In recent studies, my research group has found that time of SDS foliar symptom onset, i.e. the time when the first SDS foliar symptoms are observed, is strongly correlated with late season SDS severity and soybean yield. These results were obtained from single soybean genotypes planted in commercial and research fields, but suggest that management practices that delay disease onset should be most effective at protecting soybean yield. I am interested in testing if time of SDS onset can be a useful measure for early screening of soybean genotypes for resistance against SDS. I would monitor SDS progress over time in soybean genotypes with varying levels of resistance and determine the correlation between resistance and time of foliar disease onset. This work could include assessments of root rot progress in plants grown in infested soil or could focus on soybean resistance to F. virguliforme toxins if plants are grown fungal culture filtrates.

Dec 1, 2020 – May 30, 2021
Develop protocols and conduct 1st run of greenhouse experiments
Conduct 1st run of culture filtrate assays
Establish 1st year of field trials in May

Jun 1, 2020 – Nov 30, 2021
Continue conducting greenhouse trials
Complete 1st year of field trials
Analyze and interpret data

Dec 1, 2021 – May 30, 2022
Complete greenhouse experiments
Conduct first set of culture filtrate assays

Jun 1, 2022 – Nov 30, 2022
Complete 2nd year of field trials
Complete culture filtrate assays
Summarize data and prepare report

Deliverables:
• New knowledge on relationship between SDS and soybean yield
• Information on reliability of SDS onset for early and accurate identification of soybean resistance to SDS
• Potential use in developing expedited and more accurate resistance screening assays
• Potential use in assessing other SDS management tools

EXPANDED PROJECT October 2021 thru September 2023
Sep 30, 2021 – Mar 31, 2022
Conduct greenhouse trials
Analyze and interpret data for year 1 field trials
Plan expansion of field trials for 2022 season, including more varieties and more field locations

Apr 1, 2022 – Sept 30, 2022
Continue greenhouse trials
Conduct 2nd year of field trials – more locations in Iowa and more varieties
Summarize greenhouse data and prepare report

Sep 30, 2021 – Mar 31, 2022
Analyze and interpret data for year 2 field trials
Complete greenhouse experiments

Jun 1, 2022 – Sept 30, 2022
Conduct 3rd year of field trials
Summarize all data and prepare report

New knowledge on the relationship between SDS severity/incidence and soybean yield loss; Information on reliability of disease onset for early and accurate identification of soybean resistance to SDS; Potential use in developing expedited and more accurate resistance screening assays; Potential use in assessing other SDS management tools

Updated January 14, 2022:
Reporting Period: Jan 1, 2020 – Dec 31, 2021
Project Objective: Determine if time of SDS onset is a reliable indicator of soybean resistance against SDS

Field experiments
During the 2021 field season, field trials were planted in two locations in Central Iowa. At each location, 8 commercial soybean varieties were planted (Table 1). The soybean varieties used were obtained from three seed companies and differed in SDS resistance ranking from susceptible to resistant. The field plots were 20 ft. long and 10 ft. wide, with 30 in. spacing, and there were 4 replications per treatment. The field trial in Ames, was located at the ISU Hinds Farm and was artificially inoculated with Fusarium virguliforme and irrigated weekly. The field trial in Roland was located at a farmer field that had a history of SDS, and was not irrigated. Plots were monitored for the onset of SDS foliar symptoms. Once first symptoms were detected, the plots were rated for incidence (% plants per plot with symptoms) and severity (% leaf area with symptoms in the symptomatic plants). Plots were harvested at the end of the season.

Results: The 2021 summer was very dry and therefore not conducive to SDS development. Mean disease index and yield for each variety at the Roland and Hinds locations are shown in Tables 2 and 3, respectively. SDS foliar symptoms were first detected at Roland on Aug 3 and at Hinds on Aug 16. However, SDS incidence and severity remained low throughout the remaining season at both locations, resulting in very low disease index (a combination of severity and incidence). There were no significant differences in SDS or yield among varieties at Roland. Although there were some differences in SDS index and yield at the Hinds location, we found no correlation between SDS index, time of SDS onset, and yield. Due to the low SDS pressure observed this season, we do not think the data is reliable for determining if time of SDS onset is a reliable indicator of soybean resistance against SDS.

Greenhouse experiments
In addition to the field trials, we also conducted five greenhouse experiments. Soybeans were planted in pots, into soil infested by the SDS pathogen. After emergence, plants were assessed every two days for foliar SDS onset and disease severity; root rot and plant growth parameters were measured at the end of the experiments. In the first two experiments, we compared six soybean genotypes that are well characterized for their levels of SDS resistance and often used as checks in SDS research. However, some of these checks did not behave as expected, with resistant genotypes acting as susceptible and vice versa. These genotypes had been stored for different periods of time and had variable vigor, which may have affected their relative resistance to SDS. We now have a new batch of seed from the same genotypes that were increased in the same field in 2021, and we plan to repeat the experiment with this new seed.
We also conducted three greenhouse experiments using the same commercial varieties that were used in the field trials. In these experiments we observed that time of onset is very similar among the varieties, varying only by 2-3 days within an experiment (Fig 1). Our data to date suggests that time of onset may not be a valuable indicator of resistance in soybean seedlings. However, we will try to improve the current protocol by reducing inoculum levels or changing light intensity to determine if those factors affect time of onset.

Future plans
With additional funding generously provided by the ISRC, we plan to establish field trials at more locations (four to six) throughout Iowa in both the 2022 and 2023 seasons. This increase in locations will increase our chances to collect data from high disease environments, so we can reliably test our hypothesis. We plan to establish trials at ISU research stations and reach out to ISRC partners and council members who have offered to help us find locations for the trials and seed sources. Due to the intense labor required to detect time of onset at multiple locations, we will investigate the possibility of using remote sensing to detect initial changes in soybean canopy health that would be indicative of SDS onset. We will work in collaboration with statisticians at ISU to explore different approaches to analyze the data, possibly using thresholds to define onset and reduce variability in the data.
For the greenhouse component of this project, we plan to conduct additional experiments, using both the commercial varieties planted in the field and a new batch of seed from genotypes with well-characterized resistance levels to SDS. We will also try to optimize our protocols by reducing inoculum levels in an attempt to spread out the SDS onset time among resistance levels.

View uploaded report

Updated March 13, 2023:
Field experiments
During the 2022 field season, 11 field trials were planted throughout Iowa (Table 1). Five of the trials were established at Iowa State University (ISU) research farms; two of those trials were artificially inoculated with the SDS pathogen, Fusarium virguliforme, one of which was irrigated and the other was not. The other six trials were established at commercial farms by collaborator Nathan Schmitz from GDM seeds who coordinated the planting, maintenance and harvesting of the field trials.
At each location, nine commercial soybean varieties were planted (Table 2). The varieties used were selected to represent three levels of resistance to SDS (susceptible, moderate and resistant) from each of three seed companies. The field plots were 20 ft. long and 10 ft. wide, with 30”spacing, and there were 4 to 6 replicate plots per treatment. Plots were monitored for the onset of SDS foliar symptoms on a regular basis. Once first symptoms were detected, the plots were rated for incidence (% plants per plot with symptoms) and severity (% leaf area with symptoms in the symptomatic plants). Plots were harvested at the end of the season.


Results:
Field experiments
The 2022 summer was dry and not very conducive to SDS development. SDS symptoms developed at three pf the ISU research farm locations but SDS incidence and severity were low throughout the season. Data for each of the three locations are shown in Tables 3-5. No SDS developed at the two other locations at Curtis Farm (data not shown).
SDS developed at five of the GDM locations. Ellsworth showed the most SDS with severity ranging from 17 to 40%, but with an average of 8 to 30 plants showing symptoms per plot, by the end of the season. SDS only developed at 1 to 4 plots in the Atlantic, Cedar Rapids, Grinnell, and Winterset locations (data not shown), so no comparisons between varieties could be done. No SDS developed in Elkhart.
Overall, in the 2022 field trials disease pressure was too low to reliably study the correlation between SDS index, time of SDS onset, and yield among varieties. However, we are consulting with a statistician for a more in depth analysis to look at those relationships within variety. We are also making plans for the 2023 field season.

Greenhouse experiments
During the reporting period, we also conducted four greenhouse experiments using the same nine commercial varieties that were used in the field trials. Soybeans were planted in pots, into soil infested by the SDS pathogen. After emergence, plants were assessed every two days for foliar SDS onset and disease severity using two approaches. First, visual ratings were made based on the percent leaf area showing yellow or brown discoloration. Then, plants were scanned using a multispectral plant phenotyping equipment (Phenospex-Planteye) that measures several plant parameters including plant height, leaf area, spectral reflectance. The goal was to compare the ability of the naked eye and a digital device to detect disease onset. At the end of 4 weeks, root rot, root dry weight and shoot dry weight were measured.
We observed that time of onset is very similar among the varieties, varying only by 2-3 days within an experiment (Figures 1-4). For example, in experiment 6 (Fig. 2), most soybean varieties, including susceptible check Spencer, started showing symptoms 23 to 26 days after planting, despite having different levels of resistance to SDS. The resistant check (MN1606) and a resistant commercial line (P25T09E) started symptoms at 33 and 27 dap, respectively, and showed overall less disease at the end of the experiment. In addition, the resistant check (MN1606) showed an 8-day delay compared with the susceptible check Spencer. Both these observations show that highly resistant varieties have a slower onset of SDS compared to very susceptible varieties. A similar contrast is present in Experiments 7 and 8 (Fig. 4).
However, time of onset does not appear to be correlated with SDS resistance in varieties in the moderately susceptible or moderately resistant range, as onset varied only by 2-3 days and there was too much variation among individual plants within variety to
Our data to date suggests that time of onset may not be a valuable indicator of resistance in soybean among moderately resistant varieties in greenhouse conditions. Field trials with higher disease pressure are needed to determine in SDS onset can be useful to separate resistance levels in field conditions.
Unfortunately, the Phenospex equipment developed a problem with the software and we have not been able to download the data for our experiments. The equipment is currently being serviced at the production site in Europe to determine if it is possible to recover the digital data.

Current and future work
Plans are underway to established field trials for the 2023 season. We will plant 4-5 trials at ISU research farms, using artificial inoculation and irrigation when possible. We are also in contact with Nathan Schmitz from GDM seeds, who has kindly agreed to plant field trials at multiple locations for this project again. We have purchased soybean seed and have prepared SDS inoculum for the field trials. In addition to the visual ratings of SDS, we plan to collaborate with a colleague at ISU to obtain drone images from the Ames locations to determine if digital imaging has the ability to detect SDS onset more reliably than visual ratings.
We will also conduct greenhouse trials to compare SDS resistance levels in the varieties used in the field trials and see if they correlate with field ratings.
Finally, we will consult with a statistician to discuss possible alternative analysis that can be done of the data from 2021-23 to explore correlations of onset time and SDS resistance in more depth. We will also conducte an analysis to determine the reliability of SDS onset as an indicator of SDS final disease severity within variety.

See attached file for full report with data figures.

View uploaded report

If time of SDS onset is shown to be a reliable indicator of soybean resistance to SDS, seed companies could use this parameter to improve selection of the most resistant genotypes and varieties for commercial use. In addition, time of onset could be used as a parameter to compare other management practices, such as seed treatments, for protecting soybean yield.