2011 Plant Management Network. Accepted for publication 13 January 2011. Published. Evaluation of Soil Fumigants for Management of Verticillium Wilt of Peanut in Texas J. E. Woodward and T. A. Wheeler, Texas AgriLife Research and Extension Service, Lubbock, TX 79403; M. G. Cattaneo, Texas AgriLife Extension Service, Seminole, TX 79355; S. A. Russell, Texas AgriLife Extension Service, Brownfield, TX 79316; and T. A. Baughman, Texas AgriLife Extension, Vernon, TX 76384 Corresponding author: J. E. Woodward. jewoodward@ag.tamu.edu Woodward, J. E., Wheeler, T. A., Cattaneo, M. G., Russell, S. A., and Baughman, T. A. 2011. Evaluation of soil fumigants for management of Verticillium wilt of peanut in Texas. Online. Plant Health Progress doi:10.1094/php-2011-0323-02-rs. Abstract Field experiments were conducted to evaluate the effect of the fumigants chloropicrin and metam sodium on soil populations of Verticillium dahliae, disease incidence, and peanut yield and grade. Chloropicrin was ineffective at reducing soil populations of V. dahliae. The application of chloropicrin provided a 7 to 10% reduction in incidence of Verticillium wilt; however, there was no effect on yield or grade. Applications of metam sodium reduced soil populations of V. dahliae, but did not impact disease incidence, yield, or grade. Although fumigants had a minor effect on V. dahliae and disease incidence, the lack of a response in yield or quality limits the use of these chemicals in a production system. Introduction The soilborne fungus Verticillium dahliae Kleb., causal agent of Verticillium wilt, has a very broad host range including more than 400 plant species. Verticillium wilt is the most economically important disease of cotton on the Southern High Plains of Texas. The disease was first reported infecting peanut (Arachis hypogaea L.) in New Mexico in 1950 (11), and is responsible for significant yield losses in New Mexico, Oklahoma, and Texas (14). The fungus is capable of surviving for long periods of time in soil as microsclerotia. Pullman and DeVay (9) found that disease incidence in cotton is positively correlated with inoculum density; however, this relationship is poorly understood in peanut. Initial infections occur early in the growing season; however, symptoms are generally not observed until pod fill. Symptoms consist of marginal chlorosis or necrosis of foliage (Fig. 1), and light to dark brown discoloration in the vascular bundles of roots and leaf petioles (Fig. 2). Disease incidence levels as high as 60% have been observed in fields heavily infested with V. dahliae (J. E. Woodward, unpublished data). The integrity of peanut plants diminishes as the disease progresses and plants may die (Fig. 3). The weakening of vines often requires that peanut be dug prematurely in order to preserve yield. Fig. 1. Foliar symptoms of Verticillium wilt of peanut.
A B Fig. 2. Discoloration of peanut stems (A) and petioles (B) from Verticillium wilt infected peanut plants. Fig. 3. Large areas of peanut plants exhibiting symptoms of Verticillium wilt. Cool temperatures and high soil moisture favor fungal growth and development. Bell and Presley (1) found that temperatures between 22 and 27 C were optimal for growth and sporulation of V. dahliae. Furthermore, high levels of irrigation or rainfall are thought to decrease soil temperatures, thus increasing severity of the disease. The use of partially resistant cultivars is recommended to manage Verticillium wilt in cotton. Preliminary reports indicate that none of the commercially-available peanut cultivars used on the High Plains are resistant to the disease (14). Crop rotation is an ineffective option because of the viability of microsclerotia and the limited non-host rotation options for this region. The majority of rotation crops traditionally grown on the High Plains, such as cotton, chili pepper, and watermelon are susceptible to infection by V. dahliae. Furthermore, no fungicides are labeled for use against Verticillium wilt unlike most other diseases that affect peanut. Soil fumigants, such as chloropicrin and metam sodium, are used to manage Verticillium wilt in potato (2,8). These products are also used to manage Verticillium wilt in other crops such as strawberry, tomato, and cauliflower. Little information exists regarding the use of these fumigants against Verticillium wilt in peanut. The injection of metam sodium is recommended in Virginia (7) and Georgia (5) for the management of Cylindrocladium black rot. Krikun and Frank (6) found that metam sodium decreased soil populations of V. dahliae and reduced disease severity when applied through sprinkler irrigation compared to non-treated controls in Israel; however, the impact on yield was not reported. The lack of reliable Verticillium wilt management options warrants evaluation of tactics that may negatively impact V. dahliae and/or reduce damage caused by Verticillium wilt. The objectives of this study were to evaluate the impact of chloropicrin and metam sodium on soil populations of V. dahliae and to determine the effects of these fumigants on disease development, and peanut yield and grade. Description of Fields Used to Evaluate Fumigants Chloropicrin trials. Two field experiments were conducted in Gaines Co. Both fields had a history of severe Verticillium wilt and Sclerotinia blight. The soil type at these locations was a thick-surfaced, Brownfield, fine sand with a
slope of 1%. Chili pepper had been planted in 2007 and cotton the previous two years. Rye (Secale cereal) was planted as a cover crop in November of 2007 and was terminated with 2.24 kg ai/ha of glyphosate (Roundup WeatherMax, Monsanto Co., St. Louis, MO) 10 days prior to planting. Peanut cv. FlavorRunner 458 (19.7 seed/m of row) was strip-tilled into the terminated cover crop on 28 April 2008 in both fields. Plots were 4-rows wide planted 91.4 cm apart and 15.2 m in length. Fluazinam (Omega 500F, Syngenta Crop Protection, Greensboro, NC) at 0.625 kg a.i./ha was applied 75 and 105 days after planting for management of Sclerotinia blight. Metam sodium trials. Two field experiments were established in Gaines and Terry Counties during the 2008 growing season. The soil type at Gaines Co. was Patricia fine sand with a 1% slope. The field had been planted to cotton the previous three years. The field was prepared using a mold board plow. Peanut cv. FlavorRunner 458 (19.7 seed/m of row) was conventionally planted on 5 May 2008. Plots were 4-rows wide planted 101.6 cm apart and 15.2 m in length. The soil type at Terry Co. was Amarillo fine sandy loam with a 1-3% slope. The field had been planted to cotton in 2005 and 2006, and sorghum (Sorghum bicolor L.) in 2007. The field was prepared using a mold board plow, and seed of FlavorRunner 458 (16.4 seed/m of row) were planted on 7 May 2008. Plots were 4-rows wide planted 101.6 cm apart and 243.8 m in length. Pre-plant Applications of Fumigants Chloropicrin (Pic-Plus, Hendrix and Dail Inc., Greenville, NC) was applied at rates of 77 and 154 kg/ha into the rye cover crop on 5 April 2008 by a commercial applicator. The chemical was injected 20 cm deep using a 91.4 cm diameter coulter and trailing chisel shank on a tractor mounted applicator (Fig. 4). Metam sodium (Vapam HL, AMVAC Chemical Inc., Los Angeles, CA) was applied at rates of 25, 36, and 72 kg a.i./ha in Gaines Co., and 36 and 72 kg a.i./ha in Terry Co. The fumigant was applied in 187 liter of water per hectare by injection to a depth of 25 cm using shanks spaced 91.4 cm apart (Fig. 5). Nontreated plots were included in each trial for comparison. All applications were made 2 to 3 weeks before planting at each location. To seal the soil surface and reduce volatilization, plots were irrigated with approximately 1.3 cm of water immediately after application. Fig. 4. Injection of chloropicrin into a Secale cereal cover in a field in Gaines Co., TX, infested with Verticillium dahliae. Fig. 5. Injection of metam sodium in a field in Gaines Co., TX, infested with Verticillium dahliae. Experimental Design, Data Collection, and Analysis Treatments were arranged in a randomized complete block design with six replications, except the Terry Co. trial which had three replications. Populations of V. dahliae were enumerated 7 days prior to and 21 days after fumigants were applied. Composite soil samples (10 cores taken to a depth of 20.3 to 40.6 cm) were collected from each plot. Samples were placed on the lab bench and airdried for 1 week. Dried soil samples were assayed for microsclerotia of V. dahliae by dilution plating. In the assay, 20 cm³ soil were added to 80 ml of water, stirred vigorously and five, 1-ml aliquots added to semi-selective media (13). This process was repeated once for each sample with an additional 20 cm³ soil in 80 ml water for a total of 10 Petri dishes with 1 ml aliquots per sample. Soil was rinsed off the Petri dishes after 14 days and germinating microsclerotia were counted using a dissecting microscope.
Disease incidence was assessed starting in mid-august by visually estimating the percentage of plants exhibiting symptoms of Verticillium wilt. Plots were dug and inverted based on pod maturity and allowed to cure in windrows prior to harvest. Yields were determined by weighing harvested pods and peanut quality [% sound mature kernels + sound splits (SMK+SS)] was determined by grading sub-samples of pods and kernels from each plot (12). Data were analyzed using the ANOVA procedure (SAS v.9.1; SAS Institute Inc., Cary, NC), and means were separated using Fisher s protected LSD (P 0.05). Effects of Fumigants on V. dahliae Microsclerotia and Verticillium Wilt Chloropicrin trials. Differences in pre-fumigation microsclerotial densities were observed between the two trials (data not shown); however, the lack of a treatment by trial interaction allowed for pooling of data. Prefumigation densities of microsclerotia were similar among plots (Table 1), and were not reduced significantly by application of chloropicrin. Significant differences in incidence of Verticillium wilt were observed between treatments with and without chloropicrin (Table 1). Disease incidence was highest for the non-treated control at 54.4%, and 47.8% and 45.2% for the low and high rates of chloropicrin, respectively. Pod yields for the non-treated control (5262 kg/ha) did not differ when compared to plots treated with the low and high rates of chloropicrin, and the application of chloropicrin had no affect on % SMK+SS (Table 1), or other grade factors (data not shown). Table 1. Effect of chloropicrin on microsclerotial densities of Verticillium dahliae in soil, incidence of Verticillium wilt, and peanut yield and grade. v Verticillium dahliae (microsclerotia/cm³ soil) w Grade Treatment Prefumigation Postfumigation Verticillium wilt x (%) Yield (kg/ha) (%) y Non-treated control 14.2 13.5 54.4 a 5262 71.5 77 kg a.i./ha 9.4 13.5 47.8 b 5672 71.2 154 kg a.i./ha 12.6 10.0 45.2 b 5080 70.9 LSD (P 0.05) ns z ns 4.3 ns ns v Data are the means of two trials with 12 replications of each treatment. Values within a column followed by the same letter are not significantly different according to Fisher s protected LSD (P 0.05). w Densities were determined using a semi-selective medium 7 days before and 21 days after the application of the fumigant. x Values represent the percentage of plants/plot with symptoms of Verticillium wilt. y Grade percentages report levels of sound mature kernels plus sound splits in 500 g samples of whole pods from each plot. z ns = no significant differences. Metam sodium trials. No differences in initial populations of V. dahliae were observed between treatments in either of the two trials (data not shown); therefore, data were averaged across treatments. Baseline populations averaged 18 and 42 microsclerotia/cm³ soil for Gaines and Terry Co. Post-fumigation densities of microsclerotia were similar to baseline densities in the non-treated controls. The application of metam sodium reduced microsclerotial densities compared to the non-treated controls; however, increasing the rate of metam sodium did not further decrease microsclerotial densities. Disease incidence averaged 61.3% in Gaines Co. and 31.7% in Terry Co.; however, no treatment differences were observed (Table 2). Yields were similar for all treatments, ranging from 4656 to 5103 kg/ha and 4467 to 4654 kg/ha for each respective location (Table 2). No differences in grade were observed with increasing rates of metam sodium (Table 2).
Table 2. Effect of rates of metam sodium on microsclerotial densities of Verticillium dahliae in soil, incidence of Verticillium wilt, and peanut yield and grade. v Verticillium dahliae (microsclerotia per cm³ soil) w Trial and treatment Gaines Co. Gaines Co. Non-treated control Prefumigation Postfumigation Verticillium wilt x (%) Yield (kg/ha) Grade (%) y 20.2 12.5 a 67.7 5103 73.2 25 kg a.i./ha 21.2 5.5 b 63.7 4691 72.8 36 kg a.i./ha 17.3 5.0 b 63.3 4656 73.0 72 kg a.i./ha 14.5 4.3 b 57.7 5080 72.9 LSD (P 0.05) ns z 5.2 ns ns ns Non-treated control 37.5 30.2 a 33.8 4567 69.6 36 kg a.i./ha 45.0 16.0 b 31.5 4654 70.1 72 kg a.i./ha 43.5 13.1 b 29.8 4467 69.9 LSD (P 0.05) ns z 6.3 ns ns ns v Data are the means of six (Gaines Co.) and three (Terry Co.) replications of treatments. Values within a column followed by the same letter are not significantly different according to Fisher s protected LSD (P 0.05). w Densities were determined using a semi-selective medium 7 days before and 21 days after the application of the fumigant. x Values within a column represents the percentage of plants exhibiting symptoms of Verticillium wilt. y Grade percentages report levels of sound mature kernels plus sound splits in 500 g samples of whole pods from each plot. z ns = no significant differences. Implications of Fumigation and Verticillium Wilt in Peanut Numerous studies report on the efficacy of combinations of chloropicrin and methyl bromide in the management of Verticillium wilt; however, chloropicrin primarily served as a worker safety indicator for toxic fumes because methyl bromide alone is odorless (3). Henis and Bar (4) found that microsclerotia of V. dahliae are sensitive to chloropicrin and it has been evaluated as an alternative to methyl bromide in other crops such as potato, tomato, strawberry, and cauliflower. Results from this study indicate that chloropicrin was ineffective at reducing populations of V. dahliae in soil at the rates evaluated. Although, the use of chloropicrin reduced Verticillium wilt incidence compared to the nontreated control, the level of suppression achieved was insufficient. Metam sodium provides a wide spectrum of activity towards soilborne pests. In peanut production, fumigation with metam sodium is used primarily for management of CBR (5,7). Due to the increasing importance of Verticillium wilt on the Southern High Plains of Texas, all potential management strategies including fumigation need to be investigated. Results from this study indicate that metam sodium possesses biological activity against V. dahliae which is consistent with previous reports from Israel (6). Fumigation with metam sodium is highly effective at reducing the number of viable microsclerotia and disease incidence, while increasing yields in potato fields with a history of potato production and/or Verticillium wilt (8). Use rates in our studies ranged from 25 to 72 kg a.i./ha, which are similar to rates used for management of Cylindrocladium black rot of peanut (5,7) but well below the application rates of 135 to 270 kg a.i./ha recommended for soil injection in potato. An increase in use rate did not improve the level of activity against V. dahliae in our studies and higher rates would be cost prohibitive. Although metam sodium suppressed soil populations of V. dahliae, usage apparently did not reduce microsclerotial densities below an economic threshold. Metam
sodium may be more effective when applied in a preventative manner to fields with lower inoculum densities; however, additional studies are needed for justification. Several factors including soil type, temperature, physical properties, ph, and water holding capacity are known to impact the efficacy of metam sodium (10). Soil temperatures below 10 C will disrupt the generation and dissipation of methyl isothiocyanate (7). Soil temperatures at the time of fumigation were well above these levels (16.4 ± 1.8 C) and adequate soil moisture was present (data not shown). The application of irrigation immediately after fumigation sealed the beds, thus reducing volatilization; however, the low carrier volume used in the injection of metam sodium coupled with dry windy conditions may have adversely affected the distribution of fumigant. Furthermore, additional information regarding the relationship between microsclerotial density of V. dahliae in soil and disease development are warranted. Acknowledgments This research was supported in part by a grant from the Texas Peanut Producers Board. We thank AMVAC and Hendricks and Dail for providing the fumigants. The technical assistance of Max Batla, Mitchell Ratliff, Ira Yates, and Michael Petty is greatly appreciated. We would also like to thank grower cooperators Monty Henson, Gary Jackson, and Chuck Rowland for allowing us to conduct these trials on their farms. Literature Cited 1. Bell, A. A., and Presley, J. T. 1969. Temperature effects upon resistance and phytoalexin synthesis in cotton inoculated with Verticillium albo-atrum. Phytopathology 59:1141-1146. 2. Ben-Yephet, Y., Siti, E., and Frank, Z. 1983. Control of Verticillium dahliae by metam-sodium in loessial soil and effect on potato tuber yields. Plant Dis. 67:1223-1225. 3. Duniway, J. M. 2002. Status of chemical alternatives to methyl bromide for preplant fumigation of soil. Phytopathology 92:1337-1343. 4. Henis, Y., and Bar, J. 1973. Sensitivity to chloropicrin of microsclerotia of Verticillium dahliae as related to their germination on growth media and in soil. Phytoparasitica 1:95-99. 5. Kemerait, R. C., Brenneman, T. B., and Culbreath, A. K. 2010. Peanut disease control. Page 9 in: 2010 Georgia Pest Management Handbook, Commercial Edn. P. Guillebeau, ed. Univ. of Georgia Coop. Ext. Ser., Athens, GA. 6. Krikun, J., and Frankl, Z. R. 1982. Metham sodium applied by sprinkler irrigation to control pod rot and Verticillium wilt of peanut. Plant Dis. 66:128-130. 7. Phipps, P. M. 1990. Control of Cylindrocladium black rot of peanut with soil fumigants having methyl isothiocyanate as the active ingredient. Plant Dis. 74:438-441. 8. Powelson, R. L., and Carter, G. E. 1973. Efficacy of soil fumigants for control of Verticillium wilt of potatoes. Am. Potato J. 50:162-167. 9. Pullman, G. S., and DeVay, J. E. 1982. Epidemiology of Verticillium wilt of cotton: A relationship between propagules density and disease progression. Phytopathology 72:549-554. 10. Saeed, I. A. M., Rouse, D. I., Harkin, J. M., and Smith, K. P. 1997. Effects of soil water content and soil temperature on efficacy of metham-sodium against Verticillium dahliae. Plant Dis. 81:773-776. 11. Smith, T. E. 1960. Occurrence of Verticillium wilt of peanuts. Plant Dis. Rept. 44:435. 12. USDA-AMS. 2003. Farmers Stock Peanuts Inspection Instructions. Fruit and Vegetable Div., Fresh Products Branch, Washington, DC. 13. Wheeler, T. A., and Rowe, R. C. 1995. Influence of soil characteristics and assay techniques on quantification of Verticillium dahliae in Ohio soils. Plant Dis. 79:29-34. 14. Woodward, J. E., Batla, M. A., Wheeler, T. A., and Baughman, T. A. 2008. Response of runner-type peanut cultivars to Verticillium wilt. Proc. Am. Peanut Res. Educ. Soc. 40:73.