1989
1990
1989
1990
--
--
7.5
5.6
4.8
4.5
3140
3670
Copper hydroxide
1.68 1.68
14-day 21-day
6.4 6.9
3.1 4.4
2.3 3.0
2.8 2.8
4025 3903
3581 4125
Copper hydroxide DF
1.68 1.68
14-day 21-day
5.8 5.9
2.8 3.4
2.0 4.0
2.5 2.8
3801 3669
3970 3751
Copper resinate
0.24 0.24
14-day 21-day
6.4 6.9
3.6 5.3
4.5 2.3
4.3 5.0
3821 3354
4126 3768
Chlorothalonil
1.26 1.26
14-day 21-day
3.4 3.8
2.0 3.6
4.3 3.8
4.3 2.0
3840 3842
3687 3785
Propiconazole
0.12
21-day
4.4
3.3
3.0
3.5
3903
4020
Diniconazole
0.12
21-day
3.9
3.6
1.5
2.3
4523
4313
1.3
1.2
2.7
2.4
523
668
l.s.d. (p <0.05) a
"As in Table 1; bleafspot ratings made on 5 October using the Florida I-10 scalewhere 1= no disease and I0 = plants completelydefoliated and killed by leaf spot (Chiteka et al., 1988);Casin Table 1; aFisher's least significant differencefor comparison of means within columns
est a m o n g all t r e a t m e n t s , a l t h o u g h l e a f s p o t r a t i n g s w e r e similar to those of plants treated with chlorothalonil at e i t h e r s p r a y i n t e r v a l (Table 2).
N o s i g n i f i c a n t yield r e s p o n s e t o f u n g i c i d e t r e a t m e n t w a s detected for any treatment. Plots treated with diniconazole h a d yields t h a t w e r e n u m e r i c a l l y , b u t n o t s i g n i f i c a n t l y , higher than those of other treatments.
D i s e a s e a n d y i e l d in 1 9 9 0
L e a f s p o t i n t e n s i t y w a s l o w e r i n 1990 t h a n i n t h e p r e v i o u s two years, owing to an extended drought. Application of either copper hydroxide fungicide at 14-day intervals r e s u l t e d i n f i n a l l e a f s p o t r a t i n g s t h a t d i d n o t differ significantly from those of plants treated with chlorothal o n i l ( T a b l e 2 ). A p p l i c a t i o n o f t h e c o p p e r r e s i n a t e f u n g i c i d e at either standard or reduced application schedules result e d i n m o r e l e a f s p o t t h a n in p l o t s t r e a t e d w i t h c h l o r o t h a lonil at either of the respective spray intervals. Leaf spot r a t i n g s d i d n o t differ f o r t h e 14- o r 2 1 - d a y t r e a t m e n t s o f chlorothalonil.
Discussion U s e o f a m o d e r a t e level o f r e s i s t a n c e t o C. p e r s o n a t u m i n a cv. s u c h as S o u t h e r n R u n n e r m a y m a k e it f e a s i b l e t o u s e c o p p e r f u n g i c i d e s t h a t a r e less effective t h a n c h l o r o t h a lonil. T h i s c o u l d r e d u c e t h e a m o u n t o f s t a n d a r d o r g a n i c f u n g i c i d e s , s u c h as c h l o r o t h a l o n i l , n e c e s s a r y f o r p e a n u t p r o d u c t i o n . I n o u r tests, a p p l i c a t i o n o f c h l o r o t h a l o n i l , propiconazole and diniconazole on a 21-day schedule or copper hydroxide or copper resinate fungicides on a 14-day s c h e d u l e p r o v i d e d l e a f s p o t c o n t r o l sufficient t o p r o d u c e
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Control of peanut leaf spot: A. K. Culbreath et aL
yields comparable to those achieved by standard 14-day applications of chlorothalonil, even in years when disease pressure was severe. Gorbet, Knauft and Shokes (1990) reported yield reduction of only 337 kg h a - 1 in Southern Runner with three or four sprays of chlorothalonil and delayed application compared with the 14-day schedule. In our tests, application of four or five sprays of chlorothalonil were adequate to produce yields that were as high as those of plots treated with seven applications of chlorothalonil applied at 14-day intervals. Based on leaf spot control alone, none of the copper fungicides were as effective as chlorothalonil, propiconazole or diniconazole. This follows a trend similar to that observed with the use of these materials on susceptible cvs (Littrell, 1985). Higher yields from plants treated with diniconazole were probably due to activity of this material on soil-borne pathogens (Csinos et al., 1987), which were affected minimally by the other fungicides. Maintaining leaf spot control with four or five sprays of chlorothalonil compared with the seven typically recommended represents a significant reduction of fungicide input required for peanut production and a reduction in cost of disease control. Incorporating higher levels of leaf spot resistance into commercial peanut cvs should further reduce the chemical dependency of typical peanut production compared with cvs, such as Florunner, that require maximum management for maximum yield and grade (Gorbet, Shokes and Jackson, 1982). This represents potential economic and environmental benefits. This use of a leaf spot-resistant cv. may also provide more options for fungicide use on peanut. Alternatives to chlorothalonil may represent increases in net income and change in the types and concentrations of environmental risks associated with organic fungicide use on the crop. Although copper hydroxide fungicides are synthetically compounded materials, they are currently acceptable on a restricted basis for disease control in many certified organic crop-production programmes in the United States (Brammer, 1991). However, further clarification of the environmental risks of copper materials is needed. Copper is an essential element for plant growth, and beneficial effects of copper fungicides were observed in copper-deficient soils (Marschner, 1986). High levels of copper in the soil, however, may have direct toxic effects on plants or interfere with availability and uptake of other nutrients (Rhoads, Olson and Barnett, 1991). Furthermore, copper does not break down and does not leach easily from the soil. At present, both copper hydroxide materials evaluated in this experiment show potential for use in leaf spot control regimens on Southern Runner or a cv. with similar or greater levels of resistance. The formulation and rate of the copper resinate fungicide used in this study contained less elemental copper than did either of the other copper fungicides. Application of this material on a 14-day schedule provided sufficient leaf spot control to prevent loss of yield in 1989, a year with severe, leaf spot epidemics. The lack of significant yield response to any fungicide treatment in 1990 was due to an extended drought, which resulted in unfavourable environmental conditions for leaf spot development for much of the growing season and the
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moderate levels of resistance and tolerance of Southern Runner to C. personatum. Yield of untreated plots in 1990 was similar to that of plots receiving standard chlorothalonil treatments in the two previous years. Consideration of environmental conditions in applying fungicides may help to eliminate unnecessary applications when conditions are not conducive for disease development (Nutter and Brenneman, 1989). In many years, indicated by 1988 and 1989 results, fungicide applications would be required to prevent yield losses, even with Southern Runner. In addition to resistance to C. personatum, Southern Runner has been observed to have low to moderate levels of resistance to Puccinia arachidis (Speg.) (Gorbet et al., 1986), the cause of peanut rust, Sclerotium rolfsii (Brenneman et al., 1990), the cause of southern stem rot, and tomato spotted wilt virus (Culbreath, Demski and Todd, 1990). This multiple pathogen resistance makes this cv. well suited for use in production regimens using reduced amounts of pesticides in standard peanut production, or in organic peanut-production situations where chemical options for disease management are limited.
Notes and acknowledgements Supported by Hatch and State funds allocated to the University of Georgia College of Agriculture Experiment Stations. This work was supported in part by grants from the Georgia Agriculture Commodity Commission for Peanuts. The efforts of Mike Heath, Jimmy Laska, Fabian Tata and Fannie Fowler are gratefully acknowledged.
References Brammer, W. (1991) California Certified Organic Farmers. 1991 Certification Handbook. Ag Access, Davis, CA, 50 pp Brenneman, T. B., Branch, W. D. and Csinos, A. S. (1990) Partial resistance of Southern Runner, Arachis hypogaea, to stem rot caused by Sclerotium rolfsii. Peanut Sci. 18, 65-67 Brenneman, T. B., Chandler, L. D., Sumner, H. R. and Hammond, J. M.
(1991) Effects of application methods on efficacyof propiconazole for control of peanut diseases. Prec. Am. Peanut Res. Ed. Soc. 23, 60 (abstr.) Chiteka, Z. A., Gorbet, D. W., Shakes, F. M., Kncharek, T. A. and Knauft, D. A. (1988) Components of resistance to late leaf spot in peanut. I. Levelsand variability- implicationsfor selection. Peanut Sci.
15, 25-30 Csinos, A. S., Kvien, C. S. and LittreH, R. H. (1987) Activity of diniconazole on foliar and soilborne diseases of peanut. Appl. Agric. Res. 2, 113-116 Culbreath, A. K., Demskl, J. W. and Todd, J. W. (1990)Characterization of tomato spotted wilt epidemics in peanut. Phytopathology 80, 988
(abstr.) Gorbet, D. W., Knauft, D. A. and Shokes, F. M. (1990) Response of peanut genotypeswith different levelsof leaf spot resistanceto fungicide treatments. Crop Sci. 30, 529-533 Gorbet, D. W., Norden, A. J., Shokes, F. M. and Knauft, D. A. (1986) Southern Runner: A New Leaf Spot Resistant Peanut Variety. Univ. Fla. Agric. Exp. Stn. Circ, S-324, 13pp Gorbet, D. W., Shokes, F. M. and Jackson, L. F. (1982) Control of
peanut leaf spot with a combination of resistance and fungicide treatment. Peanut Sci. 9, 87-90
Control of peanut leaf spot: A. K. Culbreath et aL
Hammond, J. M., Baeklnan, P. A. and Lyle, J. A. (1977) Peanut foliar fungicides: relationships between leaf spot control and kernel quality. Peanut Sci. 3, 70-72 Hancock, H. G. and Weete, J. D. (1984) Performance of Tilt 3.6 EC in peanut leaf spot control. Proc. Am. Peanut Res. Ed. Soc. 16, 52 (abstr.)
Johnson, W. C., Beasley, J. P., Thompson,S. S., Womack, H., Swann, C. W. and Samples, L. E. (1987) Georgia Peanut Production Guide. Univ. Georgia Col. Agr. Coop. Ext. Serv. Bull., 54 pp Littrell, R. H. (1985) Evaluation of copper fungicides used alone and in various schedules with Bravo on peanut pod yields. Phytopathology 74, 501 (abstr.)
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Rhoads, F. M., Olson, S. M. and Barnett, R. D. (1991) Soil contamination from copper pesticides. In: Proceedings for the Environmentally Sound Agriculture Conference (Ed. by A. B. Bottcher), pp. 20-24, IFAS, Gainesville, FL Smith, D. H. and Littrell, R. H. (1980) Management of peanut foliar diseases with fungicides. Plant Dis. 64, 356-361 Steel, R. G. B. and Torrie, J. H. (1980) Principles and Procedures oJ Statistics, McGraw-Hill Book Co., New York, 633 pp
Thompson, S. S. (1990) Peanut disease losses in 1990. In: 1990 Georgia Peanut Research-Extension Report, p. 2, University of Georgia Coop. Res.-Ext. Publ. No. 2.
Marschner, H. (1986) Functions of mineral nutrients: micronutrients. In: Mineral Nutrition of Higher Plants (Ed. by H. Marschner) pp. 287-291, Academic Press, New York Nutter, F. W. and Brenneman,T. B. (1989) Development and validation of a weather-based late leaf spot spray advisory. Proc. Am. Peanut Res. Ed. Soc. 21, 24 (abstr.)
Pixley, K. V., Boote, K. J., Shokes, F. M. and Gorbet, D. W. (1990) Disease progression and leaf area dynamics of four peanut genotypes differing in resistance to late leaf spot. Crop Sci. 30, 789-796
Received 13 November 1991 Revised 4 February 1992 Accepted 4 February 1992
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Use of a resistant peanut cultivar with copper fungicides and reduced fungicide applications for control of late leaf spot A. K. C u l b r e a t h * , T. B. B r e n n e m a n a n d C. K. Kvien
University of Georgia, Coastal Plain Experiment Station, Departments of Plant Pathology and Agronomy, Tifton, Georgia, USA
Abstract
Keywords
In field tests conducted in 1988 and 1989, chlorothalonil, propiconazole, or diniconazole applied on a 2 l-day schedule provided control of late leaf spot (Cercosporidiumpersonatum) of peanut (Arachis hypogaea) that was sufficient to prevent yield losses in Southern Runner, a cultivar with moderate resistance to C. personatum. Copper hydroxide in 1988 and 1989, copper oxysulphate in 1988 and copper resinate fungicides in 1989 applied in a 14-day schedule produced yields comparable to chlorothalonii applied in a 14-day schedule, although leaf spot control provided by all of these materials was inferior to that provided by chlorothalonil, propiconazole or diniconazole. In 1990, significant yield loss due to leaf spot did not occur for even the untreated plots. In 1988 and 1989, highest yields were from plots treated with diniconazole, primarily because of superior control of soil-borne diseases by this fungicide. Resistant cultivars; Cercosporidium personatum; Arachis hypogaea; chlorothalonil; fungicidal control
Introduction
The use of cultivars (cvs) resistant to pathogens is one of the safest and most economical methods for control of plant diseases. Although peanut cvs with complete resistance to all pathogens are not available, partial resistance to one or more pathogens may be useful in reducing chemical inputs and the cost of disease control. The peanut (Arachis hypogaea L.) cv. 'Southern Runner' has a moderate level of resistance to several pathogens (Gorbet et al., 1986; Brenneman, Branch and Csinos, 1990; Culbreath, Demski and Todd, 1990), including Cercosporidium personatum (Curt. & Berk.) Deighton, the cause of late leaf spot. Late leaf spot is one of the most severe diseases of peanut world-wide and is responsible for losses totalling tens of millions of dollars annually in Georgia alone (Thompson, 1990). In the southeastern United States, leaf spot control on susceptible cvs, such as 'Florunner', is achieved by the use of multiple applications of the fungicide chlorothalonil. Excellent control of late leaf spot is achieved with chlorothalonil on susceptible cvs, but at considerable cost to growers. The fungicide applied to peanut contributes to the large pesticide load entering the environment over a large area of Georgia and the southeastern United States. Resistance in Southern Runner is not sufficient to prevent yield losses to leaf spot without application of fungicides (Gorbet et al., 1986). However, use of Southern Runner could reduce the number of *To whom correspondence should be addressed, at Plant Pathology
Department, The University of Georgia Coastal Plain Station, Tifton, GA 31793-0748, USA
fungicide applications required for peanut production. In addition to moderate resistance to infection by C. personatum, Southern Runner has continual leaf production throughout the season. This results in some replacement of leaves lost due to leaf spot disease (Pixley et al., 1990). The inorganic fungicide copper hydroxide [Cu(OH)2 ] and other copper-containing fungicides are registered for use on peanut, and the cost per application of these materials is lower than that of chlorothalonil. Copper fungicides were used extensively for leaf spot control in the southeastern United States until the development of the more effective fungicides, benomyl and chlorothalonil (Smith and Littrell, 1980). Copper fungicides, such as copper hydroxide, copper oxysulphate and copper resinate, do not provide leaf spot control that is adequate to prevent yield losses when applied to the standard susceptible cv., Florunner (Hammond, Backman and Lyle, 1977; Littrell, 1985). Copper fungicides with a moderately resistant cultivar may, however, provide adequate control of leaf spot. In addition to chlorothalonil and the copper fungicides, several experimental fungicides of the triazole (ergosterol biosynthesis inhibitors) class were evaluated for managing leaf spot diseases on peanut. Propiconazole (Hancock and Weete, 1984) and diniconazole (Csinos, Kvien and Littrell, 1987) controlled leaf spot when applied at rates of active ingredient lower than those required for chlorothalonil (Csinos et al., 1987). In addition to activity against C. personatum, these materials have potential for controlling soil-borne diseases (Csinos et al., 1987; Brenneman et al., 1991). The objective of this study was to determine the effects of 14- and 21-day ftmgicide spray
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Control of peanut leaf spot: A. K. Culbreath etaL
schedules of copper fungicides, chlorothalonil and two ergosterol biosynthesis-inhibiting fungicides on leaf spot severity on Southern Runner peanut.
Materials and methods Experiments were conducted in 1988, 1989 and 1990 at the University of Georgia Coastal Plain Experiment Station Gibbs Farm, Tifton, GA. Test sites in 1988 and 1990 were fallow and the 1989 site had been planted to cotton (Gossypium hirsutum L.) the previous growing season. Soil type in all years was a Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Paleudults) with pH of 5.7, 5.8 and 5.8 in 1988, 1989 and 1990, respectively. The experimental design was a randomized complete block with plots that each consisted of a single raised bed (7.6 m x 1.8 m) with two rows per bed. Plots were separated by one bed (1.8m wide) of two unsprayed border rows and 2.4m fallow alleys on the ends. Southern Runner was planted on 17 May, 18 May and 14 May in 1988, 1989 and 1990, respectively. Aldicarb (Temik 15G) (1.12 kg a.i. ha- 1) was applied in the planting furrow at the time of planting for insect control. Gypsum (calcium sulphate) was applied at 560 kg ha- 1 broadcast at early pegging each year. Management of weeds and insects was according to University of Georgia recommendations (Johnson et al., 1987). Treatments in all three years consisted of combinations of different fungicides and application schedules. In 1988, fungicides used included the industry standard chlorothalonil, 1.26kga.i. ha -1 (Bravo 720, ISK Biotech, Mentor, OH), copper hydroxide [Cu(On)2)] , 1.68kg metallic Cu ha- 1 (Kocide 101, 77W, Griffin Corp., Valdosta, GA), copper oxysulphate, 1.51 kg metallic Cu ha- ] (Cuproxat, Gilmore Chemical Co., Memphis, TN) and experimental organic dimethyl sterol-inhibiting fungicides, propiconazole, 0.12 kg a.i. ha- i (Tilt 3.6 EC, Ciba-Geigy Corp., Greensboro, NC) and diniconazole, 0.12kg a.i. ha- 1 (Spotless 25 WP, Valent USA Corp., Memphis, TN). Treatments in 1989 and 1990 consisted of application of chlorothalonil (1.26 kg a.i. ha- 1), Cu(OH) 2 as Kocide 101 (1.68 kg metallic Cu ha-1) and as an experimental dry flowable formulation of copper hydroxide (1.68 kg metallic Cu ha-1) (Copper Hydroxide 77 DF, Terra Corp., Riverside, CA) (henceforth referred to as copper hydroxide DF), and copper resinate, 0.24 kg metallic Cu ha- 1 (TermCop 5E, Tennessee Chemical Co., Copperhill, TN) each on 14- and 21-day spray intervals. In addition, propiconazole (0.12kg a.i. ha-1) and diniconazole (0.12kg a.i. ha-1) were applied at the 21-day spray intervals only. In all three years, fungicides were applied using a CO2-powered sprayer with a 1.9m boom, equipped with three D2-13 hollow-cone nozzles per row. Fungicides were applied in 501 water ha 1 at 345 kPa. Two spray schedule regimens were examined in all years, including a grower standard 14-day schedule and a 21-day schedule. The grower standard schedule sprays were initiated on 23 June 1988, 23 June 1989 and 29 June 1990. The 21-day schedule sprays were begun on 14 July 1988, 23 June 1989 and 29 June 1990. Seven 14-day-schedule
CROP PROTECTION Vol. 11 August 1992
applications were made in each year, and four, five and five 21-day-schedule applications were made in 1988, 1989 and 1990, respectively. Leaf spot severity ratings were made on all plots on 14 September and 14 October 1988, 24 August and 5 October 1989 and 18 September and 17 October 1990, using a scale of 1-10 where 1 represents no leaf spot and 10 represents plants completely defoliated and killed by leaf spot (Chiteka et al., 1988). Data were subjected to analysis of variance. Fisher's protected 1.s.d. values were used for evaluation of significance of treatment differences (Steel and Torrie, 1980). Differences denoted in the text are significant at p < 0.05 unless otherwise stated.
Results Disease and yield in 1988 Final leaf spot ratings for all fungicide treatments were lower than those of unsprayed plots (Table 1). Applications of copper hydroxide, copper hydroxide DF or copper oxysulphate on a 14- or 21-day schedule gave leaf spot control that was inferior to that of chlorothalonil, diniconazole or propiconazole in the corresponding spray schedules (Table 1). Plants treated with copper hydroxide or copper hydroxide DF at 14-day intervals produced yields as high as those of plants treated with diniconazole, propiconazole or chlorothalonil at that same interval. Plants treated with copper hydroxide or copper hydroxide DF at 21-day intervals, or copper oxysulphate at either interval, produced yields that did not differ significantly from yields of untreated plots (Table 1). Leaf spot ratings for plants treated with chlorothalonil at 21-day intervals were higher than for those receiving the same treatment at 14-day intervals, but yields were similar for the two treatments. Diniconazole applied on the 14-day schedule was the only treatment that reduced severity of limb rot, and incidence of stem rot was lower than for the untreated plots.
Disease and yield in 1989 Application of Cu(OH)2 at either 14- or 21-day intervals did not significantly reduce leaf spot ratings compared with unsprayed plots, whereas application of the Cu(OH)2 DF formulation reduced leaf spot severity at both spray intervals (Table 2). Neither of these provided control equal to that achieved with 14- or 21-day applications of chlorothalonil or 21-day schedule applications of propiconazole or diniconazole (Table 2). Yields of plants treated with either copper hydroxide material at either 14- or 21-day intervals did not differ from those treated with chlorothalonil, and were higher than the yield of unsprayed plants. Application of chlorothalonil at 21-day intervals resulted in leaf spot ratings that did not differ significantly from those of plants treated with chlorothalonil on a 14-day schedule, and yields for these two treatments were similar. Only diniconazole applied on the 2 l-day schedule reduced incidence of stem rot in the 1989 test. Pod yield of plants treated with diniconazole at 21-day intervals ranked high-
Control of peanut leaf spot: A. K. Culbreath et aL
363
Table 1. Comparison of fungicides applied at 14- and 21-day intervals for control of late leaf spot on Southern Runner peanut, 1988
Treatment
Rate"
Spray schedule
Leaf spot ratingb
Limb rot e
Stem rot d
Yield (kg ha- 1)
Untreated check
--
Copper hydroxide
1.68 1.68
--
7.9
28.8
5.8
2949
14-day 21-day
5.8 6.6
31.5 25.0
5.0 3.3
3694 3085
Copper oxysulphate
1.51 1.51
14-day 21-day
5.6 6.1
37.0 32.3
4.8 5.3
3185 3258
Chlorothalonil
1.26 1.26
14-day 21-day
2.5 4.3
42.3 42.3
6.3 6.0
3412 3694
Propiconazole
0.12 0.12
14-day 21-day
4.2 4.7
40.8 33.8
6.5 8.3
3630 3303
Diniconazolee
0.12 0.12
14-day 2l-day
4.0 4.0
6.3 24.0
0 5.0
3984 4365
0.6
13.5
4.3
658
1.s.d. (p <0.05) f
"Rate expressed as kg of metallic copper or a.i. ha ~; bleaf spot ratings made on 14 October using the Florida 1-10 scale where 1= no disease and 10= plants completely defoliated and killedby leaf spot (Chiteka et al., 1988);Cpercentageof limbs showingsymptoms or signsof Rhizoctonia limb rot; anumber of diseaseloci (0.3 m of affected row or less) per 13.3m of row; elUPAC chemical name: (E)-(R), E- l-(2,4-dichlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-l-yl)-l-penten-3-ol);fFisher's least significantdifference for comparison of means within columns
Table 2. Comparison of fungicides applied at 14- and 21-day intervals for control of late leaf spot on Southern Runner peanut, 1989 and 1990
Leaf spot ratingh Treatment
Rate"
Untreated
Stem rot c
Yield (kg ha 1)
Spray schedule
1989
1990
1989
1990
1989
1990
--
--
7.5
5.6
4.8
4.5
3140
3670
Copper hydroxide
1.68 1.68
14-day 21-day
6.4 6.9
3.1 4.4
2.3 3.0
2.8 2.8
4025 3903
3581 4125
Copper hydroxide DF
1.68 1.68
14-day 21-day
5.8 5.9
2.8 3.4
2.0 4.0
2.5 2.8
3801 3669
3970 3751
Copper resinate
0.24 0.24
14-day 21-day
6.4 6.9
3.6 5.3
4.5 2.3
4.3 5.0
3821 3354
4126 3768
Chlorothalonil
1.26 1.26
14-day 21-day
3.4 3.8
2.0 3.6
4.3 3.8
4.3 2.0
3840 3842
3687 3785
Propiconazole
0.12
21-day
4.4
3.3
3.0
3.5
3903
4020
Diniconazole
0.12
21-day
3.9
3.6
1.5
2.3
4523
4313
1.3
1.2
2.7
2.4
523
668
l.s.d. (p <0.05) a
"As in Table 1; bleafspot ratings made on 5 October using the Florida I-10 scalewhere 1= no disease and I0 = plants completelydefoliated and killed by leaf spot (Chiteka et al., 1988);Casin Table 1; aFisher's least significant differencefor comparison of means within columns
est a m o n g all t r e a t m e n t s , a l t h o u g h l e a f s p o t r a t i n g s w e r e similar to those of plants treated with chlorothalonil at e i t h e r s p r a y i n t e r v a l (Table 2).
N o s i g n i f i c a n t yield r e s p o n s e t o f u n g i c i d e t r e a t m e n t w a s detected for any treatment. Plots treated with diniconazole h a d yields t h a t w e r e n u m e r i c a l l y , b u t n o t s i g n i f i c a n t l y , higher than those of other treatments.
D i s e a s e a n d y i e l d in 1 9 9 0
L e a f s p o t i n t e n s i t y w a s l o w e r i n 1990 t h a n i n t h e p r e v i o u s two years, owing to an extended drought. Application of either copper hydroxide fungicide at 14-day intervals r e s u l t e d i n f i n a l l e a f s p o t r a t i n g s t h a t d i d n o t differ significantly from those of plants treated with chlorothal o n i l ( T a b l e 2 ). A p p l i c a t i o n o f t h e c o p p e r r e s i n a t e f u n g i c i d e at either standard or reduced application schedules result e d i n m o r e l e a f s p o t t h a n in p l o t s t r e a t e d w i t h c h l o r o t h a lonil at either of the respective spray intervals. Leaf spot r a t i n g s d i d n o t differ f o r t h e 14- o r 2 1 - d a y t r e a t m e n t s o f chlorothalonil.
Discussion U s e o f a m o d e r a t e level o f r e s i s t a n c e t o C. p e r s o n a t u m i n a cv. s u c h as S o u t h e r n R u n n e r m a y m a k e it f e a s i b l e t o u s e c o p p e r f u n g i c i d e s t h a t a r e less effective t h a n c h l o r o t h a lonil. T h i s c o u l d r e d u c e t h e a m o u n t o f s t a n d a r d o r g a n i c f u n g i c i d e s , s u c h as c h l o r o t h a l o n i l , n e c e s s a r y f o r p e a n u t p r o d u c t i o n . I n o u r tests, a p p l i c a t i o n o f c h l o r o t h a l o n i l , propiconazole and diniconazole on a 21-day schedule or copper hydroxide or copper resinate fungicides on a 14-day s c h e d u l e p r o v i d e d l e a f s p o t c o n t r o l sufficient t o p r o d u c e
CROP PROTECTION Vol. 11 August 1992
364
Control of peanut leaf spot: A. K. Culbreath et aL
yields comparable to those achieved by standard 14-day applications of chlorothalonil, even in years when disease pressure was severe. Gorbet, Knauft and Shokes (1990) reported yield reduction of only 337 kg h a - 1 in Southern Runner with three or four sprays of chlorothalonil and delayed application compared with the 14-day schedule. In our tests, application of four or five sprays of chlorothalonil were adequate to produce yields that were as high as those of plots treated with seven applications of chlorothalonil applied at 14-day intervals. Based on leaf spot control alone, none of the copper fungicides were as effective as chlorothalonil, propiconazole or diniconazole. This follows a trend similar to that observed with the use of these materials on susceptible cvs (Littrell, 1985). Higher yields from plants treated with diniconazole were probably due to activity of this material on soil-borne pathogens (Csinos et al., 1987), which were affected minimally by the other fungicides. Maintaining leaf spot control with four or five sprays of chlorothalonil compared with the seven typically recommended represents a significant reduction of fungicide input required for peanut production and a reduction in cost of disease control. Incorporating higher levels of leaf spot resistance into commercial peanut cvs should further reduce the chemical dependency of typical peanut production compared with cvs, such as Florunner, that require maximum management for maximum yield and grade (Gorbet, Shokes and Jackson, 1982). This represents potential economic and environmental benefits. This use of a leaf spot-resistant cv. may also provide more options for fungicide use on peanut. Alternatives to chlorothalonil may represent increases in net income and change in the types and concentrations of environmental risks associated with organic fungicide use on the crop. Although copper hydroxide fungicides are synthetically compounded materials, they are currently acceptable on a restricted basis for disease control in many certified organic crop-production programmes in the United States (Brammer, 1991). However, further clarification of the environmental risks of copper materials is needed. Copper is an essential element for plant growth, and beneficial effects of copper fungicides were observed in copper-deficient soils (Marschner, 1986). High levels of copper in the soil, however, may have direct toxic effects on plants or interfere with availability and uptake of other nutrients (Rhoads, Olson and Barnett, 1991). Furthermore, copper does not break down and does not leach easily from the soil. At present, both copper hydroxide materials evaluated in this experiment show potential for use in leaf spot control regimens on Southern Runner or a cv. with similar or greater levels of resistance. The formulation and rate of the copper resinate fungicide used in this study contained less elemental copper than did either of the other copper fungicides. Application of this material on a 14-day schedule provided sufficient leaf spot control to prevent loss of yield in 1989, a year with severe, leaf spot epidemics. The lack of significant yield response to any fungicide treatment in 1990 was due to an extended drought, which resulted in unfavourable environmental conditions for leaf spot development for much of the growing season and the
CROP PROTECTION Vol. 11 August 1992
moderate levels of resistance and tolerance of Southern Runner to C. personatum. Yield of untreated plots in 1990 was similar to that of plots receiving standard chlorothalonil treatments in the two previous years. Consideration of environmental conditions in applying fungicides may help to eliminate unnecessary applications when conditions are not conducive for disease development (Nutter and Brenneman, 1989). In many years, indicated by 1988 and 1989 results, fungicide applications would be required to prevent yield losses, even with Southern Runner. In addition to resistance to C. personatum, Southern Runner has been observed to have low to moderate levels of resistance to Puccinia arachidis (Speg.) (Gorbet et al., 1986), the cause of peanut rust, Sclerotium rolfsii (Brenneman et al., 1990), the cause of southern stem rot, and tomato spotted wilt virus (Culbreath, Demski and Todd, 1990). This multiple pathogen resistance makes this cv. well suited for use in production regimens using reduced amounts of pesticides in standard peanut production, or in organic peanut-production situations where chemical options for disease management are limited.
Notes and acknowledgements Supported by Hatch and State funds allocated to the University of Georgia College of Agriculture Experiment Stations. This work was supported in part by grants from the Georgia Agriculture Commodity Commission for Peanuts. The efforts of Mike Heath, Jimmy Laska, Fabian Tata and Fannie Fowler are gratefully acknowledged.
References Brammer, W. (1991) California Certified Organic Farmers. 1991 Certification Handbook. Ag Access, Davis, CA, 50 pp Brenneman, T. B., Branch, W. D. and Csinos, A. S. (1990) Partial resistance of Southern Runner, Arachis hypogaea, to stem rot caused by Sclerotium rolfsii. Peanut Sci. 18, 65-67 Brenneman, T. B., Chandler, L. D., Sumner, H. R. and Hammond, J. M.
(1991) Effects of application methods on efficacyof propiconazole for control of peanut diseases. Prec. Am. Peanut Res. Ed. Soc. 23, 60 (abstr.) Chiteka, Z. A., Gorbet, D. W., Shakes, F. M., Kncharek, T. A. and Knauft, D. A. (1988) Components of resistance to late leaf spot in peanut. I. Levelsand variability- implicationsfor selection. Peanut Sci.
15, 25-30 Csinos, A. S., Kvien, C. S. and LittreH, R. H. (1987) Activity of diniconazole on foliar and soilborne diseases of peanut. Appl. Agric. Res. 2, 113-116 Culbreath, A. K., Demskl, J. W. and Todd, J. W. (1990)Characterization of tomato spotted wilt epidemics in peanut. Phytopathology 80, 988
(abstr.) Gorbet, D. W., Knauft, D. A. and Shokes, F. M. (1990) Response of peanut genotypeswith different levelsof leaf spot resistanceto fungicide treatments. Crop Sci. 30, 529-533 Gorbet, D. W., Norden, A. J., Shokes, F. M. and Knauft, D. A. (1986) Southern Runner: A New Leaf Spot Resistant Peanut Variety. Univ. Fla. Agric. Exp. Stn. Circ, S-324, 13pp Gorbet, D. W., Shokes, F. M. and Jackson, L. F. (1982) Control of
peanut leaf spot with a combination of resistance and fungicide treatment. Peanut Sci. 9, 87-90
Control of peanut leaf spot: A. K. Culbreath et aL
Hammond, J. M., Baeklnan, P. A. and Lyle, J. A. (1977) Peanut foliar fungicides: relationships between leaf spot control and kernel quality. Peanut Sci. 3, 70-72 Hancock, H. G. and Weete, J. D. (1984) Performance of Tilt 3.6 EC in peanut leaf spot control. Proc. Am. Peanut Res. Ed. Soc. 16, 52 (abstr.)
Johnson, W. C., Beasley, J. P., Thompson,S. S., Womack, H., Swann, C. W. and Samples, L. E. (1987) Georgia Peanut Production Guide. Univ. Georgia Col. Agr. Coop. Ext. Serv. Bull., 54 pp Littrell, R. H. (1985) Evaluation of copper fungicides used alone and in various schedules with Bravo on peanut pod yields. Phytopathology 74, 501 (abstr.)
365
Rhoads, F. M., Olson, S. M. and Barnett, R. D. (1991) Soil contamination from copper pesticides. In: Proceedings for the Environmentally Sound Agriculture Conference (Ed. by A. B. Bottcher), pp. 20-24, IFAS, Gainesville, FL Smith, D. H. and Littrell, R. H. (1980) Management of peanut foliar diseases with fungicides. Plant Dis. 64, 356-361 Steel, R. G. B. and Torrie, J. H. (1980) Principles and Procedures oJ Statistics, McGraw-Hill Book Co., New York, 633 pp
Thompson, S. S. (1990) Peanut disease losses in 1990. In: 1990 Georgia Peanut Research-Extension Report, p. 2, University of Georgia Coop. Res.-Ext. Publ. No. 2.
Marschner, H. (1986) Functions of mineral nutrients: micronutrients. In: Mineral Nutrition of Higher Plants (Ed. by H. Marschner) pp. 287-291, Academic Press, New York Nutter, F. W. and Brenneman,T. B. (1989) Development and validation of a weather-based late leaf spot spray advisory. Proc. Am. Peanut Res. Ed. Soc. 21, 24 (abstr.)
Pixley, K. V., Boote, K. J., Shokes, F. M. and Gorbet, D. W. (1990) Disease progression and leaf area dynamics of four peanut genotypes differing in resistance to late leaf spot. Crop Sci. 30, 789-796
Received 13 November 1991 Revised 4 February 1992 Accepted 4 February 1992
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