© 2010 Plant Management Network.
Etiology of Peanut Pod Rot in Nicaragua: II.
Joao Augusto, Former Graduate Research Assistant, Timothy B. Brenneman, Professor, and Alexander S. Csinos, Professor, Department of Plant Pathology, University of Georgia Coastal Plain Experiment Station, Tifton, GA 31793
Augusto, J., Brenneman, T. B., and Csinos, A. S. 2010. Etiology of peanut pod rot in Nicaragua: II. The role of Pythium myriotylum as defined by applications of gypsum and fungicides. Online. Plant Health Progress doi:10.1094/PHP-2010-0215-02-RS.
Peanut (Arachis hypogaea L.) is monocultured in western Nicaragua on loamy-sand soils, and a pod rot of unknown etiology can greatly reduce crop yield. Pythium myriotylum was frequently isolated from symptomatic pods in fields surveyed at Cosiguina, Leon, and Chinandega regions, although Rhizoctonia and Fusarium were also common. Applications of mefenoxam (0.57 kg a.i./ha), azoxystrobin (0.34 kg a.i./ha), and gypsum (670 kg/ha) at beginning pod and 28 to 35 days later were evaluated in field trials to determine their effects on pod rot and yield. Mefenoxam consistently decreased pod rot incidence and increased yield when disease was severe at Cosiguina. In Leon and Chinandega, azoxystrobin increased yield in fields with little pod rot, apparently by controlling stem rot (Sclerotium rolfsii), but did not decrease pod rot incidence. Application of gypsum had no effect on pod rot incidence or yield, but sometimes increased calcium levels in shells. Pod mycoflora isolations and response to mefenoxam suggest P. myriotylum is the primary cause of peanut pod rot in Nicaragua, especially in Cosiguina, where pod rot incidence was high.
Peanut production in Nicaragua is concentrated in Cosiguina, Chinandega, Leon and, to a lesser extent, Managua. Soils in these regions are typically of volcanic origin (10) and are well adapted for peanut production. Peanuts are grown during the rainy season with very limited irrigated production during the dry season. Cosiguina receives the highest annual rainfall, especially from June to November when peanut is grown. Peanut pod rot is prevalent in Cosiguina and to a lesser extent in Chinandega and Leon. Symptoms included dark pericarp discoloration and moist seed decay (Fig. 1). Estimates of losses due to pod rot in Nicaragua have not been documented, but disease incidence and crop loss can be severe in some years. The etiology of this pod rot is unknown.
In other countries, pod rot can be caused by soilborne pathogens such as Pythium spp., Rhizoctonia solani, and Fusarium solani (6,7,8,15). A soil calcium imbalance can also play a significant role in the pod rot disease complex (4,9). Calcium is only available for direct uptake by pods from the soil solution and not from roots or by translocation from foliage (11). This mechanism is the basis for current recommendations to apply highly soluble calcium sources such as gypsum (23.3% calcium) during pod development.
The relationship between soil nutrient status and pod rot development in Nicaragua has not been explored, and there were questions of calcium availability in these volcanic soils exposed to frequent torrential rain. Initial studies (1) indicated that calcium deficiency may not be the primary cause of pod rot in Nicaragua, and that plant parasitic nematodes and other soilborne fungi such as R. solani and S. rolfsii were not involved. This study focused on the role of parasitic Pythium spp. and their interaction with calcium availability on pod rot incidence and peanut yield.
Isolation and Identification of Potential Pod Rot Pathogens
Ten fields in Cosiguina, four in Chinandega, and three in Leon, were surveyed in 2006 for pod rot fungi. In 2007, seven fields in Cosiguina, four in Chinandega, and three in Leon were also surveyed. The majority of fields were in the Cosiguina region where pod rot is more prevalent. The surveys were conducted at harvest between November and December by collecting freshly dug symptomatic and asymptomatic pods. Pods were considered to be rotted if they had light to dark pericarp discoloration and some degree of seed decay. All fields had been in continuous peanut cultivation for at least 5 years. At each field, approximately 30 pods with attached pegs were collected from arbitrarily selected sites. Samples were placed in plastic bags and transported to the laboratory in coolers for processing.
Pod samples were washed three times in running tap water to remove surface contaminants, and 1-cm sections were cut from the shells and pegs of rotted and asymptomatic tissues. These sections and one intact seed per pod were again washed in tap water, rinsed in sterile, distilled water and air-dried on paper towels before placement on PDA medium and incubating at 25 to 28°C for at least 48 h. Oomycetes were sub-cultured on V-8 medium (200 ml V-8 juice, 4.5 g CaCO3, 17 g Difco agar, and 800 ml distilled water) for further identification. Potential pathogens were identified, and the isolation frequency of each was recorded for each sample type, region and year.
The 2006 season was relatively dry in all regions, especially in Leon and Chinandega where total rainfall in the June to November growing season was 956 mm. Cosiguina had the highest rainfall both years averaging 2,616 mm. Pythium myriotylum and F. solani were isolated more frequently than other potential pathogens with P. myriotylum most commonly isolated from seed with wet decay. This was particularly true in 2007 with higher rainfall totals, and isolation frequencies of P. myriotylum from pegs, shells and seed were high in all regions (Table 1). Distinctive morphological characteristics of P. myriotylum included clusters of large appressoria and swollen sporangia (15) (Fig. 2). Samples with dry seed decay had more F. solani than other fungi (results not shown). Rhizoctonia solani was isolated from seed with wet and dry decay at similar frequencies (results not shown), and was more prevalent in rotted pods from Cosiguina than from the other regions in both years (Table 1). Other fungi isolated from symptomatic pegs, shells, and seed at lower frequencies included Sclerotium rolfsii, Rhizopus spp., Trichoderma spp., and Aspergillus spp. While these species were commonly isolated from asymptomatic samples, P. myriotylum was not. Results of isolation frequencies for R. solani and F. solani from rotted compared with asymptomatic pods were inconclusive (results not shown).
Table 1. Mean isolation frequencies of Pythium myriotylum, Rhizoctonia solani, Fusarium solani and other fungi from pegs, shells and seed of pods showing symptoms of pod rot in Nicaraguan peanut fields.x
x The fungal species were isolated from regular PDA medium after incubation at room temperature for at least 2 days.
y Mean isolation frequencies for P. m. (Pythium myriotylum), R. s. (Rhizoctonia solani), F. s. (Fusarium solani), and "others" (Sclerotium rolfsii, Rhizopus spp., Trichoderma spp. and Aspergillus spp.) from rotted pods.
z Means within a column with different lower-case letter(s) and means within a row with different upper-case letter(s) are significantly different according to LSD (α = 0.05).
Gypsum and Fungicide Applications
Field tests in Cosiguina, Chinandega, and Leon were conducted from 2005 to 2007 to evaluate the effectiveness of mefenoxam, azoxystrobin, and gypsum applications in reducing pod rot incidence. Mefenoxam has activity against Oomycete pathogens such as Pythium spp. (13,16), and is labeled for pod rot control on peanut. Azoxystrobin is also labeled for control of pod rot and has some activity on Pythium (16), but is used primarily to control pathogens such as S. rolfsii and R. solani. Soluble forms of calcium such as gypsum (calcium sulfate) have been shown to reduce the severity of pod rot (4,9) as discussed earlier.
Fields in Cosiguina, Leon, and Chinandega were selected based on having a history of pod rot. The fields were disk plowed to a depth of 20 to 25 cm, disk harrowed three times, and then planted to peanut cv. Georgia Green with a twin-row Cole planter at 23 to 26 seeds/m in June or early July. The experimental design was a split-plot and treatments were replicated five times at each location. Each plot consisted of double beds (two twin rows per bed), where the left bed was harvested for yield and the right bed for destructive sampling. Each plot was 10-m long and 3.6-m wide. Row spacing was 1.09 m between outside rows and 0.74 m between inside rows, and replications were separated by a 4-m fallow alley. Main plots were fungicide treatments and sub-plots were gypsum applications. Fungicide treatments were: mefenoxam at 0.57 kg a.i./ha; azoxystrobin at 0.34 kg a.i./ha; and nontreated control. The subplot treatments were: gypsum applied twice at a rate of 670 kg/ha; and no gypsum. Gypsum application was at beginning pod and beginning seed reproductive stages (2) by hand in a 50-cm band over rows without soil incorporation. Mefenoxam and azoxystrobin were applied at the same period with a CO2-pressurized beltpack sprayer set to apply 189 liter/ha at 214 kPa at 6.4 km/h using four 110 04 tips with 50-mesh screens per bed.
Standard production practices were uniformly applied to the field. All plots were sprayed every 2 weeks with chlorothalonil (1.26 kg a.i./ha) to control leaf spots and peanut rust. The herbicides pendimethalin at 0.45 to 0.82 kg a.i./ha and ammonium salt of imazapic at 23.3 g a.i./ha were applied for weed control. Carbofuran at 0.58 kg a.i./ha was incorporated at bedding to control soil insects, and plots were not fertilized or irrigated. Ten soil samples from each of four fields were taken from the 0- to 20-cm depth early in the season in plots not receiving gypsum for fertility analysis (12) at LAQUISA Laboratorios Quimicos, S.A. (Carretera Leon, Managua km 83, Apartado 154, Leon, Nicaragua). The extraction method was by ammonium acetate (5). Weather stations at Cosiguina, Chinandega, and Leon recorded daily air temperature and rainfall from 2005 to 2007.
The percent pod rot per plot was determined 3 to 4 weeks after the last application of mefenoxam and gypsum by removing all pods, including those left in the soil, from plants in 1-m sections of row and counting the number of healthy and rotted pods. The number of rotted pods was divided by total number of pods in a section and multiplied by 100 to estimate disease incidence. Three 1-m sections were sampled in each plot, and the average percentage of pod rot was recorded.
The left bed of each plot was mechanically dug and inverted with a KMC digger/inverter at 130 to 140 days after planting. Windrows were harvested with a two-row combine 5 to 10 days later. Final pod moisture after air-drying was approximately 8% (wt/wt), and peanut yield was determined after removing foreign material. Samples of peanut pods (50 to 100 pods per plot) were collected at digging and sent to the LAQUISA laboratory for analysis of calcium in shells and seed by Mehlich No. 3 extraction reagent (14).
The Statistical Analysis System PROC MIXED (SAS Institute Inc., Cary, NC) was used to analyze variation of pod rot, yield, and calcium in shells and seed in response to fungicide and gypsum applications. Data were tested for normality and homogeneity of variance prior to analysis with the mixed procedure of SAS. The interaction of year × region × treatment for pod rot, yield, and calcium in shells and seed was used to determine if data could be pooled across years and regions.
The region × fungicide × gypsum interaction was significant for pod rot (P = 0.044) and yield (P = 0.039) across years (Table 2). Results were therefore grouped by regions with high (Cosiguina) and low (Leon and Chinandega) yield and pod rot incidence. The Cosiguina region had high annual precipitation (2,616 mm) with soils that were moderately acidic and had slightly low potassium levels, medium calcium levels, and high magnesium levels (12) (Table 3). Comparatively, Leon, and Chinandega received less annual precipitation (1,471 mm) and had moderate pod rot incidence. Soils in these regions were slightly acidic with medium levels of potassium and calcium, and high magnesium levels.
Table 2. Significance of P values from Proc Mixed analysis of variance (SAS Institute Inc.) of peanut pod rot incidence, pod yield, and calcium content in shells and seed. Data were pooled across 2005 to 2007 seasons at Cosiguina, Leon, and Chinandega.x
x The interactions between year and other sources of variation were not significant (P ≤ 0.05) for pod rot, pod yield, and calcium content in shells and seed.
y ns = not significant.
Table 3. Field history, soil pH, and soil nutrients at field trial locations in Nicaragua.w
w Ten soil samples per field for analysis were arbitrarily taken early in season from nontreated plots in fields where experiments were conducted across Cosiguina, Leon, and Chinandega regions.
x All fields were monocropped with peanut for at least the 5 previous years.
y Avg. rain. = average total rainfall from June through November in 2005, 2006, and 2007.
z Pod rot was assessed in nontreated rows adjacent to plots in 12 field trials (six ratings per trial) in Cosiguina, Leon, and Chinandega.
Cosiguina region. Applications of gypsum sometimes increased calcium content in shells and total pods, but never in seed alone (Table 4). However, calcium content in shells and seed were lower than those from Leon/Chinandega regions (Tables 4 and 5) or reported from other locations (3). Fungicide treatment had no impact on the calcium content in seed.
Table 4. Effect of fungicide and gypsum applications on pod rot incidence, yield and calcium in shells and seed during 2005-2007 in Cosiguina.xy
y Numbers in the column followed by the same letter(s) are not significantly different according to the protected LSD (P ≤ 0.05).
y Mefenoxam and azoxystrobin were applied at beginning pod and beginning seed at 0.57 kg a.i./ha and 0.34 kg a.i./ha, respectively. Gypsum was applied at the same times at 670 kg/ha.
Table 5. Effect of fungicides and gypsum applications on pod rot incidence, stem rot, yield and calcium in shells and seed during 2005-2007 in Leon and Chinandega.xy
x Numbers in the column followed by different letter(s) are significantly different according to the protected LSD (P ≤ 0.05) mean separations.
y Mefenoxam and azoxystrobin were applied at beginning pod and beginning seed at 0.57 kg a.i./ha and 0.34 kg a.i./ha, respectively. Gypsum was applied at the same times at 670 kg/ha.
Applications of mefenoxam resulted in lower pod rot incidence than the nontreated control, but yield for mefenoxam was higher only when gypsum was not applied. Azoxystrobin did not significantly decrease pod rot incidence, but yield was increased although not significantly compared to the nontreated with or without gypsum. Plots receiving gypsum alone and nontreated control had similar pod rot incidence and yield (Table 4).
Leon and Chinandega regions. Calcium content in shells and total pods in gypsum-treated and nontreated plants was high in these regions compared to Cosiguina (Tables 4 and 5), but shell and seed calcium content were not increased with gypsum (Table 5). Pod rot incidence was significantly lower with applications of mefenoxam with or without supplemental gypsum (P = 0.043 and P = 0.041, respectively), but levels of pod rot were lower than in Cosiguina (Table 5). Mefenoxam application with and without gypsum did not increase pod yield. Azoxystrobin application did not decrease pod rot but increased pod yield, apparently by effective control of the relatively high levels of stem rot in these regions (Table 5). Applications of gypsum had no effect on pod rot incidence and yield.
The mycoflora found in symptomatic pods indicated that P. myriotylum had an important role in the etiology of pod rot in Nicaragua, especially in Cosiguina with most severe disease. This was supported by the reduction in pod rot incidence and increased yield obtained with mefenoxam, an Oomycete-specific fungicide known to control Pythium pod rot (16). The relative contributions of R. solani and F. solani, which have been associated with pod rot complex elsewhere, was less clear. Data showed a consistently higher isolation frequency of R. solani both years from the Cosiguina fields, which had higher pod rot incidence, than fields in the other two regions. However, there was no response to azoxystrobin or flutolanil (1) in these regions, and both fungicides have excellent activity on Rhizoctonia-incited diseases. Nevertheless, azoxystrobin was effective in controlling high levels of stem rot in Leon and Chinandega. Soil calcium levels were apparently adequate for normal pod development in all three regions since there was no effect on pod rot, yield, or seed calcium levels from added gypsum in these trials.
Funding for the research was provided in part by the Association of Peanut Growers of Nicaragua. The authors would like to thank Diego Jerez, Mario Hurtado, and Ramiro Saborio for their work in conducting these studies as well as the cooperating peanut growers.
1. Augusto, J., Brenneman, T. B., and Csinos, A. S. 2010. Etiology of peanut pod rot in Nicaragua: I. The effect of pod size, calcium, fungicide, and nematicide. Online. Plant Health Progress doi:10.1094/PHP-2010-0215-01-RS.
2. Boote, K. J. 1982. Growth stages of peanut (Arachis hypogaea L.). Peanut Sci. 9:35-40.
3. Burkhart, L., and Collins, E. R. 1941. Mineral nutrients in peanut plant growth. Soil Sci. Soc. Amer. Proc. 6:272-280.
4. Csinos, A. S., and Gaines, T. P. 1986. Peanut pod rot complex: A geocarposphere nutrient imbalance. Plant Dis. 70:525-529.
5. Doll, E. C., and Lucas, R. E. 1973. Testing soils for potassium, calcium and magnesium. Pages 133-151 in: Soil Testing and Plant Analysis. L. M. Walsh and J. D. Beaton, eds. Soil. Sci. Soc. Am., Madison, WI.
6. Frank, Z. R. 1972. Pythium myriotylum and Fusarium solani as cofactors in a pod rot complex of peanut. Phytopathology 62:1331-1334.
7. Garcia, R. 1974. Interactions of Pythium myriotylum with Fusarium solani, Rhizoctonia solani and other fungi, and with Meloidogyne arenaria in peanut pod rot and preemergence damping-off. Ph.D. diss. Univ. of Florida, Gainesville, FL.
8. Garren, K. H. 1970. Rhizoctonia solani versus Pythium myriotylum as pathogens of peanut pod breakdown. Plant Dis. Reptr. 54:542-543.
9. Hallock, D. L., and Garren, K. H. 1968. Pod breakdown, yield and grade of Virginia type peanuts as affected by Ca, Mg, and K sulfates. Agron. J. 60:253-257.
10. Joergensen, R. G., and Castillo, X. 2001. Interrelationships between microbial and soil properties in young volcanic ash soils of Nicaragua. Soil Biol. Biochem. 33:1581-1589.
11. Kvien, C. S., Branch, W. D., Summer, M. E., and Csinos, A. S. 1988. Pod characteristics influencing calcium concentrations in the seed and hull of peanut. Crop Sci. 28:666-671.
12. Marx, E. S., Hart, J., and Stevens, R. G. 1996. Soil test interpretation guide. OSU Ext. Serv. EC 1478, Oregon State Univ., Corvallis. OR.
13. Sukul, P., and Spiteller, M. 2000. Metalaxyl: Persistence, degradation, metabolism, and analytical methods. Rev. Environ. Contam. Toxicol. 164:1-26.
14. Tucker, M. R. 1992. Determination of potassium, calcium, magnesium, and sodium by Mehlich 3 extraction. Pages 9-12 in: Reference Soil and Media Diagnostic Procedures for the Southern Region of the United States. S. J. Donohue, ed. Southern Cooperative Series Bull. No. 374. Virginia Agric. Exp. Station, Blacksburg, VA.
15. Van der Plaats-Niterink, A. J. 1981. Studies in Mycology: Monograph of the genus Pythium. Centraalbureau voor Schimmelcultures, Inst. of the Royal Netherlands, The Netherlands.
16. Wheeler, T. A., Howell, C. R., Cotton, J., and Porter, D. 2005. Pythium species associated with pod rot on West Texas peanuts and in vitro sensitivity of isolates to mefenoxam and azoxystrobin. Peanut Sci. 32:9-13.