Effect of Host Resistance to Fusarium virguliforme and Heterodera glycines on Sudden Death Syndrome Disease Severity and Soybean Yield

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1 Plant Health Research Effect of Host Resistance to Fusarium virguliforme and Heterodera glycines on Sudden Death Syndrome Disease Severity and Soybean Yield L. F. Brzostowski, W. T. Schapaugh, and P. A. Rzodkiewicz, Department of Agronomy, Kansas State University, Manhattan, KS 66506; T. C. Todd and C. R. Little, Department of Plant Pathology, Kansas State University, Manhattan, KS Accepted for publication 6 November Published 27 January ABSTRACT Brzostowski, L. F., Schapaugh, W. T., Rzodkiewicz, P. A., Todd, T. C., and Little, C. R Effect of host resistance to Fusarium virguiliforme and Heterodera glycines on sudden death syndrome disease severity and soybean yield. Plant Health Progress doi: /php-rs Fusarium virguliforme, the soilborne fungus that causes sudden death syndrome (SDS), and Heterodera glycines, the soybean cyst nematode (SCN), are economically important pathogens that often occur concomitantly in Kansas soybean fields. To examine F. virguliforme and H. glycines interactions across multiple environments, four soybean genotypes with different levels of resistance to SDS and SCN were planted at three to four locations in northeastern Kansas in 2008 and Pathogen population densities were quantified at planting (Pi), midseason (Pm), and harvest (Pf). At harvest, SDS AUDPC, F. virguliforme root population densities, H. glycines reproductive factors (RF), and yield were determined. The performance of resistant (R) genotypes varied with environment and disease pressure, but SDS-R genotypes were associated with 36% greater yields than SDSsusceptible (S) genotypes in high SDS environments. Even moderate levels of SCN resistance reduced SDS disease severity in SDS-S genotypes. Negative correlations (P 0.05) were observed between yield and AUDPC, and yield and F. virguliforme root population densities. A regression model that combined both of these covariates explained 57% of the yield variation. Disease severity was positively correlated with H. glycines Pi, but negatively correlated with RF. The data emphasize the importance of combining SDS and SCN host resistance in fields with a history of both diseases. INTRODUCTION Sudden death syndrome (SDS) is an economically important disease of soybean (Glycine max (L.) Merr.) that is widely distributed across soybean growing regions in the United States (26,27). Although losses between 5 and 15% are more common, this disease may cause 100% yield loss (28). From its earliest detection, SDS has been commonly associated with high soil moisture and cool conditions (28,34). Irrigated fields with high fertility and yield potential tend to exhibit the most severe symptoms. Sudden death syndrome is an emerging disease in Kansas and information on its impact under the state s unique soybean production environments is lacking. The causal agent of SDS is Fusarium virguliforme O'Donnell & T. Aoki and was called F. solani (Mart.) Sacc. f.sp. glycines in older literature (2,24). The fungus survives in soil and soybean residue. It infects soybean roots early in the growing season, and foliar symptoms commonly develop after flowering and during pod fill (24). Foliar symptoms include interveinal chlorosis and necrosis. In severe cases, premature defoliation, pod and seed abortion, and whole-plant death occur. Roots of infected plants tend to exhibit crown necrosis and lateral root rot with gray to red-brown discoloration of the xylem. Blue-green sporulation may be seen on the taproot and lower stem (24). Growing resistant genotypes provides one management tool to help control SDS (9,24). Several moderately resistant genotypes have been identified and been made available to producers; however, none of Corresponding author: Christopher Little. crlittle@ksu.edu the current resistance packages are complete. All of the currently evaluated genotypes will display some SDS symptoms if conditions are favorable (9). As early as 1983, Hirrel (10) noted the association of the soybean cyst nematode (SCN), Heterodera glycines Ichinohe with SDS, and numerous studies since have attempted to characterize the interaction of H. glycines and F. virguliforme. While the presence of H. glycines is not necessary for F. virguliforme infection, McLean and Lawrence (16) reported that SDS foliar symptoms appeared three to seven days earlier and were more severe in field microplots infested with both pathogens compared to those where only the fungus was present. Several studies have reported a positive correlation between soil population densities of H. glycines and SDS symptoms (25,31,35,39), while others have reported a non-significant correlation between these variables (7,9). Gao et al. (7) demonstrated that while both pathogens reduced plant growth, H. glycines did not increase foliar symptoms of SDS in greenhouse evaluations and statistical interactions between H. glycines and F. virguliforme were rarely significant. Inconsistencies in the observed relationships between these two pathogens highlight the need for continued research to elucidate their interaction. The objectives of the present study were: (i) to characterize the performance of soybean genotypes with and without resistance to SDS and SCN across multiple environments in Kansas; and (ii) to evaluate relationships between H. glycines and F. virguliforme population densities and the development of SDS symptoms in these environments. doi: / PHP-RS The American Phytopathological Society PLANT HEALTH PROGRESS Vol. 15, No. 1, 2014 Page 1

2 EXPERIMENTAL DESIGN In 2008, four soybean genotypes were planted at three locations with a history of SCN and SDS: Manhattan, Rossville, and Topeka, KS. In 2009, the four soybean genotypes were planted at four locations: Manhattan, Morganville, Rossville, and Topeka, KS. The genotypes used in 2008 and 2009 were selected to represent resistance and susceptibility to both diseases based on SDS and SCN (HG Type 7) ratings from the 2007 Kansas Soybean Performance Test. Three of the genotypes were commercial cultivars developed by private companies. These genotypes have been coded: COM1, COM2, and COM3. The fourth genotype, KS3406RR, was a public release. All genotypes were in maturity group three. Preliminary SDS ratings (1 = no disease present to 5 = premature death) were 1.0 and 1.7 for COM1 and COM2, respectively, and 4.3 and 4.7 for COM3 and KS3406RR, respectively. Preliminary SCN (HG Type 7) ratings (0 = no cysts present to 100 = number of cysts on susceptible checks) were 8, 22, 23, and 74 for COM1, COM2, COM3, and KS3406RR, respectively. The SCN-resistant genotypes, COM1, COM2, and COM3, possessed resistance derived from PI and were resistant to moderately resistant based upon the rating scale of Schmitt and Shannon (36). Additionally, all soybean genotypes used in this experiment were glyphosate resistant. Soybean genotypes were planted in a randomized complete block design with four replications and in eight 3.4-meter-length rows per plot using a four-row ALMACO plot planter (ALMACO, Nevada, IA). Row spacing was 76 cm and seeding rate was 30 seeds/m. Planting dates were 16 May 2008 for Rossville and Topeka, and 10 June 2008 for Manhattan. In 2009, Rossville and Topeka were planted on 8 May. Morganville 2009 was planted on 11 May, Manhattan on 13 May, and Ottawa on 19 May. Weeds were controlled with glyphosate applications and by manual weeding. All locations received supplemental irrigation. STATISTICAL ANALYSIS Prior to statistical analysis, nematode and fungal data were transformed to log 10 (x + 1) values to reduce heterogeneity of variances. Results are reported as back-transformed values. All data were subjected to two-way analysis of variance using SAS (SAS Institute Inc., Cary, NC) PROC MIXED to determine genotype, environment, and genotype environment interactions, with genotype and environment treated as fixed effects. Soybean genotype effects were further partitioned into an SDS resistance effect (resistant vs. susceptible) and a cultivar nested within SDS resistance effect. Cultivar within main effect (SDS resistance) means were separated using single degree of freedom contrasts (α 0.05). PROC CORR was used to generate Pearson s correlation coefficients to examine the relationships among SCN, SDS, and yield. CHARACTERIZATION OF H. GLYCINES VIRULENCE H. glycines populations from each location were maintained on a susceptible soybean cultivar under greenhouse conditions prior to classification. The genetic diversity present within and among populations was characterized using the HG Type test (19). The experimental design employed a randomized complete block with seven replications, and each HG Type test was repeated once. Experimental units consisted of 6.4-cm-diameter 25-cm-deep Deepots (Stuewe & Sons, Corvallis, OR) filled with 450 cm 3 of a 50:50 mix of pasteurized sandy loam soil and sand. Each experimental unit was inoculated with approximately 10,000 H. glycines eggs and planted to one of the following soybean indicator lines: PI (Peking), PI 88788, PI 90763, PI , PI , PI 89772, PI (Cloud), and the susceptible cv. Lee 74. HG Type tests were maintained for 35 days under an ambient air temperature of 27 C and watered daily. Females were removed from the roots via water spray and mechanical manipulation, collected on a 250-μm-diameter (60- mesh) sieve, and counted at 60 magnification using an Olympus SZX16 stereomicroscope. A female index (FI) was calculated for each soybean indicator line as follows: FI = (mean number of females on test indicator soybean line / mean number of females on the susceptible check cv. Lee 74) 100. The ability of each H. glycines population to reproduce on the PI derived soybean genotypes included in the present study was also quantified using the procedures described for the HG Type test. The experimental design was a randomized complete block with six replications. Female indices were determined for the following soybean genotypes: PI 88788, COM1, COM2, COM3, and KS3406RR as the standard susceptible check. HG type determinations for the H. glycines populations at the six field sites are reported in Table 1. Female indices (FI) exceeded 10% on PI (the source for SCN resistance in this study) in five of the six fields. The PI derived cultivars used in this study generally exhibited moderately resistant reactions (FI = 10 to 30%) to SCN populations at all field sites except Rossville, where all cultivars were susceptible (data not shown). Soybean genotypes were selected to encompass different combinations of resistance and susceptibility to SDS and SCN. Based on the HG Type test, the source of SCN resistance in this study, PI 88788, was resistant or moderately resistant to the SCN populations at all six evaluation sites. Although SCN resistance in the commercial soybean lines was derived from PI 88788, the HG Type test did not provide a clear indication of the level of the resistance in these lines. Rzodkiewicz (33) showed that commercial soybean lines with resistance derived from PI were more susceptible to SCN than PI itself. While the four soybean genotypes used in this study did differ in SCN Location Year TABLE 1 HG Type determinations for field populations of Heterodera glycines. Females on Lee 74 y (1) PI (2) PI (3) PI Female Index x (4) PI (5) PI (6) PI (7) PI HG type z Manhattan Rossville Topeka Manhattan Morganville Topeka x Female index (FI) = mean number of females on test indicator soybean line / mean number of females on the susceptible check cv. Lee 74) 100. y Susceptible check. z FI values > 10 result in assignment of the indicator line number for HG type. PLANT HEALTH PROGRESS Vol. 15, No. 1, 2014 Page 2

3 resistance, the level of resistance was not high, the level of resistance varied across environment, and SCN reproduction occurred on all of the soybean genotypes in this study, especially at the Rossville site. Nonetheless, the level of resistance in our test genotypes reflects the level of resistance in genotypes currently in production. The vast majority of the commercial cultivars in Kansas derive their resistance from PI and HG Type tests on H. glycines populations across the North Central region of the United States have shown that many SCN populations are capable of effectively reproducing on this source of resistance (5,20,33). QUANTIFICATION OF PATHOGEN POPULATION DENSITIES Soil samples for enumeration of H. glycines and F. virguliforme population densities were collected from each plot at planting, midseason (soybean growth stages R2 to R3), and harvest. Each sample consisted of eight 5-cm-diameter 15-cm-deep soil cores collected in a zigzag pattern from the center six rows of each plot and bulked together in a plastic soil sample bag. Soils were stored at 6 C until processing. Cysts were extracted from 100 cm 3 subsamples by wet sieving over a 150-μm-pore sieve. Cysts were mechanically ruptured to release eggs and second-stage juveniles (J2) as described in Niblack et al. (21). Eggs and J2 were counted at 40 magnification and reported as eggs and J2 per 100 cm 3 soil. A reproductive factor (RF) was calculated using the formula: RF = Pf/Pi, where Pf = number of eggs and juveniles per 100 cm 3 soil at harvest and Pi = number of eggs and juveniles per 100 cm 3 soil at planting. Soil population densities of H. glycines varied among the environments on all sampling dates (Table 2). The largest population densities were observed at Rossville in 2008, where numbers of eggs + J2 per 100 cm 3 soil averaged 386, 1005, and 1124 at planting (Pi), midseason (Pm), and harvest (Pf), respectively (Table 3). The smallest populations occurred at Manhattan in 2008, where Pi averaged 58 per 100 cm 3 soil, and at Morganville in 2009, where Pm and Pf averaged 213 and 74 eggs + J2 per 100 cm 3 soil, respectively. Initial population densities were representative of H. glycines-infested fields in Kansas, as reflected by a recent survey of 635 soybean fields, where 75% of 124 infested fields averaged < 200 eggs per 100 cm 3 soil (unpublished data). Seasonal population increase also was significantly affected by environment (Table 3). Reproductive factors (RF) were largest at Manhattan in 2008 and Rossville in 2009, and lowest at Manhattan, Morganville, and Topeka in 2009 (Table 2). A larger final H. glycines population density did not necessarily translate into a higher reproductive factor as this measure also took into account initial nematode population densities. Table 2 Analysis of variance summary for Heterodera glycines soil population densities, Fusarium virguliforme soil and root population densities, AUDPC, and soybean yield. Heterodera glycines F value Fusarium virguiliforme Source df Pi x Pm x Pf x RF y Pi Pm Pf Root AUDPC Yield Environment 6, ** 5.69** 6.49** 6.01** 5.03** 45.54** 10.37** 4.79** 33.87** 33.37** SDS reaction 1, ** ** ** Genotype z 2, ** * ** 8.42** Env SDS 6, * 2.73* 2.45* 2.56* ** 3.70** Env Gen z 12, * 3.13** 1.97* ** 1.24 * P 0.05; ** P x Pi, Pm, Pf = soil population density at planting, midseason, and harvest, respectively. y RF = reproductive factor. z Nested within SDA reaction. Table 3 Main effect means of Heterodera glycines soil population densities and reproductive factors. Eggs + J2/100 cm 3 soil Pi x Pm x Pf x RF y Environment Manhattan c z 362 bcd 559 abc 5.3 a Rossville a 1,005 a 1,124 a 3.1 bc Topeka ab 674 ab 838 ab 4.6 b Manhattan ab 276 cd 216 cd 1.7 cd Morganville ab 213 d 74 d 0.7 d Rossville bc 504 bc 662 ab 6.3 a Topeka a 426 bc 308 bc 2.1 bcd Genotype SDS resistant COM1 (R, R) w 182 a 518 a 274 b 2.5 a COM2 (R, R) 193 a 443 ab 531 a 3.1 a SDS susceptible COM3 (S, R) 138 a 350 b 289 b 2.7 a KS3406 (S, S) 205 a 460 ab 628 a 3.4 a w Letters indicate resistance (R) or susceptibility (S) to SDS and SCN, respectively. x Pi, Pm, Pf = soil population density at planting, midseason, and harvest, respectively. y RF = reproductive factor. Means are back-transformed from log 10 -transformed data. z Means within effect and column followed by the same letter are not significantly different (P 0.05). PLANT HEALTH PROGRESS Vol. 15, No. 1, 2014 Page 3

4 No significant differences in H. glycines soil population densities at planting or midseason were observed among soybean genotypes (Table 3). Averaged across environments at harvest, however, the SCN-susceptible cultivar KS3406RR and the moderately resistant cultivar COM2 were associated with significantly larger H. glycines soil population densities than the other two moderately resistant cultivars (Table 2). Additionally, a significant genotype environment interaction was observed for H. glycines population densities at harvest (Table 3). Nematode population densities at harvest varied significantly between SDSresistant and SDS-susceptible cultivars at Morganville and Topeka in 2009, but the pattern of nematode response was not consistent between the two environments (data not shown). The varied reactions of the soybean genotypes to the SCN populations resulted in the SCN population density at harvest being significantly lower for the SCN resistant soybean genotypes than the susceptible genotype KS3406RR in only two of the seven environments. Nematode reproduction (RF) did not differ among genotypes on average, and although there was a significant genotype environment interaction (Table 3), no consistent trends in nematode reproduction could be discerned (data not shown). Population densities of F. virguliforme were determined from 1-g soil subsamples. Two hundred fifty μl of a 1:100 homogenized soil:sdh 2 O dilution was spread over four plates of modified Nash-Snyder s medium (MNSM) (3,18) for the enumeration of F. virguliforme colony forming units (CFU). Since MNSM is a semi-selective medium for Fusarium spp., identification of F. virguliforme colonies was performed by visual comparison with the standard isolate, Mont-1 (received from R. Bowen, University of Illinois, Urbana-Champaign). Plates were incubated at room temperature for 10 to 14 days in order to confirm that isolation colonies matched the Mont-1 isolate. F. virguliforme on MNSM was reported as CFU per g soil. Prior to harvest, twenty taproot samples per plot were collected for quantification of F. virguliforme population densities (per g root tissue). Ten tap roots per plot were collected, washed, and surface disinfested with 10% household bleach (v/v; approx. 0.56% NaOCl) for 5 min, rinsed with distilled water, and air dried overnight. Root samples were ground using a UDY cyclone sample mill (UDY Corporation, Fort Collins, CO), passed through a 0.5-mm-pore screen, and 0.5 g of root tissue per sample was added to 10 ml of sterile distilled water. Plating, identification, and enumeration of root CFUs were conducted in the same manner described above for soil. Soil population densities of F. virguliforme varied significantly among environments at planting, midseason, and harvest (Table 3). The largest population densities were observed for Rossville in 2008, with colony forming units (CFU) per g soil averaging 993, 2077, and 3160 for Pi, Pm, and Pf, respectively (Table 4). Rossville in 2009 consistently had the lowest population densities, with numbers of CFU per g soil averaging 167, 5, and 251 for Pi, Pm, and Pf, respectively. Significant differences among environments were also observed for F. virguliforme root population densities (Table 3). Root population densities at Topeka in 2008 were significantly lower than all other environments (Table 4). Previous studies have shown that soil populations of F. virguliforme are typically present in the hundreds to thousands of CFUs per gram of soil where the disease is prevalent. Population estimates based on plate counts typically average 1000 to 2000 CFUs per gram for the top 15 cm of soil (29,30,32). Mbofung et al. (15) used TaqMan Real-Time PCR to detect F. virguliforme in three Iowa soybean fields and reported population densities ranging from approximately 300 to CFU per gram of soil. Significant genotype main effect and genotype environment interactions were observed for F. virguliforme soil population densities at planting (Table 3), but these were due to a single environment, Rossville in Here, the fungal population density for the soybean cultivar COM1 was significantly lower than that for the other cultivars (data not shown). Similar effects were observed for the midseason sampling, but these had disappeared by harvest. Inconsistent responses to SDS resistance were also observed for F. virguliforme population densities at midseason, with SDS-susceptible genotypes displaying lower and higher levels than SDS-resistant genotypes at Rossville and Topeka, respectively, in 2009 (data not shown). A significant genotype effect or genotype environment interaction was not observed for root population densities of F. virguliforme (Table 3). DISEASE DEVELOPMENT As SDS symptoms appeared, disease incidence and severity readings were taken every two weeks at soybean growth stages R5, R6, and R7 (6). Disease incidence (DI) was measured on each Table 4 Main effect means of Fusarium virguliforme soil and root population densities. CFU/g soil Pi y Pm y Pf y CFU/g root Environment Manhattan a z 1,404 ab 2,744 ab 3,225 ab Rossville a 2,077 a 3,160 a 1,559 b Topeka a 656 bc 1,084 c 155 c Manhattan a 652 bc 1,743 abc 11,670 a Morganville a 1,017 ab 1,661 abc 4,708 ab Rossville b 5 d 251 d 5,819 ab Topeka a 348 c 1,291 bc 3,776 ab Genotype SDS resistant COM1 (R, R) x 370 b 298 b 1,375 a 2,941 ab COM2 (R, R) 453 ab 574 a 1,470 a 1,596 b SDS susceptible COM3 (S, R) 641 a 585 ab 1,269 a 2,878 ab KS3406 (S, S) 639 a 512 a 1,355 a 3,718 a x Letters indicate resistance (R) or susceptibility (S) to SDS and SCN, respectively. y Pi, Pm, Pf = soil population density at planting, midseason, and harvest, respectively. z Means within effect and column followed by the same letter are not significantly different (P 0.05). PLANT HEALTH PROGRESS Vol. 15, No. 1, 2014 Page 4

5 plot as a percentage of plants displaying SDS symptoms. Disease severity (DS) was rated on each plot on a scale from 1 to 9 according to Njiti et al. (22). Disease index (DX) was calculated as a percent with a range of 0 to 100 using the equation: DX = (DS DI)/9. Using the disease index, the area under the disease progress curve (AUDPC) of SDS was calculated according to Shaner and Finney (37): n i n1 i 2 i 1 i 1 Y Y / X X where Y i was disease index (per unit) at the ith observation, X i was time (days after planting) at the ith observation, and n was total number of observations. Foliar symptoms of SDS were observed at all environments except Topeka in Severity of foliar symptoms, as characterized by AUDPC, was strongly influenced by environment and soybean cultivar main effects and interactions (Table 3). An association between SDS epidemics and above average precipitation has been observed (12), but no such trends were identifiable across years or locations in the present study. Average monthly precipitation and maximum temperatures were similar among locations and between years, with the exception that 2009 precipitation was below the long-term average for May and September and above the long-term average for August (Fig. 1). Given the presence of supplemental irrigation, environments at all locations were likely conducive to disease development. Nonetheless, significant variability in SDS severity i across environments was observed. Symptoms were least severe at Manhattan in 2008 and at Rossville in 2009, and most severe at Rossville in 2008, and at Morganville and Topeka in 2009 (Fig. 2). In environments where SDS was severe, symptoms were observed on all soybean genotypes, but SDS-resistant cultivars (COM1 and COM2) exhibited lower AUDPC values than SDSsusceptible cultivars at Rossville in 2008, and at Manhattan, Morganville, and Topeka in 2009 (Fig. 2). Resistance to SDS has been classified as partial, polygenic, and environmentally dependent, and any genotype can display symptoms if disease conditions are optimum (9,23). AUDPC values were significantly greater for the SCNsusceptible cultivar KS3406RR compared to the moderately resistant cultivar COM3 at Rossville in 2008, and at Manhattan and Topeka in Numerous studies have reported increased SDS symptom severity in the presence of H. glycines (16,25,31,35,39), but our results suggest that even moderate levels of SCN resistance can mitigate this effect. No differences in SDS AUDPC values were observed among any of the soybean genotypes at Manhattan in 2008 or at Rossville in 2009, where SDS severity was low. RELATIONSHIPS AMONG DISEASE VARIABLES Correlation analysis of pathogen populations and foliar symptom severity (AUDPC) across environments suggested the presence of both positive and negative interactions between H. glycines and F. virguliforme (Table 5). SDS severity was positively correlated with nematode population density at planting (P = 0.003). In contrast, H. glycines reproduction (RF) was negatively correlated (P 0.05) with F. virguliforme population densities at Pm and Pf, and with AUDPC (Table 5). Significant positive correlations were observed between SDS AUDPC and F. virguliforme soil population densities from all sampling dates, but the correlation between root colonization and foliar symptoms reported in previous studies (13) was not detected. Previous research has suggested both the presence and absence of a relationship between SDS and SCN (7,8,9,24,35,39). In our study, the positive relationship (r = +0.27) observed between SDS foliar severity and H. glycines soil population densities at Pi, although not strong, does suggest that the presence of H. glycines may enhance SDS disease development with the possibility of further lowering yields. Other studies have suggested additional mechanisms of interaction. F. virguliforme has the ability to survive within H. glycines cysts, so higher numbers of cysts could result in higher fungal levels at planting (17). The nematode also may increase fungal population densities by providing wounds for F. virguliforme to enter the plant and reproduce (25,35). Our results also suggest that H. glycines reproduction was impacted by the level of SDS resistance possessed by the soybean genotypes. SDS symptoms on the susceptible soybean genotypes were severe at four locations, and it is likely that the suitability of the host to support nematode reproduction was reduced as symptoms of yellowing and defoliation of leaves in the canopy progressed on SDS-susceptible genotypes. Reduced population levels of H. glycines previously have been attributed to the root necrosis caused by the fungus (17,31,35). In our study, nematode reproduction was inversely related to soil, but not to root population densities of F. virguliforme. FIGURE 1 Average monthly precipitation and maximum temperature across locations within 2008 and PLANT HEALTH PROGRESS Vol. 15, No. 1, 2014 Page 5

6 FIGURE 2 Effect of SDS and SCN resistance on AUDPC values at seven environments in Kansas. Asterisks indicate a significant difference between resistant and susceptible SDS ( y ) or SCN ( z ) genotypes (P 0.01). MAN = Manhattan, ROS = Rossville, TOP = Topeka, and MOR = Morganville, Kansas. Table 5 Pearson correlation coefficients for Fusarium virguliforme (Fv) and Heterodera glycines (Hg) population densities, SDS foliar severity (AUDPC), and soybean yield (n = 112). Correlation coefficient Variable x Fv Pi Fv Pm Fv Pf Fv root AUDPC Hg Pi Hg Pm Hg Pf Hg RF Yield ** 0.71** * Fv Pi +0.46** +0.35** ** Fv Pm +0.65** ** ** Fv Pf ** * Fv root AUDPC +0.27** * Hg Pi +0.29** ** Hg Pm +0.70** +0.52** Hg Pf +0.68** * P 0.05, ** P x Pi, Pm, Pf = population density at planting, midseason, and harvest, respectively; RF = reproductive factor IMPACT OF ENVIRONMENT AND DISEASE VARIABLES ON SOYBEAN YIELD Soybean seed yield was collected from the center four rows of each plot at harvest using a plot combine. Significant differences in yield were observed across environments and among genotypes (Table 3). Average yields ranged from 4808 kg/ha at Topeka in 2008 to 2512 kg/ha at Morganville in 2008 (Fig. 3). The SDSresistant cultivar COM2 was the highest yielding soybean genotype, with an average yield of 3948 kg/ha across environments. The SDS- and SCN-susceptible cultivar KS3406RR had the lowest average yield of 2617 kg/ha. Soybean genotypes COM1 and COM3 yielded 3618 and 3002 kg/ha, respectively, across environments. Although the relative performance of SDS-resistant and SDSsusceptible soybean genotypes varied across environments (Table 3), it is clear that SDS resistance provided improved yield in most environments, and that the level of enhanced performance was related to SDS foliar severity. SDS-resistant soybean genotypes produced significantly greater yields than susceptible genotypes in all environments except Rossville in 2009 (Fig. 3). In low SDS environments (Manhattan and Topeka in 2008, and Rossville in 2009), SDS-resistant cultivars out-yielded susceptible cultivars by 17%, perhaps reflecting a higher yield potential for the selected cultivars in the former category, or the overall lower yields of KS3406RR, which possessed greater susceptibility to both SDS and SCN. In contrast, SDS-resistant cultivars in high SDS environments (Rossville in 2008 and Manhattan, Morganville, and Topeka in 2009) out-yielded susceptible cultivars by 59%. If the putative difference in yield potential is taken into account, the average yield benefit for growing SDSresistant cultivars in high SDS environments in Kansas is estimated to be 36% (calculated as ). Soybean yield was negatively correlated (P ) with F. virguliforme root population densities and SDS foliar disease severity (Table 5). Soybean yields were not significantly correlated with soil population densities of F. virguliforme in this study, despite weak positive correlations between fungal soil population densities and SDS foliar severity. Similar relationships were observed by Rupe et al. (30). Root population density of F. virguliforme, in contrast, was a predictor of soybean yield but not foliar disease severity, and a regression model that combined AUDPC and F. virguliforme root population densities as covariates explained 57% of the variation in soybean yield (Fig. 4). Our results suggest that these two disease variables act independently and are compatible with the findings of Kazi et al. PLANT HEALTH PROGRESS Vol. 15, No. 1, 2014 Page 6

7 FIGURE 3 Effect of SDS resistance on soybean yield at seven environments in Kansas. Asterisks indicate a significant difference between main effect means of SDS resistant and susceptible genotypes (*P 0.05, **P 0.01). Means are the average of two cultivars nested within each SDS reaction type. MAN = Manhattan, ROS = Rossville, TOP = Topeka, and MOR = Morganville, Kansas. FIGURE 4 Regression of observed vs. predicted soybean yield across seven Kansas environments. Asterisks indicate regression significance (P < ). (11), who reported that separate loci underlie resistance to root infection and leaf scorch during SDS. Soybean yields were not directly correlated with H. glycines soil population densities in the present study (Table 5). This observation conflicts with a large body of evidence for SCN involvement in soybean yield suppression (1,4,14,38), but can be explained by the combination of low H. glycines population densities in all study environments and the low to moderate level of SCN resistance exhibited by the soybean cultivars. It is clear that these results do not match expectations for situations where damaging population levels of the nematode are present, or where cultivars with high levels of resistance are available. Nonetheless, both the population densities and the virulence on PI derived cultivars used in this study are representative of H. glycines populations in Kansas (33). CONCLUSIONS Our study indicated that, given the prevalent conditions in Kansas (i.e., relatively low nematode pressure and relatively poor levels of SCN resistance), soybean yield was influenced more by SDS resistance than by SCN resistance. This observation underscores the necessity of incorporating SDS resistance into future soybean cultivars to prevent or minimize yield loss to F. virguliforme. Susceptibility to the nematode did increase the severity of SDS foliar symptoms in SDS-susceptible cultivars in three out of four high SDS environments, which in turn increased the yield loss at these sites. It remains unclear whether higher levels of SCN resistance would itself provide acceptable levels of SDS control under nematode-infested field conditions, or whether the levels of SDS resistance observed in this study would be maintained in the presence of damaging nematode population densities. PLANT HEALTH PROGRESS Vol. 15, No. 1, 2014 Page 7

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