Long-term crop rotations suppress soybean sudden death syndrome in Iowa N. ABDELSAMAD1, G. Mbofung1, A. Robertson1, M. Liebman2, and L. F. Leandro1 1 Department of Plant Pathology and Microbiology, Iowa State University, Ames, IA 50010, USA 2 Department of Agronomy, Iowa State University, Ames, IA 50010, USA Introduction Results Results Root growth and disease ratings Sudden death syndrome (SDS) of soybean is caused by the soilborne fungus Fusarium virguliforme, and is considered one of the five most economically damaging diseases in the US, causing losses in the range of 0.3 to 2.0 million tons per year for the period of 2000 to 2007 (1). Disease management is based on the use of resistant varieties, improving soil drainage, and avoiding planting in cool, wet soils. However, none of these approaches is completely effective against SDS. Crop rotations are an essential management practice against many soilborne diseases. However, there is limited information about the effects of long-term crop rotations on SDS development and F. virguliforme density in soil. Whole soybean plants were randomly sampled at R5 in 2010, and at R3 in 2011, to evaluate root rot severity and root dry weight. Foliar SDS incidence and severity ratings were taken for each plot on a percent scale. In 2010, the 2-year rotation showed the highest SDS severity and incidence (P<0.0001) compared to the 3- and 4- year rotations, on both varieties (Fig 1, and Fig 2 A-B). In 2011, SDS severity was highest (P=0.03) in the 2-year rotation, and lowest in the 4-year rotation, for the conventional variety (Fig 2 B). Incidence showed similar trends but differences were not significant (Fig 2 A). There were no significant differences in SDS severity and incidence for the Round-up ready variety in 2011 (Fig. 2 A-B). Table 1. Soybean root rot and growth, yield and SCN density in soil, in 2010, averaged over variety Rotation Root dry weight (g) Root Vigor (1-5 ) Root rot severity (%) Yield (bu/ac) SCN (eggs/ 100cc) 2-year 9.88 2.25 84.9 31.8 505 3-year 12.06 3.65 55.6 54.8 506 4-year 12.70 4.00 51.3 54.1 456 St. Error 0.88 0.24 8.8 1.6 120 3- year rotation 2- year rotation Fig. 1. Defoliation caused by SDS at Marsden farm in Fall 2010. Soybean plot in a 2-y rotation (right), and in a 3-yr rotation (left). Photo by Laura Miller, Leopold Center. In 2010, the 2-year rotation treatment showed more (P < 0.05) severe root rot, lower vigor, and smaller root dry weight compared to the 3 and 4-year rotations (Table 1). Similar differences (P<0.05) in root rot and root dry weight were observed among rotation treatments in 2011. Soybean yield was more than 20 bu/ac lower (P=0.003) in the 2-year rotation compared to 3- and 4- year rotations in 2010 (Table 1). Yield differences were also significant (P=0.018) in 2011, with 51.8, 58.7, and 61.9 bu/ac for the 2-, 3- and 4-year rotations, respectively. Yield loss was greater for the conventional variety than for the Round-up ready variety in both years. Fig. 2. SDS incidence, severity, and F. virguliforme population density in soil in 2010 and 2011. ns A b B a Fv population (2011) Fv population (2010) A Objective The objective of this study was to investigate the effect of long-term crop rotations on SDS incidence and severity, root rot and root growth, yield, and F. virguliforme and soybean cyst nematode (SCN) populations in soil. B Quantification of F. virguliforme in soil Soil samples were collected from each plot after harvest, and F. virguliforme was quantified using a Taqman protocol developed by Fakhoury et al. (3; APS poster 519P). Total genomic DNA was extracted from 250 mg of soil in triplicate. DNA concentration normalized to 5ng/ul 4ul DNA + Specific Fv primers and probe Real time PCR for Fv quantification using Taqman chemistry Methods C Conclusions The study was conducted in a field trial established in 2002, in Ames, IA, to investigate the effect of crop rotations on soybean productivity (2). The trial included three crop rotations: ns This study showed that soybean rotations with crops different from corn can suppress SDS development and reduce yield loss due to the disease. Results from quantification of Fv population density in soil suggest that the reduction in disease may have been caused by suppression of pathogen buildup in soil. This study suggests that long-term crop rotations with alfalfa, oats or clover may offer an alternative management practice for SDS. Studies on host range and inhibitory effects of alterative crops on F. virguliforme population dynamics would be valuable. Corn – Soybean 2-year (S2) Corn – Soybean – Oat/Red Clover 3-year (S3) Corn – Soybean – Oat/Alfalfa – Alfalfa 4-year (S4) F. virguliforme population density in soil did not differ significantly (P > 0.05) among rotation treatments. However, numerical trends suggest that inoculum density in soil was highest in the 2-year rotation compared to the 4-year rotation (Fig 2 C). SCN populations in soil were low (less than 2000 eggs/ cc soil), and did not differ (P = 0.05) among crop rotations, in both years of the trial (Table 1). Two soybean cultivars were included in each rotation: Round-up ready ( K287 RR) Conventional (K2918) In 2010 and 2011, the trial was used to evaluate the long-term effect of the crop rotations on SDS and soybean productivity. References Assessments: Populations of F. virguliforme and SCN in soil Root rot and root dry weight SDS incidence and severity Yield Wrather et al. 2009. Effects of diseases on soybean yields in the United States 1996 to 2007. Plant Health Progress doi:10.1094/PHP-2009-0401-01-RS Liebman et al. 2008. Agronomic and economic performance characteristics of conventional and low-external-input cropping systems in the central corn belt. Agron. J. 100: 600-610. Fakhoury et al. Profiling microbial communities in soils of SDS-infested soybean fields using next-generation sequencing. Ahmad Fakhoury. APS poster 519P. Acknowledgments We thank the Iowa Soybean Association, the USDA National Research Initiative and the Leopold Center for Sustainable Agriculture for funding this work. Thank you to Miralba Agudelo and David Sundberg for assistance with field and lab operations.