© 2008 Plant Management Network.
Efficacy of Organically Acceptable Fungicides for Management of Early and Late Leaf Spot Diseases on Partially Resistant Peanut Cultivars
E. G. Cantonwine, Assistant Professor, Department of Biology, Valdosta State University, Valdosta, GA 31698; A. K. Culbreath, Professor, Department of Plant Pathology, Coastal Plain Experiment Station, University of Georgia, Tifton 31793; B. B. Shew, Research Assistant Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695; and M. A. Boudreau, Director, Hebert Green Agroecology Inc., Asheville, NC 28801
Cantonwine, E. G., Culbreath, A. K., Shew, B. B., and Boudreau, M. A. 2008. Efficacy of organically acceptable fungicides for management of early and late leaf spot diseases on partially resistant peanut cultivars. Online. Plant Health Progress doi:10.1094/PHP-2008-0317-03-RS.
Field experiments were carried out in Georgia and North Carolina to evaluate the efficacy of fungicides approved for the organic management of early leaf spot, caused by Cercospora arachidicola, and late leaf spot, caused by Cercosporidium personatum, in peanut (Arachis hypogaea) fields planted to cultivars with partial resistance to one or both pathogens. Copper treatments alone or in mixtures resulted in less disease than a non-treated control. In Georgia, sulfur provided some disease suppression, but not as much as treatments with copper sulfate. Neem oil did not affect disease severity. Mean pod yields across years were significantly greater than the non-treated control only for copper sulfate in Georgia and cupric hydroxide in North Carolina. The minimal yield response to treatments suggests that under similar situations, the frequency of copper-based fungicide applications may be reduced with little affect on yield.
Peanut (Arachis hypogaea) is plagued by a variety of fungal diseases that can make organic production difficult, especially in the humid southeastern United States where most of the conventional domestic peanut production occurs. Two important foliar diseases of peanut are early leaf spot, caused by Cercospora arachidicola (teleomorph = Mycosphaerella arachidis ), and late leaf spot, caused by Cercosporidium personatum (teleomorph = Mycosphaerella berkeleyi). One or both diseases are common in most peanut fields throughout the southeastern US. Infections by these pathogens result in necrotic lesions on leaves, stems, and pegs which lead to premature defoliation and premature pod drop (12). The amount of defoliation at harvest is significantly correlated to the amount of pod loss, which can be 50% or more in fields where these diseases are not managed (13). In conventional peanut production systems in the southeastern US, leaf spot epidemics are managed using frequent applications of synthetic fungicides on a calendar-based or weather advisory schedule, so that five or more applications are common per growing season (9). With disease potential so great, acceptable management tools are needed to help southeastern US growers become competitive in the rapidly expanding organic peanut market.
Prior to the development of synthetic fungicides in the 1970s, copper- and sulfur-containing fungicides were used extensively for leaf spot control in the southeastern US (13). Although copper fungicides did not provide adequate leaf spot control to prevent yield loss when applied to susceptible cv Florunner (6,11), Culbreath et al. (4) demonstrated that adequate leaf spot disease control was possible when copper fungicides were applied to moderately resistant cv Southern Runner.
Experiments in Georgia and North Carolina evaluated efficacies of organically acceptable fungicides for management of early and late leaf spot diseases in conjunction with crop rotations of one or more years and resistant cultivars. The fungicides tested included copper sulfate, sulfur, cupric hydroxide, botanically-based neem oil, and biological agent Bacillus subtilis. Treatment effects on soilborne diseases endemic at each location were also evaluated.
Organic Fungicide Efficacy Trials
Georgia. Field experiments in Georgia were conducted at the University of Georgia Coastal Plain Experiment Station in Tifton GA, in 2005 (Lang Farm) and 2006 (Rigdon Farm). Soil type was a Tifton loamy sand. Fields were planted to cotton (Gossypium hirsutum) the previous year, and peanut two years prior. Peanut disease histories of these fields included severe spotted wilt, caused by Tomato spotted wilt virus, early and late leaf spot diseases, and minor incidences of stem rot, caused by Sclerotium rolfsii.
The experiment was laid out in a randomized complete block design with five replications in 2005 and four replications in 2006. Untreated peanut seed of runner-type cv Georganic (7) was planted at 15 to 16 seed/m of row on 25 May 2005 and 19 May 2006 using a vacuum planter. Plots were 7.6 to 9.5 m long, consisting of two 91-cm spaced single rows, separated by 3-m alleys. In 2005, plots were prepared using strip-tillage into a senescing cover crop of winter wheat (Triticum aestivum), as described by Cantonwine et al. (2). In 2006, plots were prepared using conventional tillage. Synthetic fertilizers were applied prior to planting based on soil analyses, and gypsum (1120 to 1344 kg/ha) was broadcast 7 July 2005 and 23 June 2006. No herbicides were used in 2005, and a pre-plant application of ethalfluralin (Sonalan HFP 3.0, DowAgrosciences LLC, Indianapolis, IN) (1.05 kg ai/ha) and S-metoalachlor (Dual Magnum 8E, Syngenta Crop Protection Inc., Greensboro, NC) (1.68 kg ai/ha) was incorporated 27 April 2006. Otherwise, weeds were maintained by hand or cultivation both years. No insecticides were applied either year. Non-organic production practices, when employed, were used to minimize variation to crop health by untested factors. The fields received 40.0 cm rainfall and 13.6 cm irrigation in 2005, and 32.9 cm rainfall and 13.6 cm irrigation in 2006.
Treatments were an un-sprayed non-treated control and 14-day-interval applications of neem oil (GOS 7 Way Neem Spray Adjuvant, Georgia Organic Solutions, Blakely, GA) (2.3 liters/ha), copper sulfate (Triangle Brand 99% Copper Sulfate, Phelps Dodge, El Paso, TX) (2.2 kg/ha), copper sulfate (2.2 kg/ha) + Bacillus subtilis (Serenade, Agraquest Inc., Davis, CA) (2.3 liters/ha) + surfactant (QRD 602, Agraquest Inc.) (0.3% V:V), sulfur (High Yield 90% Sulfur, Voluntary Purchasing Group Inc., Bonham, TX) (5.6 kg/ha), or copper sulfate (2.2 kg/ha) + sulfur (5.6 kg/ha). Fungicides were applied with a tractor-propelled boom sprayer under 345 kPa pressure in 115 liters/ha of water. Applications were made at 37, 49, 63, 76, 99, 112, and 126 days after planting (DAP) in 2005, and 28, 41, 55, 67, 81, 95, and 110 DAP in 2006.
Final percent defoliation for each plot was estimated just before peanut plants were inverted (153 DAP both years) as an interpolation of data collected using a modified version of the Florida 1 to 10 index (3) where each index value was divided into fourths (0.25). Ratings of 1 to 3 = 0% defoliation. All ratings greater than 3 were interpolated by adding the percentage represented by the index value (3 = 0%, 4 = 5%, 5 = 20%, 6 = 50%, 7 = 75%, 8 = 90%, 9 = 98%, and 10 = 100%) to the percentage of the difference represented by the appropriate quarter value. For example, a rating of 5.25 was estimated as 27.5% defoliation (20% + 7.5% defoliation). Stem rot was evaluated immediately after digging as percentage of linear plot affected by stem rot. Peanuts were harvested mechanically 5 to 7 days after inverting, and pod yields were adjusted to 10% (wt/wt) moisture.
North Carolina. The field experiment in North Carolina was carried out at the Peanut Belt Research Station at Lewiston-Woodville, NC in 2005 and at the Ben Harris Farm, Galatia, NC in 2006. The 2005 field was a Rains sandy loam soil type in a cotton, corn (Zea mays), peanut rotation with no noteworthy disease history. The 2006 field was comprised evenly of a Goldsboro sandy loam and a Norfolk sandy loam, had been planted to cotton the previous four years, and had a history of Sclerotinia blight, caused by Sclerotinia minor.
The experiment was laid out in a randomized complete block design with four replications each year. Treated (Vitavax PC, Bayer CropScience, Research Triangle Park, NC) peanut seed of the Virginia-type cv Perry (8) was planted in 0.9-m-spaced single rows at 14 seed/m on 10 May 2005. Untreated seed of the Virginia-type germplasm line GP-NC 343 (1) were similarly planted on 22 May 2006. Plots prepared using conventional tillage were four rows wide, 10.7 m long, and separated by 3-m alleys both years. No insecticides were applied either year. The herbicides S-metolachlor (Dual Magnum 8E, 1.8 liters/ha) and pendimethalin (Prowl 3.3 EC, BASF Corporation, Research Triangle Park, NC) (2.3 liters/ha) were pre-plant incorporated both years. Post-plant weed management was hand weeding. The fields received 42.7 cm rainfall and 6.4 cm irrigation from 1 May to 30 September 2005, and 55 cm rainfall from 15 June to 27 September 2006.
Treatments were a control and 14-day-interval applications of cupric hydroxide (Kocide, DuPont, Wilmington, DE) (2.2 kg/ha), or cupric hydroxide + Bacillus subtilis (Serenade, 2.2 kg/ha). Fungicides were applied with a tractor-mounted four-row sprayer calibrated to deliver 140 liters/ha of water. Applications occurred at 58, 71, 87, 101, and 114 DAP in 2005, and 63, 77, 101, and 117 DAP in 2006.
Final percent defoliation for each plot was visually estimated for all plants within a 1.2-m section of each of the center two rows 140 DAP in 2005 and 144 DAP in 2006. Incidences of Sclerotinia blight, stem rot, and Cylindrocladium black rot, caused by Cylindrocladium parasiticum, were evaluated immediately after inversion each year, 147 DAP in 2005 and 146 DAP in 2006. Pods were harvested with a combine 6 to 14 days later and yields were determined for the two center rows of the plots by weighing harvested pods after they were dried and adjusted to 7% (wt/wt) moisture.
Data were analyzed by location using Proc MIXED with ddfm = satterth option on the model statement (SAS v.8.3, SAS Institute, Cary, NC), unless otherwise stated. Main effects were considered significant when P < 0.05. Fisher’s LSD values were computed using standard error and t-values of adjusted degrees of freedom. For significant interactions, the above Fisher’s LSD was further adjusted to reflect use of the interaction term as a source of error if the F-test for the main effect using the appropriate interaction showed the main effect to be significant.
Efficacy of Organically Acceptable Fungicides
Both early and late leaf spot diseases were present at all locations each year, with disease intensities moderately high relative to comparable farmer fields. No other factors that can contribute to defoliation, such as insect damage or other foliar diseases were obvious; therefore, defoliation ratings were attributed to leaf spot diseases. No significant interactions between fungicide treatment and year occurred at either location (P > 0.05), with the exception of percent defoliation in Tifton (P = 0.03). In this case, the interaction term could be included as a source of error as explained above; therefore, all results are presented across years.
In Georgia, the mean percent defoliation due to mixed infestations of early and late leaf spot pathogens was highest for the non-treated control and neem oil treatments, lowest for plots treated with copper sulfate alone or in a mixture, and intermediate for the plots treated with sulfur (Table 1). Fungicide treatments did not affect the percentage of plot with stem rot (Table 1). Mean pod yields of fungicide treatments did not differ significantly from the non-treated control, except for the copper sulfate treatment where yield was significantly greater (Table 1).
Table 1. Effects of organically acceptable fungicides on disease ratings and pod yields in Georgia, 2005-2006.
x Combined rating of early leaf spot (Cercospora arachidicola) and late leaf spot (Cercosporidium personatum) due to mixed infections in field plots. Ratings were interpolated from a modified version of the Florida 1 to 10 index where each index value was divided into quarters. Least square means from Proc MIXED with LSD adjusted for significant treatment × year interaction (P = 0.03).
y Least square means of Sclerotium rolfsii intensities in field plots immediately after peanut inversion.
z Least square means of pod yield at 10% (wt/wt) moisture.
In North Carolina, mean defoliation was greatest in the non-treated control and similar for the cupric hydroxide and cupric hydroxide + B. subtilis treatment plots (Table 2). Soilborne diseases evaluated were not affected by fungicide treatments (Table 2). Mean pod yields were significantly greater for the cupric hydroxide treated plots than the cupric hydroxide + B. subtilis plots or non-treated control plots (Table 2).
Table 2. Effects of organically acceptable fungicides on disease ratings and pod yields in North Carolina, 2005-2006.
v Least square means of combined rating of early leaf spot (Cercospora arachidicola) and late leaf spot (Cercosporidium personatum) due to mixed infections in field plots. Ratings were visually estimated for all plants within a 1.2-m section of each of the center two rows.
w Least square means of Sclerotinia minor incidence in field plots immediately after peanut inversion.
x Least square means of Sclerotium rolfsii incidence in field plots immediately after peanut inversion.
y Least square means of Cylindrocladium parasiticum incidence in field plots immediately after peanut inversion; evaluated in 2006 only because incidence was too low to rate in 2005.
z Least square means of pod yield at 7% (wt/wt) moisture.
Implications for Organic Disease Management
Results of this study suggest that 14-day-interval applications of cupric hydroxide or copper sulfate can provide acceptable levels of early and late leaf spot diseases in rotated fields without peanut for one year with a cultivar moderately resistant to these two leaf spot pathogens. Except for Perry which is susceptible to late leaf spot, the degree of early leaf spot and late leaf spot resistances of the peanut genotypes used in this study was comparable to or better than that of the moderately resistant cv Southern Runner which was evaluated under a similar fungicide regime using copper-containing fungicides by Culbreath et al. (4). In this experiment, copper fungicides reduced leaf spot diseases as well alone as they did when mixed with sulfur or B. subtilis, and yields were significantly greater in plots sprayed with copper fungicides alone compared to the mixtures. This trend has not been consistent in other experiments that compared disease and yield responses of B. subtilis and copper fungicide mixtures to copper fungicides alone on susceptible cultivars. Experiments conducted in 2002 and 2004 resulted in comparable or better leaf spot control and significantly greater yield (P < 0.05) with a combination of B. subtilis and cupric hydroxide compared to cupric hydroxide alone. In 2005, no differences in yield were observed despite significantly less leaf spot diseases with the mixture (Culbreath, unpublished data).
Sulfur alone provided some disease management benefits, but the effects were not as great as copper sulfate alone and did not result in higher yields than the non-treated control. There was noticeable phytotoxicity (data not shown) in the sulfur and sulfur mixture treatments shortly after applications that may explain the lack of yield response compared to the non-treated control, and the reduced yield observed for the copper sulfate and sulfur mixture compared to the copper sulfate treatment alone. Based on this result, we do not recommend use of sulfur in commercial organic production unless copper toxicity levels are a concern and leaf spot diseases are in clear need of control.
Although neem oil has been reported to provide suppression of late leaf spot of peanut (5,14), and early blight and Septoria leaf spot of tomato (15), our results provide no indication that neem oil has activity against early or late leaf spot diseases of peanut in Georgia. These results corroborate results from a previous field experiment (Culbreath, unpublished data) in which neem oil provided no control of early leaf spot on a susceptible peanut cultivar.
Although the level of disease caused by soilborne pathogens was low, there were no indications that any of the fungicide treatments have activity against the diseases we evaluated. A previous study conducted by Culbreath et al. (4) also found no activity against soilborne diseases by copper-containing fungicides. Crop rotation with non-host crops and use of resistant cultivars will be critical for the management of soilborne diseases in organic production systems, since there are few other acceptable options. In conventional peanut production, two or more synthetic fungicide applications are recommended to manage soilborne diseases (9). The cultivars tested in these experiments were selected because of their resistance to multiple diseases. Georganic has partial resistance to spotted wilt, early and late leaf spot diseases, and stem rot (2,7). Perry is moderately resistant to early leaf spot, Cylindrocladium black rot and Sclerotinia blight (8), while GP-NC 343, released for insect resistance (1), has moderate resistance to early and late leaf spots (10).
The yield response of cultivars evaluated under organic fungicide applications was less than would be expected if susceptible varieties were sprayed with synthetic fungicides (2). It may be that the cultivars evaluated in these studies were tolerant to leaf spot diseases, thus minimizing yield loss. In many cases, use of a cultivar with resistance or tolerance to leaf spot pathogens may not require use of foliar fungicide applications to achieve acceptable yields. Cantonwine et al (2) reported that net economic returns of Georganic grown without fungicides were similar to or better than those of the standard cv Georgia Green treated with seven applications of chlorothalonil. However, when environmental conditions are very conducive for leaf spot development during certain years or in specific locations, use of organically-approved fungicides may be essential to protect yields. Our results indicate that copper sulfate or cupric hydroxide would be more effective for leaf spot control than the other materials evaluated in the experiments reported here.
Additional research is in progress to evaluate wide interval schedules and fungicide programs that begin when disease is first noticed, rather than relying on the calendar-based schedule currently recommended for leaf spot control in conventional production. A fungicide plan that minimizes the number of fungicide applications would fit better into an organic production plan than the calendar-based system tested in this experiment because organic producers must document need for every input. Since yield loss was minimal in this study, it is possible that under similar situations, fewer applications of copper fungicides could be used with minimal impact on pod yields.
This research was supported by a USDA SARE Southern Region, Research and Education grant.
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