Cover crop study
Over the course of this project, greenhouse experiments were conducted to determine which cover crop species can get infected by F. virguliforme (Fv), the SDS pathogen. Eighteen crops were tested, including fifteen cover crops, two soybean varieties and one corn variety. The crops were grown in soil infested with Fv for four weeks, and evaluated for root rot, foliar symptoms and biomass. Fungal DNA was quantified in roots using qPCR to determine the level of infection in the different species. The experiment was conducted three times.

We found that inoculated alfalfa, crimson clover, red clover and pea had more root rot than the non-inoculated controls, and Fv DNA quantities in roots did not differ from those found in soybean roots. Inoculated alfalfa, corn, crimson clover, oat, red clover, sorghum and turnip plants had lower biomass compared to non-inoculated controls, although corn, oat and turnip had no root necrosis. Biomass reduction and root necrosis were not observed in inoculated hairy vetch, false flax, millet, mustard, rye, ryegrass, triticale and wheat. and Fv DNA quantity in the roots of these species was lower than in soybean. Quantification of Fv in roots showed that most legumes species were infected with Fv at levels similar to soybean. Fv was detected at very low levels or not detected in brassica and grass species.

The results of our greenhouse experiments on SDS showed that the legume cover crop species tested could get infected by Fv and are therefore hosts to the SDS pathogen. These plant species might therefore help to build-up Fv inoculum in the absence of a soybean crop. On the other hand, the Brassica spp. and grasses tested were either poor or non-hosts of Fv, and may be preferred cover crop options for fields with a history of SDS. Further studies are also needed to determine if Fv inoculum in soil increases on asymptomatic hosts and if the residue derived from asymptomatic hosts plays a role in pathogen survival.

A manuscript was accepted for publication in the J. of Soil and Water Conservation. The manuscript includes the results from the cover crop host range greenhouse study, as well as data on the cover crop host range for SCN collected in Dr. Greg Tylka’s lab.


Crop Rotation Study
Over the course of this project, we investigated the effect of crop rotations on SDS. Data was collected from a long-term crop rotation study located at the ISU Marsden Farm. The plots were established in 2011 and included the following treatments: 2-year corn-soybean rotation; 3-year corn-soybean-oat/red clover rotation, and a 4-year corn-soybean-oat/alfalfa-alfalfa rotation. The 2-year rotation was conventionally managed with synthetic fertilizers, whereas the 3- and 4-year rotations received manure and low synthetic fertilizer rates. Each treatment was replicated four times, with each phase of each rotation present every year. This field has a history of SDS since 2010. The plots were monitored over the season for SDS severity and incidence, soybean root health, seed yield. Data on the density of the Fv pathogen in soil was collected by Dr. Gwyn Beattie’s lab using qPCR.

In all years of the study, SDS incidence and severity were lower, and soybean yield was higher, in 3- and 4-year rotations than in the 2-year rotation. Incidence and severity of SDS explained 45 to 87% of the variation in yield. Plants in the 2-year corn-soybean rotation generally showed more severe root rot and lower root and shoot weights than plants in the longer, diversified rotations. Fv density in soil was up to 5-fold greater in the 2-year rotation compared with the 4-year rotation.

Our findings suggest that cropping system diversification may be used to effectively and sustainably manage SDS. Diversification of the soybean-corn rotation with oat, and clover or alfalfa, in conjunction with the use of composted manure amendments, greatly suppressed SDS development and protected soybean yield. This study also demonstrated that the reduced SDS in the 4-year rotation system was associated with a lower Fv density in soil. The processes responsible for SDS suppression in the more diversified systems are unknown but may be related to the use of oat crop or manure application in these systems. These questions are currently being investigated with funding from a new USDA project.

A manuscript was accepted on the effects of cropping systems with crop rotation and compost amendments SDS. The manuscript includes the data collected at the Marsden Farm over a period of five years showing a significant suppression of SDS in the diversified rotation plots.


Tillage field trial
A field study was conducted to investigate the effect of long-term tillage on SDS and yield. This study was established in 2010, with funding from an ISA grant, in a field site with a history of SDS. The experiment was laid-out in a split split-plot design with four replicates. The main plot factor was tillage (no till, chisel plow in fall, disc and chisel plow in fall and spring, respectively), and each main plot was divided into subplots of corn or soybeans (in a 2-year rotation). Each subplot was again divided in to sub-subplots where two soybean cultivars, susceptible and moderately resistant to SDS, were planted each year. Assessments included SDS incidence and severity, root rot severity and root dry weight, F. virguliforme population densities in roots, SCN population in soil, and yield.

After six years of evaluation, root rot and SDS disease index (FDX) were different across the years meaning some years were more favorable for the disease than the others. However, tillage did not affect the risk of SDS in any year. Tillage did not affect yield either in any year. Cultivar effect was occasionally significant for all parameters. Compared to the susceptible cultivar, the moderately resistant cultivar had 61% less root rot in 2015; 61 and 62% less foliar symptoms in 2014 and 2015, respectively, and 21% greater yield in 2015. These data suggest that planting resistant cultivars can be an effective management tactic but tillage is not an option for the SDS management.