© 2012 Plant Management Network.
Peanut Genotype and Seeding Rate Effects on Spotted Wilt
A. K. Culbreath, Department of Plant Pathology, W. D. Branch, J. P. Beasley Jr., and R. S. Tubbs, Department of Crop and Soil Sciences, University of Georgia, Coastal Plain Experiment Station, 2360 Rainwater Road, Tifton, GA 31793; and C. C. Holbrook, USDA-ARS Crop Genetics and Breeding, Coastal Plain Experiment Station, Tifton, GA 31793
Culbreath, A. K., W. D. Branch, J. P. Beasley Jr., R. S. Tubbs, and C. C. Holbrook. 2011. Peanut genotype and seeding rate effects on spotted wilt. Online. Plant Health Progress doi:10.1094/PHP-2012-0227-03-RS.
Establishing high plant populations helps suppress epidemics of spotted wilt, caused by Tomato spotted wilt virus (TSWV), in peanut (Arachis hypogaea L.). However, seed costs make it desirable to minimize seeding rates used. To determine whether new resistant genotypes can allow use of reduced seeding rates, field experiments were conducted at Tifton, GA, in 2008-2009 in which seven genotypes were combined factorially with two seeding rates, 9.8 and 19.7 seed/m of row. Genotypes included three cultivars (Georgia Green, Georgia-01R, and Georgia-02C) and four advanced breeding lines (GA 052524, GA 052527, GA 052529, and C724-19-25). Across years and genotypes, final incidences of spotted wilt and standardized areas under the spotted wilt disease progress curves were lower, and yields were higher in all other cultivars and breeding lines than in the moderately TSWV-resistant cultivar Georgia Green. Final incidence of spotted wilt was lower in GA 052527 and GA 052529 than in any of the cultivar standards, and yields of all four breeding lines were higher than for any of the three cultivars. Across genotypes, final incidence of spotted wilt and standardized areas under the spotted wilt disease progress curves were lower and yield was higher for the 19.8 seed/m treatment than the 9.8 seed/m.
Plant populations or density of plants within the row is one factor that can have a large impact on spotted wilt, caused by Tomato spotted wilt virus (TSWV), in peanut (Arachis hypogaea L.), especially in susceptible cultivars or cultivars with low to moderate levels of field resistance to TSWV (10,15). Establishment of in-row populations of 13 or more plants/m of row has been recommended for management of spotted wilt in peanut in the southeastern United States, based largely on response on the moderately resistant cultivar Georgia Green (3,4,10). Branch et al. (4) evaluated the spotted wilt and yield response to seeding rates of 9.8, 16.4, and 23.0 seed/m of row in several peanut cultivars, and reported substantial reductions in incidence of spotted wilt and increases in yield in certain cultivars. However, not all of the cultivars in that study performed the same, due to seeding rate interaction with genotype. In recent years, several cultivars have been released with higher levels of resistance than that in Georgia Green (8,15,22). Cultivars with improved resistance may allow more flexibility in spotted wilt management programs, including use of lower seeding rates than needed for moderately resistant cultivars (7,22). Although prices for peanut seed vary, seed represent a major component of production costs. The price of seed for the 2011 season was approximately $2.00/kg (20). For a standard seeding rate of 19.7 seed/m, seed costs for prevalent cultivars would be approximately $273/ha. Characterization of how more highly resistant cultivars and advanced breeding lines perform relative to the standard cultivar Georgia Green across a range of seeding rates should help determine the optimum rates for minimizing production costs without increasing the risk of losses to spotted wilt.
One objective of this work was to compare the effects of seeding rate and the resultant plant population on spotted wilt in Georgia Green and new advanced breeding lines. An equally important objective was to determine if field resistance in these lines is sufficient to allow use of reduced seeding rates compared to the standard rate of approximately 20 seed/m of row typically used for Georgia Green.
Effect of Genotype and Seeding Density on Spotted Wilt and Yield
Field experiments were conducted at the University of Georgia, Coastal Plain Experiment Station, Lang-Rigdon Farm, Tifton, GA, in 2008 and 2009. Soil type in both fields was Tifton loamy sand. Fields were planted to cotton (Gossypium hirsutum L.) the preceding year and to peanut two years prior. Both preceding cotton and peanut crops were grown with conventional tillage. Disease history of fields included severe epidemics of spotted wilt in previous peanut crops.
Experimental design was randomized complete block with four replications. Fourteen treatments consisted of seven genotypes, C724-19-25 , Georgia-01R (1), Georgia-02C (2), GA 052524, GA 052527, GA 052529, and Georgia Green (3), in factorial combination with two seeding rates, 9.8, and 19.7 seed/m of row. Genotypes GA 052524, GA 052527, and GA 052529 are sister lines developed from a cross between Georgia-02C and Georgia-01R. Georgia-01R (1,6,8) and Georgia-02C (2,6) have better field resistance to TSWV than the long-time standard Georgia Green. GA 052529 has since been released as the cultivar Georgia-10T (5). The genotype C724-19-25 was developed from a cross between C-99R, a cultivar with moderate field resistance to TSWV (12,23) with COAN, a cultivar with near immunity to the peanut root-knot nematode (14,19), but that is very susceptible to TSWV (14). Plots were 11.6 m long × 0.9 m wide in 2008, and 9.8 m long × 0.9 m wide in 2009, separated by 2.4-m alleys. Peanut seed were planted in 91-cm-spaced single rows on 24 April 2008 and 27 April 2009. To determine plant populations, stand counts (number of plants/m of row) were made in each plot 14 days after planting (DAP) in 2008 and 15 DAP in 2009.
Acephate (Orthene 75W, Valent U.S.A. Corporation, Walnut Creek, CA) at 0.84 kg ai/ ha was applied 12 DAP in both years for early season control of tobacco thrips (Frankliniella fusca Hinds). Minimization of damage caused by thrips aids in early season evaluation of spotted wilt symptoms, and foliar insecticides have minimal effect on epidemics of spotted wilt (21). Calcium sulfate was applied to all plots as gypsum, 1,120 kg/ha, on 8 July 2008, and 30 June 2009.
Disease assessment. Spotted wilt was evaluated in all plots on 69, 105, and 139 DAP in 2008 and on 70, 91, 107, and 126 DAP in 2009. Spotted wilt incidence was determined by counting the number of 0.3-m portions of row containing severely stunted (Fig. 1), chlorotic (Fig. 1), wilted, or dead plants for each plot and converting that number to a percentage of total row length (11). Area under the disease progress curve was computed from spotted wilt incidence as described by Shaner and Finney (18). Since the number of evaluations and number of days during the evaluation period differed for the two years, standardized area under the disease progress curve (SAUDPC) was calculated by dividing the AUDPC by the time in days between the first and last evaluation dates (17).
Pod yields. Rainy weather delayed digging of Georgia Green and C724-19-25 in 2008, so all genotypes were dug and inverted 148 DAP in 2008. In 2009, plots of Georgia Green and C724-19-25 were dug and inverted 135 DAP, and all other genotypes were dug 147 DAP. Peanuts were harvested mechanically 5 to 8 days after inverting, and pod yields were determined by weighing harvested pods after they were dried and adjusted to 10% (wt/wt) moisture.
Statistical analysis. Data were analyzed across years using PROC MIXED with ddfm = satterth option on the model statement (SAS v.8.3, SAS Institute Inc., Cary, NC). Years and replications nested within years were considered random effects, and seeding rate and genotype were considered fixed effects. Main effects and interactions were considered significant when P ≤ 0.05. Fisher’s LSD values were computed using standard error and t-values of adjusted degrees of freedom. If seeding rate × genotype effects were significant, comparisons for one factor were made within individual levels of the other factor. Otherwise, comparisons of factor main effects were made across levels of the other factor.
Genotype and Seeding Rate Effects
Plant populations. There was no significant year × treatment effect on plant populations (plants/m of row), so data for the two years were pooled for analysis and presentation. Genotype, seeding rate, and genotype × seeding rate effects were significant for plant population. Therefore, comparisons of those two factors were made within individual levels of the other factor. Within the 9.8 seed/m seeding rate, populations ranked highest in Georgia-02C, but did not differ from GA 052527, GA 052529, or Georgia Green (Table 1). At the 19.7 seed/m seeding rate, plant population was highest for Georgia-02C and lowest for C724-19-25. Populations did not differ among the other five genotypes which were intermediate. Plant populations were higher for the 19.7 seed/m seeding rate than for the 9.8 seed/m treatment for all genotypes.
Table 1. Effect of peanut genotype and seeding rate on plant population, final incidence of spotted wilt , standardized area under the disease progress curve (SAUDPC) and pod yield, 2008-2009.
x There was a significant genotype × seeding density interaction. Therefore, comparisons of each factor were made within each level of the other factor. An asterisk indicates a significant difference (LSD = 1.2, P ≤ 0.05) between seeding rates within a genotype
y There was no significant year genotype, year × seeding rate, or year × genotype × seeding rate interaction. Therefore means presented are from analysis pooled across years. An asterisk indicates a significant (P ≤ 0.05) difference between seeding rates across genotypes based on PROC MIXED Analysis (SAS v.8.3, SAS Institute Inc., Cary, NC).
Spotted wilt. Disease progress of spotted wilt for Georgia Green, Georgia-01R, Georgia-02C, and GA 052529 for 2008 and 2009 is shown in Figure 2. There was little or no increase in incidence of spotted wilt in GA 052529 after the initial evaluation in either year.
There was no significant year × treatment interaction effect for either final
incidence of spotted wilt or SAUDPC values for the spotted wilt epidemics.
Therefore, data are presented using means from analysis across both years. For
final incidence of TSW, genotype and seeding rate were significant, but seeding
rate × genotype interaction effects were not. Final incidence of spotted wilt
was highest in Georgia Green and ranked lowest in GA 052524, GA 052527, and GA
052529. Final incidence of spotted wilt in GA 052527 and GA 052529 was lower
than in all other genotypes except GA 052524. Final incidence for Georgia Green
planted at the high seeding rate was higher than that of any of the other
genotypes planted at the low seeding rate (Table 1). Across genotypes, final
incidence of spotted wilt was lower for the 19.7 seed/m treatment than for the
9.8 seed/m treatment (Table 1). Cultivar and seeding rate effects were
significant for SAUDPC, but cultivar × seeding rate effects were not. Across
seeding rates, SAUDPC values for spotted wilt were highest in Georgia Green
(Table 1). SAUDPC values were similar for Georgia-02C, GA 052524, GA 052527, and
Yield. There was no significant year × treatment interaction or genotype × seeding rate effects for yield. Across seeding rates, yields were highest in GA 052529, GA 052524, C724-19-25, and GA 052527 (Table 1). Yields of all genotypes were higher than those of Georgia Green. Across genotypes, yields were higher for the 19.7 seed/m treatment than for the 9.8 seed/m treatment (Table 1).
Implications for Management of Spotted Wilt
Based on current spotted wilt risk index criteria, plant stands obtained with the 9.8 and 19.6 seed/m treatments represent high and low risk categories, respectively (15). Results from all genotypes evaluated in this study corroborate the relative effects of seeding rate and subsequent plant population on incidence of spotted wilt. However, final incidence of spotted wilt and SAUDPC values of Georgia-01R, Georgia-02C, and all four breeding lines planted at 9.8 seed/m of row were lower than those of Georgia Green planted at the 19.6 seed/m of row. Similarly, there was a consistent trend of higher yields with higher seeding density for all genotypes. However, the yields of the four breeding lines at the lower seeding density were greater than those of Georgia Green at the higher seeding density. Branch et al. (4) reported about 40% reduction in final incidence of spotted wilt and corresponding 30% increase in yield in Georgia Green with increasing seeding densities from 9.8 seed/m to 16.3 seed/m. In this study, increasing seeding rate in Georgia Green from 9.8 seed/m to 19.6 seed/m resulted in about 25% reduction in final incidence of spotted wilt and a 17% increase in yield. Branch et al. (4) reported reductions in spotted wilt and increases in yield for certain genotypes, but not equally among all cultivars evaluated. Likewise, significant differences for spotted wilt and yield were observed among genotypes in this study, with some genotypes responding to increasing seeding rates and others not as much. Tubbs et al. (22) reported reductions in incidence of spotted wilt across several cultivars with moderate and high levels of field resistance to TSWV with increasing seeding rates of 17, 20, and 23 seed/m of row in one of two years; however, incidence was low. In addition, seeding rate had no effect on yield in either year (22). Based on low incidence of spotted wilt and high yield observed in the breeding lines, especially GA 052529, with 9.8 seed/m rates, and little improvement in either disease control or yield with seeding rates of 19.6 seed/m, higher seeding rates likely would not provide substantial benefit, and could result in negative economic return, depending on seed cost. Before spotted wilt became a problem in Georgia, Kvien and Bergmark (16) reported yield increase in the cultivar Florunner with increasing plant populations from 5 to 20 plants/m in only one of four trials. In the current study, none of the mean plant populations were lower than 7.5 plants/m. GA 052529, released as cultivar Georgia-10T, may allow reduction in the number of seed required compared to the standard 19.8 seed/m typically recommended for Georgia Green. The minimum plant stand that can provide adequate suppression of spotted wilt and maintain high yield may be even lower than the average of 8.9 plants/m established in this study, but that minimum is still to be determined.
Corroborating a previous report (14), results from this study indicate C724-19-25 has much better field resistance to TSWV than Georgia Green. Although final incidence of spotted wilt was higher in C724-19-25 than the other breeding lines, yield of C724-19-25 was similar to yields in GA 052524, GA 052527, and GA 052529. Considering that plant populations of C724-15-25 were lower than for any other genotype across the two years, all of the breeding lines would have potential for management of spotted wilt at lower seeding rates, provided seed used have adequate rates of germination and vigor.
Planting date is another factor that can affect spotted wilt epidemics, with epidemics typically being more severe in peanut planted in April than in mid- to late-May (9,10). Culbreath et al. (9) reported that field resistance to TSWV in the cultivar AP-3 was sufficient to allow planting in late April without increasing losses to spotted wilt. Although planting date was not addressed as a factor in this study, 24 April and 27 April planting dates were utilized in effort to maximize pressure of spotted wilt epidemics. Low incidence of spotted wilt and high yields obtained with all four breeding lines evaluated represents circumstantial evidence that these lines have resistance levels sufficient to allow planting in late April without increasing risks of losses to spotted wilt.
Implications for Breeding for Field Resistance to Tomato Spotted Wilt Virus
Results from this study corroborated previous reports of field resistance to TSWV in Georgia-01R (1,6,8) and Georgia-02C (2,6) being better than that of Georgia Green. In this investigation, based on final incidence of spotted wilt, GA 052527 and GA 052529 have field resistance to TSWV and yield potential in fields with TSWV infestations better than either of their parents. Progeny lines can have better field resistance to spotted wilt than either parent. Georgia-02C has better field resistance to TSWV than either of its parents, Southern Runner and Georgia Runner. Holbrook et al. (14) reported lower incidence of TSWV in C724-19-25 used in this study, and its sister line C724-19-15, released as Tifguard (13), than in either of their parents, COAN or C-99R. However, published comparisons of both parental lines and progeny lines for reaction to TSWV have been rare. Results from this study provide additional documentation of multiple progeny lines with field resistance to TSWV better than that of either parent. This would indicate that additional progress may be possible in breeding for field resistance to TSWV, due to transgressive segregation, even if specific sources of higher levels of field resistance are not found.
The authors thank Michael Heath, Sam Holbrook, Ron Hooks, Jonathan Roberts and Matthew Wiggins for their field assistance. This research was supported in part by the Georgia Peanut Commission and the National Peanut Board.
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